3. The GNAT Compilation Model

This chapter describes the compilation model used by GNAT. Although similar to that used by other languages such as C and C++, this model is substantially different from the traditional Ada compilation models, which are based on a centralized program library. The chapter covers the following material:

3.1. Source Representation

Ada source programs are represented in standard text files, using Latin-1 coding. Latin-1 is an 8-bit code that includes the familiar 7-bit ASCII set, plus additional characters used for representing foreign languages (see Foreign Language Representation for support of non-USA character sets). The format effector characters are represented using their standard ASCII encodings, as follows:





Vertical tab



Horizontal tab



Carriage return



Line feed



Form feed


Source files are in standard text file format. In addition, GNAT will recognize a wide variety of stream formats, in which the end of physical lines is marked by any of the following sequences: LF, CR, CR-LF, or LF-CR. This is useful in accommodating files that are imported from other operating systems.

The end of a source file is normally represented by the physical end of file. However, the control character 16#1A# (SUB) is also recognized as signalling the end of the source file. Again, this is provided for compatibility with other operating systems where this code is used to represent the end of file.

Each file contains a single Ada compilation unit, including any pragmas associated with the unit. For example, this means you must place a package declaration (a package spec) and the corresponding body in separate files. An Ada compilation (which is a sequence of compilation units) is represented using a sequence of files. Similarly, you will place each subunit or child unit in a separate file.

3.2. Foreign Language Representation

GNAT supports the standard character sets defined in Ada as well as several other non-standard character sets for use in localized versions of the compiler (Character Set Control).

3.2.1. Latin-1

The basic character set is Latin-1. This character set is defined by ISO standard 8859, part 1. The lower half (character codes 16#00#16#7F#) is identical to standard ASCII coding, but the upper half is used to represent additional characters. These include extended letters used by European languages, such as French accents, the vowels with umlauts used in German, and the extra letter A-ring used in Swedish.

For a complete list of Latin-1 codes and their encodings, see the source file of library unit Ada.Characters.Latin_1 in file a-chlat1.ads. You may use any of these extended characters freely in character or string literals. In addition, the extended characters that represent letters can be used in identifiers.

3.2.2. Other 8-Bit Codes

GNAT also supports several other 8-bit coding schemes:

ISO 8859-2 (Latin-2)

Latin-2 letters allowed in identifiers, with uppercase and lowercase equivalence.

ISO 8859-3 (Latin-3)

Latin-3 letters allowed in identifiers, with uppercase and lowercase equivalence.

ISO 8859-4 (Latin-4)

Latin-4 letters allowed in identifiers, with uppercase and lowercase equivalence.

ISO 8859-5 (Cyrillic)

ISO 8859-5 letters (Cyrillic) allowed in identifiers, with uppercase and lowercase equivalence.

ISO 8859-15 (Latin-9)

ISO 8859-15 (Latin-9) letters allowed in identifiers, with uppercase and lowercase equivalence

IBM PC (code page 437)

This code page is the normal default for PCs in the U.S. It corresponds to the original IBM PC character set. This set has some, but not all, of the extended Latin-1 letters, but these letters do not have the same encoding as Latin-1. In this mode, these letters are allowed in identifiers with uppercase and lowercase equivalence.

IBM PC (code page 850)

This code page is a modification of 437 extended to include all the Latin-1 letters, but still not with the usual Latin-1 encoding. In this mode, all these letters are allowed in identifiers with uppercase and lowercase equivalence.

Full Upper 8-bit

Any character in the range 80-FF allowed in identifiers, and all are considered distinct. In other words, there are no uppercase and lowercase equivalences in this range. This is useful in conjunction with certain encoding schemes used for some foreign character sets (e.g., the typical method of representing Chinese characters on the PC).

No Upper-Half

No upper-half characters in the range 80-FF are allowed in identifiers. This gives Ada 83 compatibility for identifier names.

For precise data on the encodings permitted, and the uppercase and lowercase equivalences that are recognized, see the file csets.adb in the GNAT compiler sources. You will need to obtain a full source release of GNAT to obtain this file.

3.2.3. Wide_Character Encodings

GNAT allows wide character codes to appear in character and string literals, and also optionally in identifiers, by means of the following possible encoding schemes:

Hex Coding

In this encoding, a wide character is represented by the following five character sequence:

ESC a b c d

where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ESC A345 is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set.

Upper-Half Coding

The wide character with encoding 16#abcd# where the upper bit is on (in other words, ‘a’ is in the range 8-F) is represented as two bytes, 16#ab# and 16#cd#. The second byte cannot be a format control character, but is not required to be in the upper half. This method can be also used for shift-JIS or EUC, where the internal coding matches the external coding.

Shift JIS Coding

A wide character is represented by a two-character sequence, 16#ab# and 16#cd#, with the restrictions described for upper-half encoding as described above. The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion. Only characters defined in the JIS code set table can be used with this encoding method.

EUC Coding

A wide character is represented by a two-character sequence 16#ab# and 16#cd#, with both characters being in the upper half. The internal character code is the corresponding JIS character according to the EUC encoding algorithm. Only characters defined in the JIS code set table can be used with this encoding method.

UTF-8 Coding

A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2. Depending on the character value, the representation is a one, two, or three byte sequence:

16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

where the xxx bits correspond to the left-padded bits of the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, and in the following section on wide wide characters, the use of these sequences is documented).

Brackets Coding

In this encoding, a wide character is represented by the following eight character sequence:

[ " a b c d " ]

where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, [‘A345’] is used to represent the wide character with code 16#A345#. It is also possible (though not required) to use the Brackets coding for upper half characters. For example, the code 16#A3# can be represented as ['A3'].

This scheme is compatible with use of the full Wide_Character set, and is also the method used for wide character encoding in some standard ACATS (Ada Conformity Assessment Test Suite) test suite distributions.


Some of these coding schemes do not permit the full use of the Ada character set. For example, neither Shift JIS nor EUC allow the use of the upper half of the Latin-1 set.

3.2.4. Wide_Wide_Character Encodings

GNAT allows wide wide character codes to appear in character and string literals, and also optionally in identifiers, by means of the following possible encoding schemes:

UTF-8 Coding

A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2. Depending on the character value, the representation of character codes with values greater than 16#FFFF# is a is a four, five, or six byte sequence:

16#01_0000#-16#10_FFFF#:     11110xxx 10xxxxxx 10xxxxxx
16#0020_0000#-16#03FF_FFFF#: 111110xx 10xxxxxx 10xxxxxx
                             10xxxxxx 10xxxxxx
16#0400_0000#-16#7FFF_FFFF#: 1111110x 10xxxxxx 10xxxxxx
                             10xxxxxx 10xxxxxx 10xxxxxx

where the xxx bits correspond to the left-padded bits of the 32-bit character value.

Brackets Coding

In this encoding, a wide wide character is represented by the following ten or twelve byte character sequence:

[ " a b c d e f " ]
[ " a b c d e f g h " ]

where a-h are the six or eight hexadecimal characters (using uppercase letters) of the wide wide character code. For example, [“1F4567”] is used to represent the wide wide character with code 16#001F_4567#.

This scheme is compatible with use of the full Wide_Wide_Character set, and is also the method used for wide wide character encoding in some standard ACATS (Ada Conformity Assessment Test Suite) test suite distributions.

3.3. File Naming Topics and Utilities

GNAT has a default file naming scheme and also provides the user with a high degree of control over how the names and extensions of the source files correspond to the Ada compilation units that they contain.

3.3.1. File Naming Rules

The default file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters.

An exception arises if the file name generated by the above rules starts with one of the characters a, g, i, or s, and the second character is a minus. In this case, the character tilde is used in place of the minus. The reason for this special rule is to avoid clashes with the standard names for child units of the packages System, Ada, Interfaces, and GNAT, which use the prefixes s-, a-, i-, and g-, respectively.

The file extension is .ads for a spec and .adb for a body. The following table shows some examples of these rules.

Source File

Ada Compilation Unit


Main (spec)


Main (body)


Arith_Functions (package spec)


Arith_Functions (package body)


Func.Spec (child package spec)


Func.Spec (child package body)


Sub (subunit of Main)


A.Bad (child package body)

Following these rules can result in excessively long file names if corresponding unit names are long (for example, if child units or subunits are heavily nested). An option is available to shorten such long file names (called file name ‘krunching’). This may be particularly useful when programs being developed with GNAT are to be used on operating systems with limited file name lengths. Using gnatkr.

Of course, no file shortening algorithm can guarantee uniqueness over all possible unit names; if file name krunching is used, it is your responsibility to ensure no name clashes occur. Alternatively you can specify the exact file names that you want used, as described in the next section. Finally, if your Ada programs are migrating from a compiler with a different naming convention, you can use the gnatchop utility to produce source files that follow the GNAT naming conventions. (For details see Renaming Files with gnatchop.)

Note: in the case of Windows or Mac OS operating systems, case is not significant. So for example on Windows if the canonical name is main-sub.adb, you can use the file name Main-Sub.adb instead. However, case is significant for other operating systems, so for example, if you want to use other than canonically cased file names on a Unix system, you need to follow the procedures described in the next section.

3.3.2. Using Other File Names

In the previous section, we have described the default rules used by GNAT to determine the file name in which a given unit resides. It is often convenient to follow these default rules, and if you follow them, the compiler knows without being explicitly told where to find all the files it needs.

However, in some cases, particularly when a program is imported from another Ada compiler environment, it may be more convenient for the programmer to specify which file names contain which units. GNAT allows arbitrary file names to be used by means of the Source_File_Name pragma. The form of this pragma is as shown in the following examples:

pragma Source_File_Name (My_Utilities.Stacks,
  Spec_File_Name => "myutilst_a.ada");
pragma Source_File_name (My_Utilities.Stacks,
  Body_File_Name => "myutilst.ada");

As shown in this example, the first argument for the pragma is the unit name (in this example a child unit). The second argument has the form of a named association. The identifier indicates whether the file name is for a spec or a body; the file name itself is given by a string literal.

The source file name pragma is a configuration pragma, which means that normally it will be placed in the gnat.adc file used to hold configuration pragmas that apply to a complete compilation environment. For more details on how the gnat.adc file is created and used see Handling of Configuration Pragmas.

GNAT allows completely arbitrary file names to be specified using the source file name pragma. However, if the file name specified has an extension other than .ads or .adb it is necessary to use a special syntax when compiling the file. The name in this case must be preceded by the special sequence -x followed by a space and the name of the language, here ada, as in:

$ gcc -c -x ada peculiar_file_name.sim

gnatmake handles non-standard file names in the usual manner (the non-standard file name for the main program is simply used as the argument to gnatmake). Note that if the extension is also non-standard, then it must be included in the gnatmake command, it may not be omitted.

3.3.3. Alternative File Naming Schemes

The previous section described the use of the Source_File_Name pragma to allow arbitrary names to be assigned to individual source files. However, this approach requires one pragma for each file, and especially in large systems can result in very long gnat.adc files, and also create a maintenance problem.

GNAT also provides a facility for specifying systematic file naming schemes other than the standard default naming scheme previously described. An alternative scheme for naming is specified by the use of Source_File_Name pragmas having the following format:

pragma Source_File_Name (
   Spec_File_Name  => FILE_NAME_PATTERN
 [ , Casing          => CASING_SPEC]
 [ , Dot_Replacement => STRING_LITERAL ] );

pragma Source_File_Name (
   Body_File_Name  => FILE_NAME_PATTERN
 [ , Casing          => CASING_SPEC ]
 [ , Dot_Replacement => STRING_LITERAL ] ) ;

pragma Source_File_Name (
   Subunit_File_Name  => FILE_NAME_PATTERN
 [ , Casing          => CASING_SPEC ]
 [ , Dot_Replacement => STRING_LITERAL ] ) ;

CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

The FILE_NAME_PATTERN string shows how the file name is constructed. It contains a single asterisk character, and the unit name is substituted systematically for this asterisk. The optional parameter Casing indicates whether the unit name is to be all upper-case letters, all lower-case letters, or mixed-case. If no Casing parameter is used, then the default is all lower-case.

The optional Dot_Replacement string is used to replace any periods that occur in subunit or child unit names. If no Dot_Replacement argument is used then separating dots appear unchanged in the resulting file name. Although the above syntax indicates that the Casing argument must appear before the Dot_Replacement argument, but it is also permissible to write these arguments in the opposite order.

As indicated, it is possible to specify different naming schemes for bodies, specs, and subunits. Quite often the rule for subunits is the same as the rule for bodies, in which case, there is no need to give a separate Subunit_File_Name rule, and in this case the Body_File_name rule is used for subunits as well.

The separate rule for subunits can also be used to implement the rather unusual case of a compilation environment (e.g., a single directory) which contains a subunit and a child unit with the same unit name. Although both units cannot appear in the same partition, the Ada Reference Manual allows (but does not require) the possibility of the two units coexisting in the same environment.

The file name translation works in the following steps:

  • If there is a specific Source_File_Name pragma for the given unit, then this is always used, and any general pattern rules are ignored.

  • If there is a pattern type Source_File_Name pragma that applies to the unit, then the resulting file name will be used if the file exists. If more than one pattern matches, the latest one will be tried first, and the first attempt resulting in a reference to a file that exists will be used.

  • If no pattern type Source_File_Name pragma that applies to the unit for which the corresponding file exists, then the standard GNAT default naming rules are used.

As an example of the use of this mechanism, consider a commonly used scheme in which file names are all lower case, with separating periods copied unchanged to the resulting file name, and specs end with .1.ada, and bodies end with .2.ada. GNAT will follow this scheme if the following two pragmas appear:

pragma Source_File_Name
  (Spec_File_Name => ".1.ada");
pragma Source_File_Name
  (Body_File_Name => ".2.ada");

The default GNAT scheme is actually implemented by providing the following default pragmas internally:

pragma Source_File_Name
  (Spec_File_Name => ".ads", Dot_Replacement => "-");
pragma Source_File_Name
  (Body_File_Name => ".adb", Dot_Replacement => "-");

Our final example implements a scheme typically used with one of the Ada 83 compilers, where the separator character for subunits was ‘__’ (two underscores), specs were identified by adding _.ADA, bodies by adding .ADA, and subunits by adding .SEP. All file names were upper case. Child units were not present of course since this was an Ada 83 compiler, but it seems reasonable to extend this scheme to use the same double underscore separator for child units.

pragma Source_File_Name
  (Spec_File_Name => "_.ADA",
   Dot_Replacement => "__",
   Casing = Uppercase);
pragma Source_File_Name
  (Body_File_Name => ".ADA",
   Dot_Replacement => "__",
   Casing = Uppercase);
pragma Source_File_Name
  (Subunit_File_Name => ".SEP",
   Dot_Replacement => "__",
   Casing = Uppercase);

3.3.4. Handling Arbitrary File Naming Conventions with gnatname Arbitrary File Naming Conventions

The GNAT compiler must be able to know the source file name of a compilation unit. When using the standard GNAT default file naming conventions (.ads for specs, .adb for bodies), the GNAT compiler does not need additional information.

When the source file names do not follow the standard GNAT default file naming conventions, the GNAT compiler must be given additional information through a configuration pragmas file (Configuration Pragmas) or a project file. When the non-standard file naming conventions are well-defined, a small number of pragmas Source_File_Name specifying a naming pattern (Alternative File Naming Schemes) may be sufficient. However, if the file naming conventions are irregular or arbitrary, a number of pragma Source_File_Name for individual compilation units must be defined. To help maintain the correspondence between compilation unit names and source file names within the compiler, GNAT provides a tool gnatname to generate the required pragmas for a set of files. Running gnatname

The usual form of the gnatname command is:

$ gnatname [ switches ]  naming_pattern  [ naming_patterns ]
    [--and [ switches ]  naming_pattern  [ naming_patterns ]]

All of the arguments are optional. If invoked without any argument, gnatname will display its usage.

When used with at least one naming pattern, gnatname will attempt to find all the compilation units in files that follow at least one of the naming patterns. To find these compilation units, gnatname will use the GNAT compiler in syntax-check-only mode on all regular files.

One or several Naming Patterns may be given as arguments to gnatname. Each Naming Pattern is enclosed between double quotes (or single quotes on Windows). A Naming Pattern is a regular expression similar to the wildcard patterns used in file names by the Unix shells or the DOS prompt.

gnatname may be called with several sections of directories/patterns. Sections are separated by the switch --and. In each section, there must be at least one pattern. If no directory is specified in a section, the current directory (or the project directory if -P is used) is implied. The options other that the directory switches and the patterns apply globally even if they are in different sections.

Examples of Naming Patterns are:

"body_*"    "spec_*"

For a more complete description of the syntax of Naming Patterns, see the second kind of regular expressions described in g-regexp.ads (the ‘Glob’ regular expressions).

When invoked without the switch -P, gnatname will create a configuration pragmas file gnat.adc in the current working directory, with pragmas Source_File_Name for each file that contains a valid Ada unit. Switches for gnatname

Switches for gnatname must precede any specified Naming Pattern.

You may specify any of the following switches to gnatname:


Display Copyright and version, then exit disregarding all other options.


If --version was not used, display usage, then exit disregarding all other options.


Real object, library or exec directories are subdirectories <dir> of the specified ones.


Do not create a backup copy of an existing project file.


Start another section of directories/patterns.


Create a configuration pragmas file filename (instead of the default gnat.adc). There may be zero, one or more space between -c and filename. filename may include directory information. filename must be writable. There may be only one switch -c. When a switch -c is specified, no switch -P may be specified (see below).


Look for source files in directory dir. There may be zero, one or more spaces between -d and dir. dir may end with /**, that is it may be of the form root_dir/**. In this case, the directory root_dir and all of its subdirectories, recursively, have to be searched for sources. When a switch -d is specified, the current working directory will not be searched for source files, unless it is explicitly specified with a -d or -D switch. Several switches -d may be specified. If dir is a relative path, it is relative to the directory of the configuration pragmas file specified with switch -c, or to the directory of the project file specified with switch -P or, if neither switch -c nor switch -P are specified, it is relative to the current working directory. The directory specified with switch -d must exist and be readable.


Look for source files in all directories listed in text file filename. There may be zero, one or more spaces between -D and filename. filename must be an existing, readable text file. Each nonempty line in filename must be a directory. Specifying switch -D is equivalent to specifying as many switches -d as there are nonempty lines in file.


Follow symbolic links when processing project files.


Foreign patterns. Using this switch, it is possible to add sources of languages other than Ada to the list of sources of a project file. It is only useful if a -P switch is used. For example,

gnatname -Pprj -f"*.c" "*.ada"

will look for Ada units in all files with the .ada extension, and will add to the list of file for project prj.gpr the C files with extension .c.


Output usage (help) information. The output is written to stdout.


Create or update project file proj. There may be zero, one or more space between -P and proj. proj may include directory information. proj must be writable. There may be only one switch -P. When a switch -P is specified, no switch -c may be specified. On all platforms, except on VMS, when gnatname is invoked for an existing project file <proj>.gpr, a backup copy of the project file is created in the project directory with file name <proj>.gpr.saved_x. ‘x’ is the first non negative number that makes this backup copy a new file.


Verbose mode. Output detailed explanation of behavior to stdout. This includes name of the file written, the name of the directories to search and, for each file in those directories whose name matches at least one of the Naming Patterns, an indication of whether the file contains a unit, and if so the name of the unit.

-v -v

Very Verbose mode. In addition to the output produced in verbose mode, for each file in the searched directories whose name matches none of the Naming Patterns, an indication is given that there is no match.


Excluded patterns. Using this switch, it is possible to exclude some files that would match the name patterns. For example,

gnatname -x "*_nt.ada" "*.ada"

will look for Ada units in all files with the .ada extension, except those whose names end with _nt.ada. Examples of gnatname Usage

$ gnatname -c /home/me/names.adc -d sources "[a-z]*.ada*"

In this example, the directory /home/me must already exist and be writable. In addition, the directory /home/me/sources (specified by -d sources) must exist and be readable.

Note the optional spaces after -c and -d.

$ gnatname -P/home/me/proj -x "*_nt_body.ada"
-dsources -dsources/plus -Dcommon_dirs.txt "body_*" "spec_*"

Note that several switches -d may be used, even in conjunction with one or several switches -D. Several Naming Patterns and one excluded pattern are used in this example.

3.3.5. File Name Krunching with gnatkr

This section discusses the method used by the compiler to shorten the default file names chosen for Ada units so that they do not exceed the maximum length permitted. It also describes the gnatkr utility that can be used to determine the result of applying this shortening. About gnatkr

The default file naming rule in GNAT is that the file name must be derived from the unit name. The exact default rule is as follows:

  • Take the unit name and replace all dots by hyphens.

  • If such a replacement occurs in the second character position of a name, and the first character is a, g, s, or i, then replace the dot by the character ~ (tilde) instead of a minus.

    The reason for this exception is to avoid clashes with the standard names for children of System, Ada, Interfaces, and GNAT, which use the prefixes s-, a-, i-, and g-, respectively.

The -gnatknn switch of the compiler activates a ‘krunching’ circuit that limits file names to nn characters (where nn is a decimal integer).

The gnatkr utility can be used to determine the krunched name for a given file, when krunched to a specified maximum length. Using gnatkr

The gnatkr command has the form:

$ gnatkr name [ length ]

name is the uncrunched file name, derived from the name of the unit in the standard manner described in the previous section (i.e., in particular all dots are replaced by hyphens). The file name may or may not have an extension (defined as a suffix of the form period followed by arbitrary characters other than period). If an extension is present then it will be preserved in the output. For example, when krunching hellofile.ads to eight characters, the result will be hellofil.ads.

Note: for compatibility with previous versions of gnatkr dots may appear in the name instead of hyphens, but the last dot will always be taken as the start of an extension. So if gnatkr is given an argument such as Hello.World.adb it will be treated exactly as if the first period had been a hyphen, and for example krunching to eight characters gives the result hellworl.adb.

Note that the result is always all lower case. Characters of the other case are folded as required.

length represents the length of the krunched name. The default when no argument is given is 8 characters. A length of zero stands for unlimited, in other words do not chop except for system files where the implied crunching length is always eight characters.

The output is the krunched name. The output has an extension only if the original argument was a file name with an extension. Krunching Method

The initial file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters, except that a hyphen in the second character position is replaced by a tilde if the first character is a, i, g, or s. The extension is .ads for a spec and .adb for a body. Krunching does not affect the extension, but the file name is shortened to the specified length by following these rules:

  • The name is divided into segments separated by hyphens, tildes or underscores and all hyphens, tildes, and underscores are eliminated. If this leaves the name short enough, we are done.

  • If the name is too long, the longest segment is located (left-most if there are two of equal length), and shortened by dropping its last character. This is repeated until the name is short enough.

    As an example, consider the krunching of our-strings-wide_fixed.adb to fit the name into 8 characters as required by some operating systems:

    our-strings-wide_fixed 22
    our strings wide fixed 19
    our string  wide fixed 18
    our strin   wide fixed 17
    our stri    wide fixed 16
    our stri    wide fixe  15
    our str     wide fixe  14
    our str     wid  fixe  13
    our str     wid  fix   12
    ou  str     wid  fix   11
    ou  st      wid  fix   10
    ou  st      wi   fix   9
    ou  st      wi   fi    8
    Final file name: oustwifi.adb
  • The file names for all predefined units are always krunched to eight characters. The krunching of these predefined units uses the following special prefix replacements:







    interfac es-




    These system files have a hyphen in the second character position. That is why normal user files replace such a character with a tilde, to avoid confusion with system file names.

    As an example of this special rule, consider ada-strings-wide_fixed.adb, which gets krunched as follows:

    ada-strings-wide_fixed 22
    a-  strings wide fixed 18
    a-  string  wide fixed 17
    a-  strin   wide fixed 16
    a-  stri    wide fixed 15
    a-  stri    wide fixe  14
    a-  str     wide fixe  13
    a-  str     wid  fixe  12
    a-  str     wid  fix   11
    a-  st      wid  fix   10
    a-  st      wi   fix   9
    a-  st      wi   fi    8
    Final file name: a-stwifi.adb

Of course no file shortening algorithm can guarantee uniqueness over all possible unit names, and if file name krunching is used then it is your responsibility to ensure that no name clashes occur. The utility program gnatkr is supplied for conveniently determining the krunched name of a file. Examples of gnatkr Usage

$ gnatkr very_long_unit_name.ads      --> velounna.ads
$ gnatkr grandparent-parent-child.ads --> grparchi.ads
$ gnatkr Grandparent.Parent.Child.ads --> grparchi.ads
$ gnatkr grandparent-parent-child     --> grparchi
$ gnatkr very_long_unit_name.ads/count=6 --> vlunna.ads
$ gnatkr very_long_unit_name.ads/count=0 --> very_long_unit_name.ads

3.3.6. Renaming Files with gnatchop

This section discusses how to handle files with multiple units by using the gnatchop utility. This utility is also useful in renaming files to meet the standard GNAT default file naming conventions. Handling Files with Multiple Units

The basic compilation model of GNAT requires that a file submitted to the compiler have only one unit and there be a strict correspondence between the file name and the unit name.

If you want to keep your files with multiple units, perhaps to maintain compatibility with some other Ada compilation system, you can use gnatname to generate or update your project files. Generated or modified project files can be processed by GNAT.

See Handling Arbitrary File Naming Conventions with gnatname for more details on how to use gnatname.

Alternatively, if you want to permanently restructure a set of ‘foreign’ files so that they match the GNAT rules, and do the remaining development using the GNAT structure, you can simply use gnatchop once, generate the new set of files and work with them from that point on.

Note that if your file containing multiple units starts with a byte order mark (BOM) specifying UTF-8 encoding, then the files generated by gnatchop will each start with a copy of this BOM, meaning that they can be compiled automatically in UTF-8 mode without needing to specify an explicit encoding. Operating gnatchop in Compilation Mode

The basic function of gnatchop is to take a file with multiple units and split it into separate files. The boundary between files is reasonably clear, except for the issue of comments and pragmas. In default mode, the rule is that any pragmas between units belong to the previous unit, except that configuration pragmas always belong to the following unit. Any comments belong to the following unit. These rules almost always result in the right choice of the split point without needing to mark it explicitly and most users will find this default to be what they want. In this default mode it is incorrect to submit a file containing only configuration pragmas, or one that ends in configuration pragmas, to gnatchop.

However, using a special option to activate ‘compilation mode’, gnatchop can perform another function, which is to provide exactly the semantics required by the RM for handling of configuration pragmas in a compilation. In the absence of configuration pragmas (at the main file level), this option has no effect, but it causes such configuration pragmas to be handled in a quite different manner.

First, in compilation mode, if gnatchop is given a file that consists of only configuration pragmas, then this file is appended to the gnat.adc file in the current directory. This behavior provides the required behavior described in the RM for the actions to be taken on submitting such a file to the compiler, namely that these pragmas should apply to all subsequent compilations in the same compilation environment. Using GNAT, the current directory, possibly containing a gnat.adc file is the representation of a compilation environment. For more information on the gnat.adc file, see Handling of Configuration Pragmas.

Second, in compilation mode, if gnatchop is given a file that starts with configuration pragmas, and contains one or more units, then these configuration pragmas are prepended to each of the chopped files. This behavior provides the required behavior described in the RM for the actions to be taken on compiling such a file, namely that the pragmas apply to all units in the compilation, but not to subsequently compiled units.

Finally, if configuration pragmas appear between units, they are appended to the previous unit. This results in the previous unit being illegal, since the compiler does not accept configuration pragmas that follow a unit. This provides the required RM behavior that forbids configuration pragmas other than those preceding the first compilation unit of a compilation.

For most purposes, gnatchop will be used in default mode. The compilation mode described above is used only if you need exactly accurate behavior with respect to compilations, and you have files that contain multiple units and configuration pragmas. In this circumstance the use of gnatchop with the compilation mode switch provides the required behavior, and is for example the mode in which GNAT processes the ACVC tests. Command Line for gnatchop

The gnatchop command has the form:

$ gnatchop switches file_name [file_name ...]

The only required argument is the file name of the file to be chopped. There are no restrictions on the form of this file name. The file itself contains one or more Ada units, in normal GNAT format, concatenated together. As shown, more than one file may be presented to be chopped.

When run in default mode, gnatchop generates one output file in the current directory for each unit in each of the files.

directory, if specified, gives the name of the directory to which the output files will be written. If it is not specified, all files are written to the current directory.

For example, given a file called hellofiles containing

procedure Hello;

with Ada.Text_IO; use Ada.Text_IO;
procedure Hello is
   Put_Line ("Hello");
end Hello;

the command

$ gnatchop hellofiles

generates two files in the current directory, one called hello.ads containing the single line that is the procedure spec, and the other called hello.adb containing the remaining text. The original file is not affected. The generated files can be compiled in the normal manner.

When gnatchop is invoked on a file that is empty or that contains only empty lines and/or comments, gnatchop will not fail, but will not produce any new sources.

For example, given a file called toto.txt containing

--  Just a comment

the command

$ gnatchop toto.txt

will not produce any new file and will result in the following warnings:

toto.txt:1:01: warning: empty file, contains no compilation units
no compilation units found
no source files written Switches for gnatchop

gnatchop recognizes the following switches:


Display Copyright and version, then exit disregarding all other options.


If --version was not used, display usage, then exit disregarding all other options.


Causes gnatchop to operate in compilation mode, in which configuration pragmas are handled according to strict RM rules. See previous section for a full description of this mode.


This passes the given -gnatxxx switch to gnat which is used to parse the given file. Not all xxx options make sense, but for example, the use of -gnati2 allows gnatchop to process a source file that uses Latin-2 coding for identifiers.


Causes gnatchop to generate a brief help summary to the standard output file showing usage information.


Limit generated file names to the specified number mm of characters. This is useful if the resulting set of files is required to be interoperable with systems which limit the length of file names. No space is allowed between the -k and the numeric value. The numeric value may be omitted in which case a default of -k8, suitable for use with DOS-like file systems, is used. If no -k switch is present then there is no limit on the length of file names.


Causes the file modification time stamp of the input file to be preserved and used for the time stamp of the output file(s). This may be useful for preserving coherency of time stamps in an environment where gnatchop is used as part of a standard build process.


Causes output of informational messages indicating the set of generated files to be suppressed. Warnings and error messages are unaffected.


Generate Source_Reference pragmas. Use this switch if the output files are regarded as temporary and development is to be done in terms of the original unchopped file. This switch causes Source_Reference pragmas to be inserted into each of the generated files to refers back to the original file name and line number. The result is that all error messages refer back to the original unchopped file. In addition, the debugging information placed into the object file (when the -g switch of gcc or gnatmake is specified) also refers back to this original file so that tools like profilers and debuggers will give information in terms of the original unchopped file.

If the original file to be chopped itself contains a Source_Reference pragma referencing a third file, then gnatchop respects this pragma, and the generated Source_Reference pragmas in the chopped file refer to the original file, with appropriate line numbers. This is particularly useful when gnatchop is used in conjunction with gnatprep to compile files that contain preprocessing statements and multiple units.


Causes gnatchop to operate in verbose mode. The version number and copyright notice are output, as well as exact copies of the gnat1 commands spawned to obtain the chop control information.


Overwrite existing file names. Normally gnatchop regards it as a fatal error if there is already a file with the same name as a file it would otherwise output, in other words if the files to be chopped contain duplicated units. This switch bypasses this check, and causes all but the last instance of such duplicated units to be skipped.


Specify the path of the GNAT parser to be used. When this switch is used, no attempt is made to add the prefix to the GNAT parser executable. Examples of gnatchop Usage

$ gnatchop -w hello_s.ada prerelease/files

Chops the source file hello_s.ada. The output files will be placed in the directory prerelease/files, overwriting any files with matching names in that directory (no files in the current directory are modified).

$ gnatchop archive

Chops the source file archive into the current directory. One useful application of gnatchop is in sending sets of sources around, for example in email messages. The required sources are simply concatenated (for example, using a Unix cat command), and then gnatchop is used at the other end to reconstitute the original file names.

$ gnatchop file1 file2 file3 direc

Chops all units in files file1, file2, file3, placing the resulting files in the directory direc. Note that if any units occur more than once anywhere within this set of files, an error message is generated, and no files are written. To override this check, use the -w switch, in which case the last occurrence in the last file will be the one that is output, and earlier duplicate occurrences for a given unit will be skipped.

3.4. Configuration Pragmas

Configuration pragmas include those pragmas described as such in the Ada Reference Manual, as well as implementation-dependent pragmas that are configuration pragmas. See the Implementation_Defined_Pragmas chapter in the GNAT_Reference_Manual for details on these additional GNAT-specific configuration pragmas. Most notably, the pragma Source_File_Name, which allows specifying non-default names for source files, is a configuration pragma. The following is a complete list of configuration pragmas recognized by GNAT:


3.4.1. Handling of Configuration Pragmas

Configuration pragmas may either appear at the start of a compilation unit, or they can appear in a configuration pragma file to apply to all compilations performed in a given compilation environment.

GNAT also provides the gnatchop utility to provide an automatic way to handle configuration pragmas following the semantics for compilations (that is, files with multiple units), described in the RM. See Operating gnatchop in Compilation Mode for details. However, for most purposes, it will be more convenient to edit the gnat.adc file that contains configuration pragmas directly, as described in the following section.

In the case of Restrictions pragmas appearing as configuration pragmas in individual compilation units, the exact handling depends on the type of restriction.

Restrictions that require partition-wide consistency (like No_Tasking) are recognized wherever they appear and can be freely inherited, e.g. from a withed unit to the withing unit. This makes sense since the binder will in any case insist on seeing consistent use, so any unit not conforming to any restrictions that are anywhere in the partition will be rejected, and you might as well find that out at compile time rather than at bind time.

For restrictions that do not require partition-wide consistency, e.g. SPARK or No_Implementation_Attributes, in general the restriction applies only to the unit in which the pragma appears, and not to any other units.

The exception is No_Elaboration_Code which always applies to the entire object file from a compilation, i.e. to the body, spec, and all subunits. This restriction can be specified in a configuration pragma file, or it can be on the body and/or the spec (in eithe case it applies to all the relevant units). It can appear on a subunit only if it has previously appeared in the body of spec.

3.4.2. The Configuration Pragmas Files

In GNAT a compilation environment is defined by the current directory at the time that a compile command is given. This current directory is searched for a file whose name is gnat.adc. If this file is present, it is expected to contain one or more configuration pragmas that will be applied to the current compilation. However, if the switch -gnatA is used, gnat.adc is not considered. When taken into account, gnat.adc is added to the dependencies, so that if gnat.adc is modified later, an invocation of gnatmake will recompile the source.

Configuration pragmas may be entered into the gnat.adc file either by running gnatchop on a source file that consists only of configuration pragmas, or more conveniently by direct editing of the gnat.adc file, which is a standard format source file.

Besides gnat.adc, additional files containing configuration pragmas may be applied to the current compilation using the switch -gnatec=path where path must designate an existing file that contains only configuration pragmas. These configuration pragmas are in addition to those found in gnat.adc (provided gnat.adc is present and switch -gnatA is not used).

It is allowable to specify several switches -gnatec=, all of which will be taken into account.

Files containing configuration pragmas specified with switches -gnatec= are added to the dependencies, unless they are temporary files. A file is considered temporary if its name ends in .tmp or .TMP. Certain tools follow this naming convention because they pass information to gcc via temporary files that are immediately deleted; it doesn’t make sense to depend on a file that no longer exists. Such tools include gprbuild, gnatmake, and gnatcheck.

By default, configuration pragma files are stored by their absolute paths in ALI files. You can use the -gnateb switch in order to store them by their basename instead.

If you are using project file, a separate mechanism is provided using project attributes.

3.5. Generating Object Files

An Ada program consists of a set of source files, and the first step in compiling the program is to generate the corresponding object files. These are generated by compiling a subset of these source files. The files you need to compile are the following:

  • If a package spec has no body, compile the package spec to produce the object file for the package.

  • If a package has both a spec and a body, compile the body to produce the object file for the package. The source file for the package spec need not be compiled in this case because there is only one object file, which contains the code for both the spec and body of the package.

  • For a subprogram, compile the subprogram body to produce the object file for the subprogram. The spec, if one is present, is as usual in a separate file, and need not be compiled.

  • In the case of subunits, only compile the parent unit. A single object file is generated for the entire subunit tree, which includes all the subunits.

  • Compile child units independently of their parent units (though, of course, the spec of all the ancestor unit must be present in order to compile a child unit).

  • Compile generic units in the same manner as any other units. The object files in this case are small dummy files that contain at most the flag used for elaboration checking. This is because GNAT always handles generic instantiation by means of macro expansion. However, it is still necessary to compile generic units, for dependency checking and elaboration purposes.

The preceding rules describe the set of files that must be compiled to generate the object files for a program. Each object file has the same name as the corresponding source file, except that the extension is .o as usual.

You may wish to compile other files for the purpose of checking their syntactic and semantic correctness. For example, in the case where a package has a separate spec and body, you would not normally compile the spec. However, it is convenient in practice to compile the spec to make sure it is error-free before compiling clients of this spec, because such compilations will fail if there is an error in the spec.

GNAT provides an option for compiling such files purely for the purposes of checking correctness; such compilations are not required as part of the process of building a program. To compile a file in this checking mode, use the -gnatc switch.

3.6. Source Dependencies

A given object file clearly depends on the source file which is compiled to produce it. Here we are using “depends” in the sense of a typical make utility; in other words, an object file depends on a source file if changes to the source file require the object file to be recompiled. In addition to this basic dependency, a given object may depend on additional source files as follows:

  • If a file being compiled withs a unit X, the object file depends on the file containing the spec of unit X. This includes files that are withed implicitly either because they are parents of withed child units or they are run-time units required by the language constructs used in a particular unit.

  • If a file being compiled instantiates a library level generic unit, the object file depends on both the spec and body files for this generic unit.

  • If a file being compiled instantiates a generic unit defined within a package, the object file depends on the body file for the package as well as the spec file.

  • If a file being compiled contains a call to a subprogram for which pragma Inline applies and inlining is activated with the -gnatn switch, the object file depends on the file containing the body of this subprogram as well as on the file containing the spec. Note that for inlining to actually occur as a result of the use of this switch, it is necessary to compile in optimizing mode.

    The use of -gnatN activates inlining optimization that is performed by the front end of the compiler. This inlining does not require that the code generation be optimized. Like -gnatn, the use of this switch generates additional dependencies.

    When using a gcc-based back end, then the use of -gnatN is deprecated, and the use of -gnatn is preferred. Historically front end inlining was more extensive than the gcc back end inlining, but that is no longer the case.

  • If an object file O depends on the proper body of a subunit through inlining or instantiation, it depends on the parent unit of the subunit. This means that any modification of the parent unit or one of its subunits affects the compilation of O.

  • The object file for a parent unit depends on all its subunit body files.

  • The previous two rules meant that for purposes of computing dependencies and recompilation, a body and all its subunits are treated as an indivisible whole.

    These rules are applied transitively: if unit A withs unit B, whose elaboration calls an inlined procedure in package C, the object file for unit A will depend on the body of C, in file c.adb.

    The set of dependent files described by these rules includes all the files on which the unit is semantically dependent, as dictated by the Ada language standard. However, it is a superset of what the standard describes, because it includes generic, inline, and subunit dependencies.

    An object file must be recreated by recompiling the corresponding source file if any of the source files on which it depends are modified. For example, if the make utility is used to control compilation, the rule for an Ada object file must mention all the source files on which the object file depends, according to the above definition. The determination of the necessary recompilations is done automatically when one uses gnatmake.

3.7. The Ada Library Information Files

Each compilation actually generates two output files. The first of these is the normal object file that has a .o extension. The second is a text file containing full dependency information. It has the same name as the source file, but an .ali extension. This file is known as the Ada Library Information (ALI) file. The following information is contained in the ALI file.

  • Version information (indicates which version of GNAT was used to compile the unit(s) in question)

  • Main program information (including priority and time slice settings, as well as the wide character encoding used during compilation).

  • List of arguments used in the gcc command for the compilation

  • Attributes of the unit, including configuration pragmas used, an indication of whether the compilation was successful, exception model used etc.

  • A list of relevant restrictions applying to the unit (used for consistency) checking.

  • Categorization information (e.g., use of pragma Pure).

  • Information on all withed units, including presence of Elaborate or Elaborate_All pragmas.

  • Information from any Linker_Options pragmas used in the unit

  • Information on the use of Body_Version or Version attributes in the unit.

  • Dependency information. This is a list of files, together with time stamp and checksum information. These are files on which the unit depends in the sense that recompilation is required if any of these units are modified.

  • Cross-reference data. Contains information on all entities referenced in the unit. Used by tools like gnatxref and gnatfind to provide cross-reference information.

For a full detailed description of the format of the ALI file, see the source of the body of unit Lib.Writ, contained in file lib-writ.adb in the GNAT compiler sources.

3.8. Binding an Ada Program

When using languages such as C and C++, once the source files have been compiled the only remaining step in building an executable program is linking the object modules together. This means that it is possible to link an inconsistent version of a program, in which two units have included different versions of the same header.

The rules of Ada do not permit such an inconsistent program to be built. For example, if two clients have different versions of the same package, it is illegal to build a program containing these two clients. These rules are enforced by the GNAT binder, which also determines an elaboration order consistent with the Ada rules.

The GNAT binder is run after all the object files for a program have been created. It is given the name of the main program unit, and from this it determines the set of units required by the program, by reading the corresponding ALI files. It generates error messages if the program is inconsistent or if no valid order of elaboration exists.

If no errors are detected, the binder produces a main program, in Ada by default, that contains calls to the elaboration procedures of those compilation unit that require them, followed by a call to the main program. This Ada program is compiled to generate the object file for the main program. The name of the Ada file is b~xxx.adb` (with the corresponding spec b~xxx.ads`) where xxx is the name of the main program unit.

Finally, the linker is used to build the resulting executable program, using the object from the main program from the bind step as well as the object files for the Ada units of the program.

3.9. GNAT and Libraries

This section describes how to build and use libraries with GNAT, and also shows how to recompile the GNAT run-time library. You should be familiar with the Project Manager facility (see the GNAT_Project_Manager chapter of the GPRbuild User’s Guide) before reading this chapter.

3.9.1. Introduction to Libraries in GNAT

A library is, conceptually, a collection of objects which does not have its own main thread of execution, but rather provides certain services to the applications that use it. A library can be either statically linked with the application, in which case its code is directly included in the application, or, on platforms that support it, be dynamically linked, in which case its code is shared by all applications making use of this library.

GNAT supports both types of libraries. In the static case, the compiled code can be provided in different ways. The simplest approach is to provide directly the set of objects resulting from compilation of the library source files. Alternatively, you can group the objects into an archive using whatever commands are provided by the operating system. For the latter case, the objects are grouped into a shared library.

In the GNAT environment, a library has three types of components:

A GNAT library may expose all its source files, which is useful for documentation purposes. Alternatively, it may expose only the units needed by an external user to make use of the library. That is to say, the specs reflecting the library services along with all the units needed to compile those specs, which can include generic bodies or any body implementing an inlined routine. In the case of stand-alone libraries those exposed units are called interface units (Stand-alone Ada Libraries).

All compilation units comprising an application, including those in a library, need to be elaborated in an order partially defined by Ada’s semantics. GNAT computes the elaboration order from the ALI files and this is why they constitute a mandatory part of GNAT libraries. Stand-alone libraries are the exception to this rule because a specific library elaboration routine is produced independently of the application(s) using the library.

3.9.2. General Ada Libraries Building a library

The easiest way to build a library is to use the Project Manager, which supports a special type of project called a Library Project (see the Library Projects section in the GNAT Project Manager chapter of the GPRbuild User’s Guide).

A project is considered a library project, when two project-level attributes are defined in it: Library_Name and Library_Dir. In order to control different aspects of library configuration, additional optional project-level attributes can be specified:

  • Library_Kind

    This attribute controls whether the library is to be static or dynamic

  • Library_Version

    This attribute specifies the library version; this value is used during dynamic linking of shared libraries to determine if the currently installed versions of the binaries are compatible.

  • Library_Options

  • Library_GCC

    These attributes specify additional low-level options to be used during library generation, and redefine the actual application used to generate library.

The GNAT Project Manager takes full care of the library maintenance task, including recompilation of the source files for which objects do not exist or are not up to date, assembly of the library archive, and installation of the library (i.e., copying associated source, object and ALI files to the specified location).

Here is a simple library project file:

project My_Lib is
  for Source_Dirs use ("src1", "src2");
  for Object_Dir use "obj";
  for Library_Name use "mylib";
  for Library_Dir use "lib";
  for Library_Kind use "dynamic";
end My_lib;

and the compilation command to build and install the library:

$ gnatmake -Pmy_lib

It is not entirely trivial to perform manually all the steps required to produce a library. We recommend that you use the GNAT Project Manager for this task. In special cases where this is not desired, the necessary steps are discussed below.

There are various possibilities for compiling the units that make up the library: for example with a Makefile (Using the GNU make Utility) or with a conventional script. For simple libraries, it is also possible to create a dummy main program which depends upon all the packages that comprise the interface of the library. This dummy main program can then be given to gnatmake, which will ensure that all necessary objects are built.

After this task is accomplished, you should follow the standard procedure of the underlying operating system to produce the static or shared library.

Here is an example of such a dummy program:

with My_Lib.Service1;
with My_Lib.Service2;
with My_Lib.Service3;
procedure My_Lib_Dummy is

Here are the generic commands that will build an archive or a shared library.

# compiling the library
$ gnatmake -c my_lib_dummy.adb

# we don't need the dummy object itself
$ rm my_lib_dummy.o my_lib_dummy.ali

# create an archive with the remaining objects
$ ar rc libmy_lib.a *.o
# some systems may require "ranlib" to be run as well

# or create a shared library
$ gcc -shared -o libmy_lib.so *.o
# some systems may require the code to have been compiled with -fPIC

# remove the object files that are now in the library
$ rm *.o

# Make the ALI files read-only so that gnatmake will not try to
# regenerate the objects that are in the library
$ chmod -w *.ali

Please note that the library must have a name of the form libxxx.a or libxxx.so (or libxxx.dll on Windows) in order to be accessed by the directive -lxxx at link time. Installing a library

If you use project files, library installation is part of the library build process (see the Installing a Library with Project Files section of the GNAT Project Manager chapter of the GPRbuild User’s Guide).

When project files are not an option, it is also possible, but not recommended, to install the library so that the sources needed to use the library are on the Ada source path and the ALI files & libraries be on the Ada Object path (see Search Paths and the Run-Time Library (RTL). Alternatively, the system administrator can place general-purpose libraries in the default compiler paths, by specifying the libraries’ location in the configuration files ada_source_path and ada_object_path. These configuration files must be located in the GNAT installation tree at the same place as the gcc spec file. The location of the gcc spec file can be determined as follows:

$ gcc -v

The configuration files mentioned above have a simple format: each line must contain one unique directory name. Those names are added to the corresponding path in their order of appearance in the file. The names can be either absolute or relative; in the latter case, they are relative to where theses files are located.

The files ada_source_path and ada_object_path might not be present in a GNAT installation, in which case, GNAT will look for its run-time library in the directories adainclude (for the sources) and adalib (for the objects and ALI files). When the files exist, the compiler does not look in adainclude and adalib, and thus the ada_source_path file must contain the location for the GNAT run-time sources (which can simply be adainclude). In the same way, the ada_object_path file must contain the location for the GNAT run-time objects (which can simply be adalib).

You can also specify a new default path to the run-time library at compilation time with the switch --RTS=rts-path. You can thus choose / change the run-time library you want your program to be compiled with. This switch is recognized by gcc, gnatmake, gnatbind, gnatls, gnatfind and gnatxref.

It is possible to install a library before or after the standard GNAT library, by reordering the lines in the configuration files. In general, a library must be installed before the GNAT library if it redefines any part of it. Using a library

Once again, the project facility greatly simplifies the use of libraries. In this context, using a library is just a matter of adding a with clause in the user project. For instance, to make use of the library My_Lib shown in examples in earlier sections, you can write:

with "my_lib";
project My_Proj is
end My_Proj;

Even if you have a third-party, non-Ada library, you can still use GNAT’s Project Manager facility to provide a wrapper for it. For example, the following project, when withed by your main project, will link with the third-party library liba.a:

project Liba is
   for Externally_Built use "true";
   for Source_Files use ();
   for Library_Dir use "lib";
   for Library_Name use "a";
   for Library_Kind use "static";
end Liba;

This is an alternative to the use of pragma Linker_Options. It is especially interesting in the context of systems with several interdependent static libraries where finding a proper linker order is not easy and best be left to the tools having visibility over project dependence information.

In order to use an Ada library manually, you need to make sure that this library is on both your source and object path (see Search Paths and the Run-Time Library (RTL) and Search Paths for gnatbind). Furthermore, when the objects are grouped in an archive or a shared library, you need to specify the desired library at link time.

For example, you can use the library mylib installed in /dir/my_lib_src and /dir/my_lib_obj with the following commands:

$ gnatmake -aI/dir/my_lib_src -aO/dir/my_lib_obj my_appl \\
  -largs -lmy_lib

This can be expressed more simply:

$ gnatmake my_appl

when the following conditions are met:

  • /dir/my_lib_src has been added by the user to the environment variable ADA_INCLUDE_PATH, or by the administrator to the file ada_source_path

  • /dir/my_lib_obj has been added by the user to the environment variable ADA_OBJECTS_PATH, or by the administrator to the file ada_object_path

  • a pragma Linker_Options has been added to one of the sources. For example:

    pragma Linker_Options ("-lmy_lib");

Note that you may also load a library dynamically at run time given its filename, as illustrated in the GNAT plugins example in the directory share/examples/gnat/plugins within the GNAT install area.

3.9.3. Stand-alone Ada Libraries Introduction to Stand-alone Libraries

A Stand-alone Library (abbreviated ‘SAL’) is a library that contains the necessary code to elaborate the Ada units that are included in the library. In contrast with an ordinary library, which consists of all sources, objects and ALI files of the library, a SAL may specify a restricted subset of compilation units to serve as a library interface. In this case, the fully self-sufficient set of files will normally consist of an objects archive, the sources of interface units’ specs, and the ALI files of interface units. If an interface spec contains a generic unit or an inlined subprogram, the body’s source must also be provided; if the units that must be provided in the source form depend on other units, the source and ALI files of those must also be provided.

The main purpose of a SAL is to minimize the recompilation overhead of client applications when a new version of the library is installed. Specifically, if the interface sources have not changed, client applications do not need to be recompiled. If, furthermore, a SAL is provided in the shared form and its version, controlled by Library_Version attribute, is not changed, then the clients do not need to be relinked.

SALs also allow the library providers to minimize the amount of library source text exposed to the clients. Such ‘information hiding’ might be useful or necessary for various reasons.

Stand-alone libraries are also well suited to be used in an executable whose main routine is not written in Ada. Building a Stand-alone Library

GNAT’s Project facility provides a simple way of building and installing stand-alone libraries; see the Stand-alone Library Projects section in the GNAT Project Manager chapter of the GPRbuild User’s Guide. To be a Stand-alone Library Project, in addition to the two attributes that make a project a Library Project (Library_Name and Library_Dir; see the Library Projects section in the GNAT Project Manager chapter of the GPRbuild User’s Guide), the attribute Library_Interface must be defined. For example:

for Library_Dir use "lib_dir";
for Library_Name use "dummy";
for Library_Interface use ("int1", "int1.child");

Attribute Library_Interface has a non-empty string list value, each string in the list designating a unit contained in an immediate source of the project file.

When a Stand-alone Library is built, first the binder is invoked to build a package whose name depends on the library name (b~dummy.ads/b in the example above). This binder-generated package includes initialization and finalization procedures whose names depend on the library name (dummyinit and dummyfinal in the example above). The object corresponding to this package is included in the library.

You must ensure timely (e.g., prior to any use of interfaces in the SAL) calling of these procedures if a static SAL is built, or if a shared SAL is built with the project-level attribute Library_Auto_Init set to "false".

For a Stand-Alone Library, only the ALI files of the Interface Units (those that are listed in attribute Library_Interface) are copied to the Library Directory. As a consequence, only the Interface Units may be imported from Ada units outside of the library. If other units are imported, the binding phase will fail.

It is also possible to build an encapsulated library where not only the code to elaborate and finalize the library is embedded but also ensuring that the library is linked only against static libraries. So an encapsulated library only depends on system libraries, all other code, including the GNAT runtime, is embedded. To build an encapsulated library the attribute Library_Standalone must be set to encapsulated:

for Library_Dir use "lib_dir";
for Library_Name use "dummy";
for Library_Kind use "dynamic";
for Library_Interface use ("int1", "int1.child");
for Library_Standalone use "encapsulated";

The default value for this attribute is standard in which case a stand-alone library is built.

The attribute Library_Src_Dir may be specified for a Stand-Alone Library. Library_Src_Dir is a simple attribute that has a single string value. Its value must be the path (absolute or relative to the project directory) of an existing directory. This directory cannot be the object directory or one of the source directories, but it can be the same as the library directory. The sources of the Interface Units of the library that are needed by an Ada client of the library will be copied to the designated directory, called the Interface Copy directory. These sources include the specs of the Interface Units, but they may also include bodies and subunits, when pragmas Inline or Inline_Always are used, or when there is a generic unit in the spec. Before the sources are copied to the Interface Copy directory, an attempt is made to delete all files in the Interface Copy directory.

Building stand-alone libraries by hand is somewhat tedious, but for those occasions when it is necessary here are the steps that you need to perform:

  • Compile all library sources.

  • Invoke the binder with the switch -n (No Ada main program), with all the ALI files of the interfaces, and with the switch -L to give specific names to the init and final procedures. For example:

    $ gnatbind -n int1.ali int2.ali -Lsal1
  • Compile the binder generated file:

    $ gcc -c b~int2.adb
  • Link the dynamic library with all the necessary object files, indicating to the linker the names of the init (and possibly final) procedures for automatic initialization (and finalization). The built library should be placed in a directory different from the object directory.

  • Copy the ALI files of the interface to the library directory, add in this copy an indication that it is an interface to a SAL (i.e., add a word SL on the line in the ALI file that starts with letter ‘P’) and make the modified copy of the ALI file read-only.

Using SALs is not different from using other libraries (see Using a library). Creating a Stand-alone Library to be used in a non-Ada context

It is easy to adapt the SAL build procedure discussed above for use of a SAL in a non-Ada context.

The only extra step required is to ensure that library interface subprograms are compatible with the main program, by means of pragma Export or pragma Convention.

Here is an example of simple library interface for use with C main program:

package My_Package is

   procedure Do_Something;
   pragma Export (C, Do_Something, "do_something");

   procedure Do_Something_Else;
   pragma Export (C, Do_Something_Else, "do_something_else");

end My_Package;

On the foreign language side, you must provide a ‘foreign’ view of the library interface; remember that it should contain elaboration routines in addition to interface subprograms.

The example below shows the content of mylib_interface.h (note that there is no rule for the naming of this file, any name can be used)

/* the library elaboration procedure */
extern void mylibinit (void);

/* the library finalization procedure */
extern void mylibfinal (void);

/* the interface exported by the library */
extern void do_something (void);
extern void do_something_else (void);

Libraries built as explained above can be used from any program, provided that the elaboration procedures (named mylibinit in the previous example) are called before the library services are used. Any number of libraries can be used simultaneously, as long as the elaboration procedure of each library is called.

Below is an example of a C program that uses the mylib library.

#include "mylib_interface.h"

main (void)
   /* First, elaborate the library before using it */
   mylibinit ();

   /* Main program, using the library exported entities */
   do_something ();
   do_something_else ();

   /* Library finalization at the end of the program */
   mylibfinal ();
   return 0;

Note that invoking any library finalization procedure generated by gnatbind shuts down the Ada run-time environment. Consequently, the finalization of all Ada libraries must be performed at the end of the program. No call to these libraries or to the Ada run-time library should be made after the finalization phase.

Note also that special care must be taken with multi-tasks applications. The initialization and finalization routines are not protected against concurrent access. If such requirement is needed it must be ensured at the application level using a specific operating system services like a mutex or a critical-section. Restrictions in Stand-alone Libraries

The pragmas listed below should be used with caution inside libraries, as they can create incompatibilities with other Ada libraries:

  • pragma Locking_Policy

  • pragma Partition_Elaboration_Policy

  • pragma Queuing_Policy

  • pragma Task_Dispatching_Policy

  • pragma Unreserve_All_Interrupts

When using a library that contains such pragmas, the user must make sure that all libraries use the same pragmas with the same values. Otherwise, Program_Error will be raised during the elaboration of the conflicting libraries. The usage of these pragmas and its consequences for the user should therefore be well documented.

Similarly, the traceback in the exception occurrence mechanism should be enabled or disabled in a consistent manner across all libraries. Otherwise, Program_Error will be raised during the elaboration of the conflicting libraries.

If the Version or Body_Version attributes are used inside a library, then you need to perform a gnatbind step that specifies all ALI files in all libraries, so that version identifiers can be properly computed. In practice these attributes are rarely used, so this is unlikely to be a consideration.

3.9.4. Rebuilding the GNAT Run-Time Library

It may be useful to recompile the GNAT library in various debugging or experimentation contexts. A project file called libada.gpr is provided to that effect and can be found in the directory containing the GNAT library. The location of this directory depends on the way the GNAT environment has been installed and can be determined by means of the command:

$ gnatls -v

The last entry in the source search path usually contains the gnat library (the adainclude directory). This project file contains its own documentation and in particular the set of instructions needed to rebuild a new library and to use it.

Note that rebuilding the GNAT Run-Time is only recommended for temporary experiments or debugging, and is not supported.

3.10. Conditional Compilation

This section presents some guidelines for modeling conditional compilation in Ada and describes the gnatprep preprocessor utility.

3.10.1. Modeling Conditional Compilation in Ada

It is often necessary to arrange for a single source program to serve multiple purposes, where it is compiled in different ways to achieve these different goals. Some examples of the need for this feature are

  • Adapting a program to a different hardware environment

  • Adapting a program to a different target architecture

  • Turning debugging features on and off

  • Arranging for a program to compile with different compilers

In C, or C++, the typical approach would be to use the preprocessor that is defined as part of the language. The Ada language does not contain such a feature. This is not an oversight, but rather a very deliberate design decision, based on the experience that overuse of the preprocessing features in C and C++ can result in programs that are extremely difficult to maintain. For example, if we have ten switches that can be on or off, this means that there are a thousand separate programs, any one of which might not even be syntactically correct, and even if syntactically correct, the resulting program might not work correctly. Testing all combinations can quickly become impossible.

Nevertheless, the need to tailor programs certainly exists, and in this section we will discuss how this can be achieved using Ada in general, and GNAT in particular. Use of Boolean Constants

In the case where the difference is simply which code sequence is executed, the cleanest solution is to use Boolean constants to control which code is executed.

FP_Initialize_Required : constant Boolean := True;
if FP_Initialize_Required then
end if;

Not only will the code inside the if statement not be executed if the constant Boolean is False, but it will also be completely deleted from the program. However, the code is only deleted after the if statement has been checked for syntactic and semantic correctness. (In contrast, with preprocessors the code is deleted before the compiler ever gets to see it, so it is not checked until the switch is turned on.)

Typically the Boolean constants will be in a separate package, something like:

package Config is
   FP_Initialize_Required : constant Boolean := True;
   Reset_Available        : constant Boolean := False;
end Config;

The Config package exists in multiple forms for the various targets, with an appropriate script selecting the version of Config needed. Then any other unit requiring conditional compilation can do a with of Config to make the constants visible. Debugging - A Special Case

A common use of conditional code is to execute statements (for example dynamic checks, or output of intermediate results) under control of a debug switch, so that the debugging behavior can be turned on and off. This can be done using a Boolean constant to control whether the code is active:

if Debugging then
   Put_Line ("got to the first stage!");
end if;


if Debugging and then Temperature > 999.0 then
   raise Temperature_Crazy;
end if;

Since this is a common case, there are special features to deal with this in a convenient manner. For the case of tests, Ada 2005 has added a pragma Assert that can be used for such tests. This pragma is modeled on the Assert pragma that has always been available in GNAT, so this feature may be used with GNAT even if you are not using Ada 2005 features. The use of pragma Assert is described in the GNAT_Reference_Manual, but as an example, the last test could be written:

pragma Assert (Temperature <= 999.0, "Temperature Crazy");

or simply

pragma Assert (Temperature <= 999.0);

In both cases, if assertions are active and the temperature is excessive, the exception Assert_Failure will be raised, with the given string in the first case or a string indicating the location of the pragma in the second case used as the exception message.

You can turn assertions on and off by using the Assertion_Policy pragma.

This is an Ada 2005 pragma which is implemented in all modes by GNAT. Alternatively, you can use the -gnata switch to enable assertions from the command line, which applies to all versions of Ada.

For the example above with the Put_Line, the GNAT-specific pragma Debug can be used:

pragma Debug (Put_Line ("got to the first stage!"));

If debug pragmas are enabled, the argument, which must be of the form of a procedure call, is executed (in this case, Put_Line will be called). Only one call can be present, but of course a special debugging procedure containing any code you like can be included in the program and then called in a pragma Debug argument as needed.

One advantage of pragma Debug over the if Debugging then construct is that pragma Debug can appear in declarative contexts, such as at the very beginning of a procedure, before local declarations have been elaborated.

Debug pragmas are enabled using either the -gnata switch that also controls assertions, or with a separate Debug_Policy pragma.

The latter pragma is new in the Ada 2005 versions of GNAT (but it can be used in Ada 95 and Ada 83 programs as well), and is analogous to pragma Assertion_Policy to control assertions.

Assertion_Policy and Debug_Policy are configuration pragmas, and thus they can appear in gnat.adc if you are not using a project file, or in the file designated to contain configuration pragmas in a project file. They then apply to all subsequent compilations. In practice the use of the -gnata switch is often the most convenient method of controlling the status of these pragmas.

Note that a pragma is not a statement, so in contexts where a statement sequence is required, you can’t just write a pragma on its own. You have to add a null statement.

if ... then
   ... -- some statements
   pragma Assert (Num_Cases < 10);
end if; Conditionalizing Declarations

In some cases it may be necessary to conditionalize declarations to meet different requirements. For example we might want a bit string whose length is set to meet some hardware message requirement.

This may be possible using declare blocks controlled by conditional constants:

if Small_Machine then
      X : Bit_String (1 .. 10);
      X : Large_Bit_String (1 .. 1000);
end if;

Note that in this approach, both declarations are analyzed by the compiler so this can only be used where both declarations are legal, even though one of them will not be used.

Another approach is to define integer constants, e.g., Bits_Per_Word, or Boolean constants, e.g., Little_Endian, and then write declarations that are parameterized by these constants. For example

for Rec use
  Field1 at 0 range Boolean'Pos (Little_Endian) * 10 .. Bits_Per_Word;
end record;

If Bits_Per_Word is set to 32, this generates either

for Rec use
  Field1 at 0 range 0 .. 32;
end record;

for the big endian case, or

for Rec use record
    Field1 at 0 range 10 .. 32;
end record;

for the little endian case. Since a powerful subset of Ada expression notation is usable for creating static constants, clever use of this feature can often solve quite difficult problems in conditionalizing compilation (note incidentally that in Ada 95, the little endian constant was introduced as System.Default_Bit_Order, so you do not need to define this one yourself). Use of Alternative Implementations

In some cases, none of the approaches described above are adequate. This can occur for example if the set of declarations required is radically different for two different configurations.

In this situation, the official Ada way of dealing with conditionalizing such code is to write separate units for the different cases. As long as this does not result in excessive duplication of code, this can be done without creating maintenance problems. The approach is to share common code as far as possible, and then isolate the code and declarations that are different. Subunits are often a convenient method for breaking out a piece of a unit that is to be conditionalized, with separate files for different versions of the subunit for different targets, where the build script selects the right one to give to the compiler.

As an example, consider a situation where a new feature in Ada 2005 allows something to be done in a really nice way. But your code must be able to compile with an Ada 95 compiler. Conceptually you want to say:

if Ada_2005 then
   ... neat Ada 2005 code
   ... not quite as neat Ada 95 code
end if;

where Ada_2005 is a Boolean constant.

But this won’t work when Ada_2005 is set to False, since the then clause will be illegal for an Ada 95 compiler. (Recall that although such unreachable code would eventually be deleted by the compiler, it still needs to be legal. If it uses features introduced in Ada 2005, it will be illegal in Ada 95.)

So instead we write

procedure Insert is separate;

Then we have two files for the subunit Insert, with the two sets of code. If the package containing this is called File_Queries, then we might have two files

  • file_queries-insert-2005.adb

  • file_queries-insert-95.adb

and the build script renames the appropriate file to file_queries-insert.adb and then carries out the compilation.

This can also be done with project files’ naming schemes. For example:

for body ("File_Queries.Insert") use "file_queries-insert-2005.ada";

Note also that with project files it is desirable to use a different extension than ads / adb for alternative versions. Otherwise a naming conflict may arise through another commonly used feature: to declare as part of the project a set of directories containing all the sources obeying the default naming scheme.

The use of alternative units is certainly feasible in all situations, and for example the Ada part of the GNAT run-time is conditionalized based on the target architecture using this approach. As a specific example, consider the implementation of the AST feature in VMS. There is one spec: s-asthan.ads which is the same for all architectures, and three bodies:

  • s-asthan.adb

    used for all non-VMS operating systems

  • s-asthan-vms-alpha.adb

    used for VMS on the Alpha

  • s-asthan-vms-ia64.adb

    used for VMS on the ia64

The dummy version s-asthan.adb simply raises exceptions noting that this operating system feature is not available, and the two remaining versions interface with the corresponding versions of VMS to provide VMS-compatible AST handling. The GNAT build script knows the architecture and operating system, and automatically selects the right version, renaming it if necessary to s-asthan.adb before the run-time build.

Another style for arranging alternative implementations is through Ada’s access-to-subprogram facility. In case some functionality is to be conditionally included, you can declare an access-to-procedure variable Ref that is initialized to designate a ‘do nothing’ procedure, and then invoke Ref.all when appropriate. In some library package, set Ref to Proc'Access for some procedure Proc that performs the relevant processing. The initialization only occurs if the library package is included in the program. The same idea can also be implemented using tagged types and dispatching calls. Preprocessing

Although it is quite possible to conditionalize code without the use of C-style preprocessing, as described earlier in this section, it is nevertheless convenient in some cases to use the C approach. Moreover, older Ada compilers have often provided some preprocessing capability, so legacy code may depend on this approach, even though it is not standard.

To accommodate such use, GNAT provides a preprocessor (modeled to a large extent on the various preprocessors that have been used with legacy code on other compilers, to enable easier transition).

The preprocessor may be used in two separate modes. It can be used quite separately from the compiler, to generate a separate output source file that is then fed to the compiler as a separate step. This is the gnatprep utility, whose use is fully described in Preprocessing with gnatprep.

The preprocessing language allows such constructs as

#if DEBUG or else (PRIORITY > 4) then
   sequence of declarations
   completely different sequence of declarations
#end if;

The values of the symbols DEBUG and PRIORITY can be defined either on the command line or in a separate file.

The other way of running the preprocessor is even closer to the C style and often more convenient. In this approach the preprocessing is integrated into the compilation process. The compiler is given the preprocessor input which includes #if lines etc, and then the compiler carries out the preprocessing internally and processes the resulting output. For more details on this approach, see Integrated Preprocessing.

3.10.2. Preprocessing with gnatprep

This section discusses how to use GNAT’s gnatprep utility for simple preprocessing. Although designed for use with GNAT, gnatprep does not depend on any special GNAT features. For further discussion of conditional compilation in general, see Conditional Compilation. Preprocessing Symbols

Preprocessing symbols are defined in definition files and referenced in the sources to be preprocessed. A preprocessing symbol is an identifier, following normal Ada (case-insensitive) rules for its syntax, with the restriction that all characters need to be in the ASCII set (no accented letters). Using gnatprep

To call gnatprep use:

$ gnatprep [ switches ] infile outfile [ deffile ]


  • switches

    is an optional sequence of switches as described in the next section.

  • infile

    is the full name of the input file, which is an Ada source file containing preprocessor directives.

  • outfile

    is the full name of the output file, which is an Ada source in standard Ada form. When used with GNAT, this file name will normally have an ads or adb suffix.

  • deffile

    is the full name of a text file containing definitions of preprocessing symbols to be referenced by the preprocessor. This argument is optional, and can be replaced by the use of the -D switch. Switches for gnatprep


Display Copyright and version, then exit disregarding all other options.


If --version was not used, display usage and then exit disregarding all other options.


Causes both preprocessor lines and the lines deleted by preprocessing to be replaced by blank lines in the output source file, preserving line numbers in the output file.


Causes both preprocessor lines and the lines deleted by preprocessing to be retained in the output source as comments marked with the special string "--! ". This option will result in line numbers being preserved in the output file.


Causes comments to be scanned. Normally comments are ignored by gnatprep. If this option is specified, then comments are scanned and any $symbol substitutions performed as in program text. This is particularly useful when structured comments are used (e.g., for programs written in a pre-2014 version of the SPARK Ada subset). Note that this switch is not available when doing integrated preprocessing (it would be useless in this context since comments are ignored by the compiler in any case).


Defines a new preprocessing symbol with the specified value. If no value is given on the command line, then symbol is considered to be True. This switch can be used in place of a definition file.


Causes a Source_Reference pragma to be generated that references the original input file, so that error messages will use the file name of this original file. The use of this switch implies that preprocessor lines are not to be removed from the file, so its use will force -b mode if -c has not been specified explicitly.

Note that if the file to be preprocessed contains multiple units, then it will be necessary to gnatchop the output file from gnatprep. If a Source_Reference pragma is present in the preprocessed file, it will be respected by gnatchop -r so that the final chopped files will correctly refer to the original input source file for gnatprep.


Causes a sorted list of symbol names and values to be listed on the standard output file.


Use LF as line terminators when writing files. By default the line terminator of the host (LF under unix, CR/LF under Windows) is used.


Causes undefined symbols to be treated as having the value FALSE in the context of a preprocessor test. In the absence of this option, an undefined symbol in a #if or #elsif test will be treated as an error.


Verbose mode: generates more output about work done.

Note: if neither -b nor -c is present, then preprocessor lines and deleted lines are completely removed from the output, unless -r is specified, in which case -b is assumed. Form of Definitions File

The definitions file contains lines of the form:

symbol := value

where symbol is a preprocessing symbol, and value is one of the following:

  • Empty, corresponding to a null substitution,

  • A string literal using normal Ada syntax, or

  • Any sequence of characters from the set {letters, digits, period, underline}.

Comment lines may also appear in the definitions file, starting with the usual --, and comments may be added to the definitions lines. Form of Input Text for gnatprep

The input text may contain preprocessor conditional inclusion lines, as well as general symbol substitution sequences.

The preprocessor conditional inclusion commands have the form:

#if <expression> [then]
#elsif <expression> [then]
#elsif <expression> [then]
#end if;

In this example, <expression> is defined by the following grammar:

<expression> ::=  <symbol>
<expression> ::=  <symbol> = "<value>"
<expression> ::=  <symbol> = <symbol>
<expression> ::=  <symbol> = <integer>
<expression> ::=  <symbol> > <integer>
<expression> ::=  <symbol> >= <integer>
<expression> ::=  <symbol> < <integer>
<expression> ::=  <symbol> <= <integer>
<expression> ::=  <symbol> 'Defined
<expression> ::=  not <expression>
<expression> ::=  <expression> and <expression>
<expression> ::=  <expression> or <expression>
<expression> ::=  <expression> and then <expression>
<expression> ::=  <expression> or else <expression>
<expression> ::=  ( <expression> )

Note the following restriction: it is not allowed to have “and” or “or” following “not” in the same expression without parentheses. For example, this is not allowed:

not X or Y

This can be expressed instead as one of the following forms:

(not X) or Y
not (X or Y)

For the first test (<expression> ::= <symbol>) the symbol must have either the value true or false, that is to say the right-hand of the symbol definition must be one of the (case-insensitive) literals True or False. If the value is true, then the corresponding lines are included, and if the value is false, they are excluded.

When comparing a symbol to an integer, the integer is any non negative literal integer as defined in the Ada Reference Manual, such as 3, 16#FF# or 2#11#. The symbol value must also be a non negative integer. Integer values in the range 0 .. 2**31-1 are supported.

The test (<expression> ::= <symbol>’Defined) is true only if the symbol has been defined in the definition file or by a -D switch on the command line. Otherwise, the test is false.

The equality tests are case insensitive, as are all the preprocessor lines.

If the symbol referenced is not defined in the symbol definitions file, then the effect depends on whether or not switch -u is specified. If so, then the symbol is treated as if it had the value false and the test fails. If this switch is not specified, then it is an error to reference an undefined symbol. It is also an error to reference a symbol that is defined with a value other than True or False.

The use of the not operator inverts the sense of this logical test. The not operator cannot be combined with the or or and operators, without parentheses. For example, “if not X or Y then” is not allowed, but “if (not X) or Y then” and “if not (X or Y) then” are.

The then keyword is optional as shown

The # must be the first non-blank character on a line, but otherwise the format is free form. Spaces or tabs may appear between the # and the keyword. The keywords and the symbols are case insensitive as in normal Ada code. Comments may be used on a preprocessor line, but other than that, no other tokens may appear on a preprocessor line. Any number of elsif clauses can be present, including none at all. The else is optional, as in Ada.

The # marking the start of a preprocessor line must be the first non-blank character on the line, i.e., it must be preceded only by spaces or horizontal tabs.

Symbol substitution outside of preprocessor lines is obtained by using the sequence:


anywhere within a source line, except in a comment or within a string literal. The identifier following the $ must match one of the symbols defined in the symbol definition file, and the result is to substitute the value of the symbol in place of $symbol in the output file.

Note that although the substitution of strings within a string literal is not possible, it is possible to have a symbol whose defined value is a string literal. So instead of setting XYZ to hello and writing:

Header : String := "$XYZ";

you should set XYZ to "hello" and write:

Header : String := $XYZ;

and then the substitution will occur as desired.

3.10.3. Integrated Preprocessing

As noted above, a file to be preprocessed consists of Ada source code in which preprocessing lines have been inserted. However, instead of using gnatprep to explicitly preprocess a file as a separate step before compilation, you can carry out the preprocessing implicitly as part of compilation. Such integrated preprocessing, which is the common style with C, is performed when either or both of the following switches are passed to the compiler:

  • -gnatep, which specifies the preprocessor data file. This file dictates how the source files will be preprocessed (e.g., which symbol definition files apply to which sources).

  • -gnateD, which defines values for preprocessing symbols.

Integrated preprocessing applies only to Ada source files, it is not available for configuration pragma files.

With integrated preprocessing, the output from the preprocessor is not, by default, written to any external file. Instead it is passed internally to the compiler. To preserve the result of preprocessing in a file, either run gnatprep in standalone mode or else supply the -gnateG switch (described below) to the compiler.

When using project files:

  • the builder switch -x should be used if any Ada source is compiled with gnatep=, so that the compiler finds the preprocessor data file.

  • the preprocessing data file and the symbol definition files should be located in the source directories of the project.

Note that the gnatmake switch -m will almost always trigger recompilation for sources that are preprocessed, because gnatmake cannot compute the checksum of the source after preprocessing.

The actual preprocessing function is described in detail in Preprocessing with gnatprep. This section explains the switches that relate to integrated preprocessing.


This switch specifies the file name (without directory information) of the preprocessor data file. Either place this file in one of the source directories, or, when using project files, reference the project file’s directory via the project_name'Project_Dir project attribute; e.g:

project Prj is
   package Compiler is
      for Switches ("Ada") use
        ("-gnatep=" & Prj'Project_Dir & "prep.def");
   end Compiler;
end Prj;

A preprocessor data file is a text file that contains preprocessor control lines. A preprocessor control line directs the preprocessing of either a particular source file, or, analogous to others in Ada, all sources not specified elsewhere in the preprocessor data file. A preprocessor control line can optionally identify a definition file that assigns values to preprocessor symbols, as well as a list of switches that relate to preprocessing. Empty lines and comments (using Ada syntax) are also permitted, with no semantic effect.

Here’s an example of a preprocessor data file:

"toto.adb"  "prep.def" -u
--  Preprocess toto.adb, using definition file prep.def
--  Undefined symbols are treated as False

* -c -DVERSION=V101
--  Preprocess all other sources without using a definition file
--  Suppressed lined are commented
--  Symbol VERSION has the value V101

"tata.adb" "prep2.def" -s
--  Preprocess tata.adb, using definition file prep2.def
--  List all symbols with their values

A preprocessor control line has the following syntax:

<preprocessor_control_line> ::=
   <preprocessor_input> [ <definition_file_name> ] { <switch> }

<preprocessor_input> ::= <source_file_name> | '*'

<definition_file_name> ::= <string_literal>

<source_file_name> := <string_literal>

<switch> := (See below for list)

Thus each preprocessor control line starts with either a literal string or the character ‘*’:

  • A literal string is the file name (without directory information) of the source file that will be input to the preprocessor.

  • The character ‘*’ is a wild-card indicator; the additional parameters on the line indicate the preprocessing for all the sources that are not specified explicitly on other lines (the order of the lines is not significant).

It is an error to have two lines with the same file name or two lines starting with the character ‘*’.

After the file name or ‘*’, an optional literal string specifies the name of the definition file to be used for preprocessing (Form of Definitions File). The definition files are found by the compiler in one of the source directories. In some cases, when compiling a source in a directory other than the current directory, if the definition file is in the current directory, it may be necessary to add the current directory as a source directory through the -I switch; otherwise the compiler would not find the definition file.

Finally, switches similar to those of gnatprep may optionally appear:


Causes both preprocessor lines and the lines deleted by preprocessing to be replaced by blank lines, preserving the line number. This switch is always implied; however, if specified after -c it cancels the effect of -c.


Causes both preprocessor lines and the lines deleted by preprocessing to be retained as comments marked with the special string ‘–!’.


Define or redefine symbol to have new_value as its value. The permitted form for symbol is either an Ada identifier, or any Ada reserved word aside from if, else, elsif, end, and, or and then. The permitted form for new_value is a literal string, an Ada identifier or any Ada reserved word. A symbol declared with this switch replaces a symbol with the same name defined in a definition file.


Causes a sorted list of symbol names and values to be listed on the standard output file.


Causes undefined symbols to be treated as having the value FALSE in the context of a preprocessor test. In the absence of this option, an undefined symbol in a #if or #elsif test will be treated as an error.


Define or redefine symbol to have new_value as its value. If no value is supplied, then the value of symbol is True. The form of symbol is an identifier, following normal Ada (case-insensitive) rules for its syntax, and new_value is either an arbitrary string between double quotes or any sequence (including an empty sequence) of characters from the set (letters, digits, period, underline). Ada reserved words may be used as symbols, with the exceptions of if, else, elsif, end, and, or and then.



A symbol declared with this switch on the command line replaces a symbol with the same name either in a definition file or specified with a switch -D in the preprocessor data file.

This switch is similar to switch -D of gnatprep.


When integrated preprocessing is performed on source file filename.extension, create or overwrite filename.extension.prep to contain the result of the preprocessing. For example if the source file is foo.adb then the output file will be foo.adb.prep.

3.11. Mixed Language Programming

This section describes how to develop a mixed-language program, with a focus on combining Ada with C or C++.

3.11.1. Interfacing to C

Interfacing Ada with a foreign language such as C involves using compiler directives to import and/or export entity definitions in each language – using extern statements in C, for instance, and the Import, Export, and Convention pragmas in Ada. A full treatment of these topics is provided in Appendix B, section 1 of the Ada Reference Manual.

There are two ways to build a program using GNAT that contains some Ada sources and some foreign language sources, depending on whether or not the main subprogram is written in Ada. Here is a source example with the main subprogram in Ada:

/* file1.c */
#include <stdio.h>

void print_num (int num)
  printf ("num is %d.\\n", num);
/* file2.c */

/* num_from_Ada is declared in my_main.adb */
extern int num_from_Ada;

int get_num (void)
  return num_from_Ada;
--  my_main.adb
procedure My_Main is

   --  Declare then export an Integer entity called num_from_Ada
   My_Num : Integer := 10;
   pragma Export (C, My_Num, "num_from_Ada");

   --  Declare an Ada function spec for Get_Num, then use
   --  C function get_num for the implementation.
   function Get_Num return Integer;
   pragma Import (C, Get_Num, "get_num");

   --  Declare an Ada procedure spec for Print_Num, then use
   --  C function print_num for the implementation.
   procedure Print_Num (Num : Integer);
   pragma Import (C, Print_Num, "print_num");

   Print_Num (Get_Num);
end My_Main;

To build this example:

  • First compile the foreign language files to generate object files:

    $ gcc -c file1.c
    $ gcc -c file2.c
  • Then, compile the Ada units to produce a set of object files and ALI files:

    $ gnatmake -c my_main.adb
  • Run the Ada binder on the Ada main program:

    $ gnatbind my_main.ali
  • Link the Ada main program, the Ada objects and the other language objects:

    $ gnatlink my_main.ali file1.o file2.o

The last three steps can be grouped in a single command:

$ gnatmake my_main.adb -largs file1.o file2.o

If the main program is in a language other than Ada, then you may have more than one entry point into the Ada subsystem. You must use a special binder option to generate callable routines that initialize and finalize the Ada units (Binding with Non-Ada Main Programs). Calls to the initialization and finalization routines must be inserted in the main program, or some other appropriate point in the code. The call to initialize the Ada units must occur before the first Ada subprogram is called, and the call to finalize the Ada units must occur after the last Ada subprogram returns. The binder will place the initialization and finalization subprograms into the b~xxx.adb file where they can be accessed by your C sources. To illustrate, we have the following example:

/* main.c */
extern void adainit (void);
extern void adafinal (void);
extern int add (int, int);
extern int sub (int, int);

int main (int argc, char *argv[])
   int a = 21, b = 7;


   /* Should print "21 + 7 = 28" */
   printf ("%d + %d = %d\\n", a, b, add (a, b));

   /* Should print "21 - 7 = 14" */
   printf ("%d - %d = %d\\n", a, b, sub (a, b));

--  unit1.ads
package Unit1 is
   function Add (A, B : Integer) return Integer;
   pragma Export (C, Add, "add");
end Unit1;
--  unit1.adb
package body Unit1 is
   function Add (A, B : Integer) return Integer is
      return A + B;
   end Add;
end Unit1;
--  unit2.ads
package Unit2 is
   function Sub (A, B : Integer) return Integer;
   pragma Export (C, Sub, "sub");
end Unit2;
--  unit2.adb
package body Unit2 is
   function Sub (A, B : Integer) return Integer is
      return A - B;
   end Sub;
end Unit2;

The build procedure for this application is similar to the last example’s:

  • First, compile the foreign language files to generate object files:

    $ gcc -c main.c
  • Next, compile the Ada units to produce a set of object files and ALI files:

    $ gnatmake -c unit1.adb
    $ gnatmake -c unit2.adb
  • Run the Ada binder on every generated ALI file. Make sure to use the -n option to specify a foreign main program:

    $ gnatbind -n unit1.ali unit2.ali
  • Link the Ada main program, the Ada objects and the foreign language objects. You need only list the last ALI file here:

    $ gnatlink unit2.ali main.o -o exec_file

    This procedure yields a binary executable called exec_file.

Depending on the circumstances (for example when your non-Ada main object does not provide symbol main), you may also need to instruct the GNAT linker not to include the standard startup objects by passing the -nostartfiles switch to gnatlink.

3.11.2. Calling Conventions

GNAT follows standard calling sequence conventions and will thus interface to any other language that also follows these conventions. The following Convention identifiers are recognized by GNAT:


This indicates that the standard Ada calling sequence will be used and all Ada data items may be passed without any limitations in the case where GNAT is used to generate both the caller and callee. It is also possible to mix GNAT generated code and code generated by another Ada compiler. In this case, the data types should be restricted to simple cases, including primitive types. Whether complex data types can be passed depends on the situation. Probably it is safe to pass simple arrays, such as arrays of integers or floats. Records may or may not work, depending on whether both compilers lay them out identically. Complex structures involving variant records, access parameters, tasks, or protected types, are unlikely to be able to be passed.

Note that in the case of GNAT running on a platform that supports HP Ada 83, a higher degree of compatibility can be guaranteed, and in particular records are laid out in an identical manner in the two compilers. Note also that if output from two different compilers is mixed, the program is responsible for dealing with elaboration issues. Probably the safest approach is to write the main program in the version of Ada other than GNAT, so that it takes care of its own elaboration requirements, and then call the GNAT-generated adainit procedure to ensure elaboration of the GNAT components. Consult the documentation of the other Ada compiler for further details on elaboration.

However, it is not possible to mix the tasking run time of GNAT and HP Ada 83, All the tasking operations must either be entirely within GNAT compiled sections of the program, or entirely within HP Ada 83 compiled sections of the program.


Specifies assembler as the convention. In practice this has the same effect as convention Ada (but is not equivalent in the sense of being considered the same convention).


Equivalent to Assembler.


Data will be passed according to the conventions described in section B.4 of the Ada Reference Manual.


Data will be passed according to the conventions described in section B.3 of the Ada Reference Manual.

A note on interfacing to a C ‘varargs’ function:

In C, varargs allows a function to take a variable number of arguments. There is no direct equivalent in this to Ada. One approach that can be used is to create a C wrapper for each different profile and then interface to this C wrapper. For example, to print an int value using printf, create a C function printfi that takes two arguments, a pointer to a string and an int, and calls printf. Then in the Ada program, use pragma Import to interface to printfi.

It may work on some platforms to directly interface to a varargs function by providing a specific Ada profile for a particular call. However, this does not work on all platforms, since there is no guarantee that the calling sequence for a two argument normal C function is the same as for calling a varargs C function with the same two arguments.


Equivalent to C.


Equivalent to C.

C_Plus_Plus (or CPP)

This stands for C++. For most purposes this is identical to C. See the separate description of the specialized GNAT pragmas relating to C++ interfacing for further details.


Data will be passed according to the conventions described in section B.5 of the Ada Reference Manual.


This applies to an intrinsic operation, as defined in the Ada Reference Manual. If a pragma Import (Intrinsic) applies to a subprogram, this means that the body of the subprogram is provided by the compiler itself, usually by means of an efficient code sequence, and that the user does not supply an explicit body for it. In an application program, the pragma may be applied to the following sets of names:

  • Rotate_Left, Rotate_Right, Shift_Left, Shift_Right, Shift_Right_Arithmetic. The corresponding subprogram declaration must have two formal parameters. The first one must be a signed integer type or a modular type with a binary modulus, and the second parameter must be of type Natural. The return type must be the same as the type of the first argument. The size of this type can only be 8, 16, 32, or 64.

  • Binary arithmetic operators: ‘+’, ‘-‘, ‘*’, ‘/’. The corresponding operator declaration must have parameters and result type that have the same root numeric type (for example, all three are long_float types). This simplifies the definition of operations that use type checking to perform dimensional checks:

  type Distance is new Long_Float;
  type Time     is new Long_Float;
  type Velocity is new Long_Float;
  function "/" (D : Distance; T : Time)
    return Velocity;
  pragma Import (Intrinsic, "/");

This common idiom is often programmed with a generic definition and an
explicit body. The pragma makes it simpler to introduce such declarations.
It incurs no overhead in compilation time or code size, because it is
implemented as a single machine instruction.
  • General subprogram entities. This is used to bind an Ada subprogram declaration to a compiler builtin by name with back-ends where such interfaces are available. A typical example is the set of __builtin functions exposed by the GCC back-end, as in the following example:

    function builtin_sqrt (F : Float) return Float;
    pragma Import (Intrinsic, builtin_sqrt, "__builtin_sqrtf");

    Most of the GCC builtins are accessible this way, and as for other import conventions (e.g. C), it is the user’s responsibility to ensure that the Ada subprogram profile matches the underlying builtin expectations.


This is relevant only to Windows implementations of GNAT, and specifies that the Stdcall calling sequence will be used, as defined by the NT API. Nevertheless, to ease building cross-platform bindings this convention will be handled as a C calling convention on non-Windows platforms.


This is equivalent to Stdcall.


This is equivalent to Stdcall.


This is a special convention that indicates that the compiler should provide a stub body that raises Program_Error.

GNAT additionally provides a useful pragma Convention_Identifier that can be used to parameterize conventions and allow additional synonyms to be specified. For example if you have legacy code in which the convention identifier Fortran77 was used for Fortran, you can use the configuration pragma:

pragma Convention_Identifier (Fortran77, Fortran);

And from now on the identifier Fortran77 may be used as a convention identifier (for example in an Import pragma) with the same meaning as Fortran.

3.11.3. Building Mixed Ada and C++ Programs

A programmer inexperienced with mixed-language development may find that building an application containing both Ada and C++ code can be a challenge. This section gives a few hints that should make this task easier. Interfacing to C++

GNAT supports interfacing with the G++ compiler (or any C++ compiler generating code that is compatible with the G++ Application Binary Interface —see http://www.codesourcery.com/archives/cxx-abi).

Interfacing can be done at 3 levels: simple data, subprograms, and classes. In the first two cases, GNAT offers a specific Convention C_Plus_Plus (or CPP) that behaves exactly like Convention C. Usually, C++ mangles the names of subprograms. To generate proper mangled names automatically, see Generating Ada Bindings for C and C++ headers). This problem can also be addressed manually in two ways:

  • by modifying the C++ code in order to force a C convention using the extern "C" syntax.

  • by figuring out the mangled name (using e.g. nm) and using it as the Link_Name argument of the pragma import.

Interfacing at the class level can be achieved by using the GNAT specific pragmas such as CPP_Constructor. See the GNAT_Reference_Manual for additional information. Linking a Mixed C++ & Ada Program

Usually the linker of the C++ development system must be used to link mixed applications because most C++ systems will resolve elaboration issues (such as calling constructors on global class instances) transparently during the link phase. GNAT has been adapted to ease the use of a foreign linker for the last phase. Three cases can be considered:

  • Using GNAT and G++ (GNU C++ compiler) from the same GCC installation: The C++ linker can simply be called by using the C++ specific driver called g++.

    Note that if the C++ code uses inline functions, you will need to compile your C++ code with the -fkeep-inline-functions switch in order to provide an existing function implementation that the Ada code can link with.

    $ g++ -c -fkeep-inline-functions file1.C
    $ g++ -c -fkeep-inline-functions file2.C
    $ gnatmake ada_unit -largs file1.o file2.o --LINK=g++
  • Using GNAT and G++ from two different GCC installations: If both compilers are on the :envvar`PATH`, the previous method may be used. It is important to note that environment variables such as C_INCLUDE_PATH, GCC_EXEC_PREFIX, BINUTILS_ROOT, and GCC_ROOT will affect both compilers at the same time and may make one of the two compilers operate improperly if set during invocation of the wrong compiler. It is also very important that the linker uses the proper libgcc.a GCC library – that is, the one from the C++ compiler installation. The implicit link command as suggested in the gnatmake command from the former example can be replaced by an explicit link command with the full-verbosity option in order to verify which library is used:

    $ gnatbind ada_unit
    $ gnatlink -v -v ada_unit file1.o file2.o --LINK=c++

    If there is a problem due to interfering environment variables, it can be worked around by using an intermediate script. The following example shows the proper script to use when GNAT has not been installed at its default location and g++ has been installed at its default location:

    $ cat ./my_script
    unset GCC_ROOT
    c++ $*
    $ gnatlink -v -v ada_unit file1.o file2.o --LINK=./my_script
  • Using a non-GNU C++ compiler: The commands previously described can be used to insure that the C++ linker is used. Nonetheless, you need to add a few more parameters to the link command line, depending on the exception mechanism used.

    If the setjmp / longjmp exception mechanism is used, only the paths to the libgcc libraries are required:

    $ cat ./my_script
    CC $* gcc -print-file-name=libgcc.a gcc -print-file-name=libgcc_eh.a
    $ gnatlink ada_unit file1.o file2.o --LINK=./my_script

    where CC is the name of the non-GNU C++ compiler.

    If the “zero cost” exception mechanism is used, and the platform supports automatic registration of exception tables (e.g., Solaris), paths to more objects are required:

    $ cat ./my_script
    CC gcc -print-file-name=crtbegin.o $* \\
    gcc -print-file-name=libgcc.a gcc -print-file-name=libgcc_eh.a \\
    gcc -print-file-name=crtend.o
    $ gnatlink ada_unit file1.o file2.o --LINK=./my_script

    If the “zero cost exception” mechanism is used, and the platform doesn’t support automatic registration of exception tables (e.g., HP-UX or AIX), the simple approach described above will not work and a pre-linking phase using GNAT will be necessary.

Another alternative is to use the gprbuild multi-language builder which has a large knowledge base and knows how to link Ada and C++ code together automatically in most cases. A Simple Example

The following example, provided as part of the GNAT examples, shows how to achieve procedural interfacing between Ada and C++ in both directions. The C++ class A has two methods. The first method is exported to Ada by the means of an extern C wrapper function. The second method calls an Ada subprogram. On the Ada side, the C++ calls are modelled by a limited record with a layout comparable to the C++ class. The Ada subprogram, in turn, calls the C++ method. So, starting from the C++ main program, the process passes back and forth between the two languages.

Here are the compilation commands:

$ gnatmake -c simple_cpp_interface
$ g++ -c cpp_main.C
$ g++ -c ex7.C
$ gnatbind -n simple_cpp_interface
$ gnatlink simple_cpp_interface -o cpp_main --LINK=g++ -lstdc++ ex7.o cpp_main.o

Here are the corresponding sources:


#include "ex7.h"

extern "C" {
  void adainit (void);
  void adafinal (void);
  void method1 (A *t);

void method1 (A *t)
  t->method1 ();

int main ()
  A obj;
  adainit ();
  obj.method2 (3030);
  adafinal ();

class Origin {
  int o_value;
class A : public Origin {
  void method1 (void);
  void method2 (int v);
  int   a_value;

#include "ex7.h"
#include <stdio.h>

extern "C" { void ada_method2 (A *t, int v);}

void A::method1 (void)
  a_value = 2020;
  printf ("in A::method1, a_value = %d \\n",a_value);

void A::method2 (int v)
   ada_method2 (this, v);
   printf ("in A::method2, a_value = %d \\n",a_value);

   a_value = 1010;
  printf ("in A::A, a_value = %d \\n",a_value);
-- simple_cpp_interface.ads
with System;
package Simple_Cpp_Interface is
   type A is limited
         Vptr    : System.Address;
         O_Value : Integer;
         A_Value : Integer;
      end record;
   pragma Convention (C, A);

   procedure Method1 (This : in out A);
   pragma Import (C, Method1);

   procedure Ada_Method2 (This : in out A; V : Integer);
   pragma Export (C, Ada_Method2);

end Simple_Cpp_Interface;
-- simple_cpp_interface.adb
package body Simple_Cpp_Interface is

   procedure Ada_Method2 (This : in out A; V : Integer) is
      Method1 (This);
      This.A_Value := V;
   end Ada_Method2;

end Simple_Cpp_Interface; Interfacing with C++ constructors

In order to interface with C++ constructors GNAT provides the pragma CPP_Constructor (see the GNAT_Reference_Manual for additional information). In this section we present some common uses of C++ constructors in mixed-languages programs in GNAT.

Let us assume that we need to interface with the following C++ class:

class Root {
  int  a_value;
  int  b_value;
  virtual int Get_Value ();
  Root();              // Default constructor
  Root(int v);         // 1st non-default constructor
  Root(int v, int w);  // 2nd non-default constructor

For this purpose we can write the following package spec (further information on how to build this spec is available in Interfacing with C++ at the Class Level and Generating Ada Bindings for C and C++ headers).

with Interfaces.C; use Interfaces.C;
package Pkg_Root is
  type Root is tagged limited record
     A_Value : int;
     B_Value : int;
  end record;
  pragma Import (CPP, Root);

  function Get_Value (Obj : Root) return int;
  pragma Import (CPP, Get_Value);

  function Constructor return Root;
  pragma Cpp_Constructor (Constructor, "_ZN4RootC1Ev");

  function Constructor (v : Integer) return Root;
  pragma Cpp_Constructor (Constructor, "_ZN4RootC1Ei");

  function Constructor (v, w : Integer) return Root;
  pragma Cpp_Constructor (Constructor, "_ZN4RootC1Eii");
end Pkg_Root;

On the Ada side the constructor is represented by a function (whose name is arbitrary) that returns the classwide type corresponding to the imported C++ class. Although the constructor is described as a function, it is typically a procedure with an extra implicit argument (the object being initialized) at the implementation level. GNAT issues the appropriate call, whatever it is, to get the object properly initialized.

Constructors can only appear in the following contexts:

  • On the right side of an initialization of an object of type T.

  • On the right side of an initialization of a record component of type T.

  • In an Ada 2005 limited aggregate.

  • In an Ada 2005 nested limited aggregate.

  • In an Ada 2005 limited aggregate that initializes an object built in place by an extended return statement.

In a declaration of an object whose type is a class imported from C++, either the default C++ constructor is implicitly called by GNAT, or else the required C++ constructor must be explicitly called in the expression that initializes the object. For example:

Obj1 : Root;
Obj2 : Root := Constructor;
Obj3 : Root := Constructor (v => 10);
Obj4 : Root := Constructor (30, 40);

The first two declarations are equivalent: in both cases the default C++ constructor is invoked (in the former case the call to the constructor is implicit, and in the latter case the call is explicit in the object declaration). Obj3 is initialized by the C++ non-default constructor that takes an integer argument, and Obj4 is initialized by the non-default C++ constructor that takes two integers.

Let us derive the imported C++ class in the Ada side. For example:

type DT is new Root with record
   C_Value : Natural := 2009;
end record;

In this case the components DT inherited from the C++ side must be initialized by a C++ constructor, and the additional Ada components of type DT are initialized by GNAT. The initialization of such an object is done either by default, or by means of a function returning an aggregate of type DT, or by means of an extension aggregate.

Obj5 : DT;
Obj6 : DT := Function_Returning_DT (50);
Obj7 : DT := (Constructor (30,40) with C_Value => 50);

The declaration of Obj5 invokes the default constructors: the C++ default constructor of the parent type takes care of the initialization of the components inherited from Root, and GNAT takes care of the default initialization of the additional Ada components of type DT (that is, C_Value is initialized to value 2009). The order of invocation of the constructors is consistent with the order of elaboration required by Ada and C++. That is, the constructor of the parent type is always called before the constructor of the derived type.

Let us now consider a record that has components whose type is imported from C++. For example:

type Rec1 is limited record
   Data1 : Root := Constructor (10);
   Value : Natural := 1000;
end record;

type Rec2 (D : Integer := 20) is limited record
   Rec   : Rec1;
   Data2 : Root := Constructor (D, 30);
end record;

The initialization of an object of type Rec2 will call the non-default C++ constructors specified for the imported components. For example:

Obj8 : Rec2 (40);

Using Ada 2005 we can use limited aggregates to initialize an object invoking C++ constructors that differ from those specified in the type declarations. For example:

Obj9 : Rec2 := (Rec => (Data1 => Constructor (15, 16),
                        others => <>),
                others => <>);

The above declaration uses an Ada 2005 limited aggregate to initialize Obj9, and the C++ constructor that has two integer arguments is invoked to initialize the Data1 component instead of the constructor specified in the declaration of type Rec1. In Ada 2005 the box in the aggregate indicates that unspecified components are initialized using the expression (if any) available in the component declaration. That is, in this case discriminant D is initialized to value 20, Value is initialized to value 1000, and the non-default C++ constructor that handles two integers takes care of initializing component Data2 with values 20,30.

In Ada 2005 we can use the extended return statement to build the Ada equivalent to C++ non-default constructors. For example:

function Constructor (V : Integer) return Rec2 is
   return Obj : Rec2 := (Rec => (Data1  => Constructor (V, 20),
                                 others => <>),
                         others => <>) do
      --  Further actions required for construction of
      --  objects of type Rec2
   end record;
end Constructor;

In this example the extended return statement construct is used to build in place the returned object whose components are initialized by means of a limited aggregate. Any further action associated with the constructor can be placed inside the construct. Interfacing with C++ at the Class Level

In this section we demonstrate the GNAT features for interfacing with C++ by means of an example making use of Ada 2005 abstract interface types. This example consists of a classification of animals; classes have been used to model our main classification of animals, and interfaces provide support for the management of secondary classifications. We first demonstrate a case in which the types and constructors are defined on the C++ side and imported from the Ada side, and latter the reverse case.

The root of our derivation will be the Animal class, with a single private attribute (the Age of the animal), a constructor, and two public primitives to set and get the value of this attribute.

class Animal {
   virtual void Set_Age (int New_Age);
   virtual int Age ();
   Animal() {Age_Count = 0;};
   int Age_Count;

Abstract interface types are defined in C++ by means of classes with pure virtual functions and no data members. In our example we will use two interfaces that provide support for the common management of Carnivore and Domestic animals:

class Carnivore {
   virtual int Number_Of_Teeth () = 0;

class Domestic {
   virtual void Set_Owner (char* Name) = 0;

Using these declarations, we can now say that a Dog is an animal that is both Carnivore and Domestic, that is:

class Dog : Animal, Carnivore, Domestic {
   virtual int  Number_Of_Teeth ();
   virtual void Set_Owner (char* Name);

   Dog(); // Constructor
   int  Tooth_Count;
   char *Owner;

In the following examples we will assume that the previous declarations are located in a file named animals.h. The following package demonstrates how to import these C++ declarations from the Ada side:

with Interfaces.C.Strings; use Interfaces.C.Strings;
package Animals is
  type Carnivore is limited interface;
  pragma Convention (C_Plus_Plus, Carnivore);
  function Number_Of_Teeth (X : Carnivore)
     return Natural is abstract;

  type Domestic is limited interface;
  pragma Convention (C_Plus_Plus, Domestic);
  procedure Set_Owner
    (X    : in out Domestic;
     Name : Chars_Ptr) is abstract;

  type Animal is tagged limited record
    Age : Natural;
  end record;
  pragma Import (C_Plus_Plus, Animal);

  procedure Set_Age (X : in out Animal; Age : Integer);
  pragma Import (C_Plus_Plus, Set_Age);

  function Age (X : Animal) return Integer;
  pragma Import (C_Plus_Plus, Age);

  function New_Animal return Animal;
  pragma CPP_Constructor (New_Animal);
  pragma Import (CPP, New_Animal, "_ZN6AnimalC1Ev");

  type Dog is new Animal and Carnivore and Domestic with record
    Tooth_Count : Natural;
    Owner       : Chars_Ptr;
  end record;
  pragma Import (C_Plus_Plus, Dog);

  function Number_Of_Teeth (A : Dog) return Natural;
  pragma Import (C_Plus_Plus, Number_Of_Teeth);

  procedure Set_Owner (A : in out Dog; Name : Chars_Ptr);
  pragma Import (C_Plus_Plus, Set_Owner);

  function New_Dog return Dog;
  pragma CPP_Constructor (New_Dog);
  pragma Import (CPP, New_Dog, "_ZN3DogC2Ev");
end Animals;

Thanks to the compatibility between GNAT run-time structures and the C++ ABI, interfacing with these C++ classes is easy. The only requirement is that all the primitives and components must be declared exactly in the same order in the two languages.

Regarding the abstract interfaces, we must indicate to the GNAT compiler by means of a pragma Convention (C_Plus_Plus), the convention used to pass the arguments to the called primitives will be the same as for C++. For the imported classes we use pragma Import with convention C_Plus_Plus to indicate that they have been defined on the C++ side; this is required because the dispatch table associated with these tagged types will be built in the C++ side and therefore will not contain the predefined Ada primitives which Ada would otherwise expect.

As the reader can see there is no need to indicate the C++ mangled names associated with each subprogram because it is assumed that all the calls to these primitives will be dispatching calls. The only exception is the constructor, which must be registered with the compiler by means of pragma CPP_Constructor and needs to provide its associated C++ mangled name because the Ada compiler generates direct calls to it.

With the above packages we can now declare objects of type Dog on the Ada side and dispatch calls to the corresponding subprograms on the C++ side. We can also extend the tagged type Dog with further fields and primitives, and override some of its C++ primitives on the Ada side. For example, here we have a type derivation defined on the Ada side that inherits all the dispatching primitives of the ancestor from the C++ side.

with Animals; use Animals;
package Vaccinated_Animals is
  type Vaccinated_Dog is new Dog with null record;
  function Vaccination_Expired (A : Vaccinated_Dog) return Boolean;
end Vaccinated_Animals;

It is important to note that, because of the ABI compatibility, the programmer does not need to add any further information to indicate either the object layout or the dispatch table entry associated with each dispatching operation.

Now let us define all the types and constructors on the Ada side and export them to C++, using the same hierarchy of our previous example:

with Interfaces.C.Strings;
use Interfaces.C.Strings;
package Animals is
  type Carnivore is limited interface;
  pragma Convention (C_Plus_Plus, Carnivore);
  function Number_Of_Teeth (X : Carnivore)
     return Natural is abstract;

  type Domestic is limited interface;
  pragma Convention (C_Plus_Plus, Domestic);
  procedure Set_Owner
    (X    : in out Domestic;
     Name : Chars_Ptr) is abstract;

  type Animal is tagged record
    Age : Natural;
  end record;
  pragma Convention (C_Plus_Plus, Animal);

  procedure Set_Age (X : in out Animal; Age : Integer);
  pragma Export (C_Plus_Plus, Set_Age);

  function Age (X : Animal) return Integer;
  pragma Export (C_Plus_Plus, Age);

  function New_Animal return Animal'Class;
  pragma Export (C_Plus_Plus, New_Animal);

  type Dog is new Animal and Carnivore and Domestic with record
    Tooth_Count : Natural;
    Owner       : String (1 .. 30);
  end record;
  pragma Convention (C_Plus_Plus, Dog);

  function Number_Of_Teeth (A : Dog) return Natural;
  pragma Export (C_Plus_Plus, Number_Of_Teeth);

  procedure Set_Owner (A : in out Dog; Name : Chars_Ptr);
  pragma Export (C_Plus_Plus, Set_Owner);

  function New_Dog return Dog'Class;
  pragma Export (C_Plus_Plus, New_Dog);
end Animals;

Compared with our previous example the only differences are the use of pragma Convention (instead of pragma Import), and the use of pragma Export to indicate to the GNAT compiler that the primitives will be available to C++. Thanks to the ABI compatibility, on the C++ side there is nothing else to be done; as explained above, the only requirement is that all the primitives and components are declared in exactly the same order.

For completeness, let us see a brief C++ main program that uses the declarations available in animals.h (presented in our first example) to import and use the declarations from the Ada side, properly initializing and finalizing the Ada run-time system along the way:

#include "animals.h"
#include <iostream>
using namespace std;

void Check_Carnivore (Carnivore *obj) {...}
void Check_Domestic (Domestic *obj)   {...}
void Check_Animal (Animal *obj)       {...}
void Check_Dog (Dog *obj)             {...}

extern "C" {
  void adainit (void);
  void adafinal (void);
  Dog* new_dog ();

void test ()
  Dog *obj = new_dog();  // Ada constructor
  Check_Carnivore (obj); // Check secondary DT
  Check_Domestic (obj);  // Check secondary DT
  Check_Animal (obj);    // Check primary DT
  Check_Dog (obj);       // Check primary DT

int main ()
  adainit ();  test();  adafinal ();
  return 0;

3.11.4. Generating Ada Bindings for C and C++ headers

GNAT includes a binding generator for C and C++ headers which is intended to do 95% of the tedious work of generating Ada specs from C or C++ header files.

Note that this capability is not intended to generate 100% correct Ada specs, and will is some cases require manual adjustments, although it can often be used out of the box in practice.

Some of the known limitations include:

  • only very simple character constant macros are translated into Ada constants. Function macros (macros with arguments) are partially translated as comments, to be completed manually if needed.

  • some extensions (e.g. vector types) are not supported

  • pointers to pointers or complex structures are mapped to System.Address

  • identifiers with identical name (except casing) will generate compilation errors (e.g. shm_get vs SHM_GET).

The code is generated using Ada 2012 syntax, which makes it easier to interface with other languages. In most cases you can still use the generated binding even if your code is compiled using earlier versions of Ada (e.g. -gnat95). Running the Binding Generator

The binding generator is part of the gcc compiler and can be invoked via the -fdump-ada-spec switch, which will generate Ada spec files for the header files specified on the command line, and all header files needed by these files transitively. For example:

$ g++ -c -fdump-ada-spec -C /usr/include/time.h
$ gcc -c *.ads

will generate, under GNU/Linux, the following files: time_h.ads, bits_time_h.ads, stddef_h.ads, bits_types_h.ads which correspond to the files /usr/include/time.h, /usr/include/bits/time.h, etc…, and will then compile these Ada specs in Ada 2005 mode.

The -C switch tells gcc to extract comments from headers, and will attempt to generate corresponding Ada comments.

If you want to generate a single Ada file and not the transitive closure, you can use instead the -fdump-ada-spec-slim switch.

You can optionally specify a parent unit, of which all generated units will be children, using -fada-spec-parent=unit.

Note that we recommend when possible to use the g++ driver to generate bindings, even for most C headers, since this will in general generate better Ada specs. For generating bindings for C++ headers, it is mandatory to use the g++ command, or gcc -x c++ which is equivalent in this case. If g++ cannot work on your C headers because of incompatibilities between C and C++, then you can fallback to gcc instead.

For an example of better bindings generated from the C++ front-end, the name of the parameters (when available) are actually ignored by the C front-end. Consider the following C header:

extern void foo (int variable);

with the C front-end, variable is ignored, and the above is handled as:

extern void foo (int);

generating a generic:

procedure foo (param1 : int);

with the C++ front-end, the name is available, and we generate:

procedure foo (variable : int);

In some cases, the generated bindings will be more complete or more meaningful when defining some macros, which you can do via the -D switch. This is for example the case with Xlib.h under GNU/Linux:

$ g++ -c -fdump-ada-spec -DXLIB_ILLEGAL_ACCESS -C /usr/include/X11/Xlib.h

The above will generate more complete bindings than a straight call without the -DXLIB_ILLEGAL_ACCESS switch.

In other cases, it is not possible to parse a header file in a stand-alone manner, because other include files need to be included first. In this case, the solution is to create a small header file including the needed #include and possible #define directives. For example, to generate Ada bindings for readline/readline.h, you need to first include stdio.h, so you can create a file with the following two lines in e.g. readline1.h:

#include <stdio.h>
#include <readline/readline.h>

and then generate Ada bindings from this file:

$ g++ -c -fdump-ada-spec readline1.h Generating Bindings for C++ Headers

Generating bindings for C++ headers is done using the same options, always with the g++ compiler. Note that generating Ada spec from C++ headers is a much more complex job and support for C++ headers is much more limited that support for C headers. As a result, you will need to modify the resulting bindings by hand more extensively when using C++ headers.

In this mode, C++ classes will be mapped to Ada tagged types, constructors will be mapped using the CPP_Constructor pragma, and when possible, multiple inheritance of abstract classes will be mapped to Ada interfaces (see the Interfacing to C++ section in the GNAT Reference Manual for additional information on interfacing to C++).

For example, given the following C++ header file:

class Carnivore {
   virtual int Number_Of_Teeth () = 0;

class Domestic {
   virtual void Set_Owner (char* Name) = 0;

class Animal {
  int Age_Count;
  virtual void Set_Age (int New_Age);

class Dog : Animal, Carnivore, Domestic {
  int  Tooth_Count;
  char *Owner;

  virtual int  Number_Of_Teeth ();
  virtual void Set_Owner (char* Name);


The corresponding Ada code is generated:

package Class_Carnivore is
  type Carnivore is limited interface;
  pragma Import (CPP, Carnivore);

  function Number_Of_Teeth (this : access Carnivore) return int is abstract;
use Class_Carnivore;

package Class_Domestic is
  type Domestic is limited interface;
  pragma Import (CPP, Domestic);

  procedure Set_Owner
    (this : access Domestic;
     Name : Interfaces.C.Strings.chars_ptr) is abstract;
use Class_Domestic;

package Class_Animal is
  type Animal is tagged limited record
    Age_Count : aliased int;
  end record;
  pragma Import (CPP, Animal);

  procedure Set_Age (this : access Animal; New_Age : int);
  pragma Import (CPP, Set_Age, "_ZN6Animal7Set_AgeEi");
use Class_Animal;

package Class_Dog is
  type Dog is new Animal and Carnivore and Domestic with record
    Tooth_Count : aliased int;
    Owner : Interfaces.C.Strings.chars_ptr;
  end record;
  pragma Import (CPP, Dog);

  function Number_Of_Teeth (this : access Dog) return int;
  pragma Import (CPP, Number_Of_Teeth, "_ZN3Dog15Number_Of_TeethEv");

  procedure Set_Owner
    (this : access Dog; Name : Interfaces.C.Strings.chars_ptr);
  pragma Import (CPP, Set_Owner, "_ZN3Dog9Set_OwnerEPc");

  function New_Dog return Dog;
  pragma CPP_Constructor (New_Dog);
  pragma Import (CPP, New_Dog, "_ZN3DogC1Ev");
use Class_Dog; Switches


Generate Ada spec files for the given header files transitively (including all header files that these headers depend upon).


Generate Ada spec files for the header files specified on the command line only.


Specifies that all files generated by -fdump-ada-spec are to be child units of the specified parent unit.


Extract comments from headers and generate Ada comments in the Ada spec files.

3.11.5. Generating C Headers for Ada Specifications

GNAT includes a C header generator for Ada specifications which supports Ada types that have a direct mapping to C types. This includes in particular support for:

  • Scalar types

  • Constrained arrays

  • Records (untagged)

  • Composition of the above types

  • Constant declarations

  • Object declarations

  • Subprogram declarations Running the C Header Generator

The C header generator is part of the GNAT compiler and can be invoked via the -gnatceg combination of switches, which will generate a .h file corresponding to the given input file (Ada spec or body). Note that only spec files are processed in any case, so giving a spec or a body file as input is equivalent. For example:

$ gcc -c -gnatceg pack1.ads

will generate a self-contained file called pack1.h including common definitions from the Ada Standard package, followed by the definitions included in pack1.ads, as well as all the other units withed by this file.

For instance, given the following Ada files:

package Pack2 is
   type Int is range 1 .. 10;
end Pack2;
with Pack2;

package Pack1 is
   type Rec is record
      Field1, Field2 : Pack2.Int;
   end record;

   Global : Rec := (1, 2);

   procedure Proc1 (R : Rec);
   procedure Proc2 (R : in out Rec);
end Pack1;

The above gcc command will generate the following pack1.h file:

/* Standard definitions skipped */
#ifndef PACK2_ADS
#define PACK2_ADS
typedef short_short_integer pack2__TintB;
typedef pack2__TintB pack2__int;
#endif /* PACK2_ADS */

#ifndef PACK1_ADS
#define PACK1_ADS
typedef struct _pack1__rec {
  pack2__int field1;
  pack2__int field2;
} pack1__rec;
extern pack1__rec pack1__global;
extern void pack1__proc1(const pack1__rec r);
extern void pack1__proc2(pack1__rec *r);
#endif /* PACK1_ADS */

You can then include pack1.h from a C source file and use the types, call subprograms, reference objects, and constants.

3.12. GNAT and Other Compilation Models

This section compares the GNAT model with the approaches taken in other environents, first the C/C++ model and then the mechanism that has been used in other Ada systems, in particular those traditionally used for Ada 83.

3.12.1. Comparison between GNAT and C/C++ Compilation Models

The GNAT model of compilation is close to the C and C++ models. You can think of Ada specs as corresponding to header files in C. As in C, you don’t need to compile specs; they are compiled when they are used. The Ada with is similar in effect to the #include of a C header.

One notable difference is that, in Ada, you may compile specs separately to check them for semantic and syntactic accuracy. This is not always possible with C headers because they are fragments of programs that have less specific syntactic or semantic rules.

The other major difference is the requirement for running the binder, which performs two important functions. First, it checks for consistency. In C or C++, the only defense against assembling inconsistent programs lies outside the compiler, in a makefile, for example. The binder satisfies the Ada requirement that it be impossible to construct an inconsistent program when the compiler is used in normal mode.

The other important function of the binder is to deal with elaboration issues. There are also elaboration issues in C++ that are handled automatically. This automatic handling has the advantage of being simpler to use, but the C++ programmer has no control over elaboration. Where gnatbind might complain there was no valid order of elaboration, a C++ compiler would simply construct a program that malfunctioned at run time.

3.12.2. Comparison between GNAT and Conventional Ada Library Models

This section is intended for Ada programmers who have used an Ada compiler implementing the traditional Ada library model, as described in the Ada Reference Manual.

In GNAT, there is no ‘library’ in the normal sense. Instead, the set of source files themselves acts as the library. Compiling Ada programs does not generate any centralized information, but rather an object file and a ALI file, which are of interest only to the binder and linker. In a traditional system, the compiler reads information not only from the source file being compiled, but also from the centralized library. This means that the effect of a compilation depends on what has been previously compiled. In particular:

  • When a unit is withed, the unit seen by the compiler corresponds to the version of the unit most recently compiled into the library.

  • Inlining is effective only if the necessary body has already been compiled into the library.

  • Compiling a unit may obsolete other units in the library.

In GNAT, compiling one unit never affects the compilation of any other units because the compiler reads only source files. Only changes to source files can affect the results of a compilation. In particular:

  • When a unit is withed, the unit seen by the compiler corresponds to the source version of the unit that is currently accessible to the compiler.

  • Inlining requires the appropriate source files for the package or subprogram bodies to be available to the compiler. Inlining is always effective, independent of the order in which units are compiled.

  • Compiling a unit never affects any other compilations. The editing of sources may cause previous compilations to be out of date if they depended on the source file being modified.

The most important result of these differences is that order of compilation is never significant in GNAT. There is no situation in which one is required to do one compilation before another. What shows up as order of compilation requirements in the traditional Ada library becomes, in GNAT, simple source dependencies; in other words, there is only a set of rules saying what source files must be present when a file is compiled.

3.13. Using GNAT Files with External Tools

This section explains how files that are produced by GNAT may be used with tools designed for other languages.

3.13.1. Using Other Utility Programs with GNAT

The object files generated by GNAT are in standard system format and in particular the debugging information uses this format. This means programs generated by GNAT can be used with existing utilities that depend on these formats.

In general, any utility program that works with C will also often work with Ada programs generated by GNAT. This includes software utilities such as gprof (a profiling program), gdb (the FSF debugger), and utilities such as Purify.

3.13.2. The External Symbol Naming Scheme of GNAT

In order to interpret the output from GNAT, when using tools that are originally intended for use with other languages, it is useful to understand the conventions used to generate link names from the Ada entity names.

All link names are in all lowercase letters. With the exception of library procedure names, the mechanism used is simply to use the full expanded Ada name with dots replaced by double underscores. For example, suppose we have the following package spec:

package QRS is
   MN : Integer;
end QRS;

The variable MN has a full expanded Ada name of QRS.MN, so the corresponding link name is qrs__mn. Of course if a pragma Export is used this may be overridden:

package Exports is
   Var1 : Integer;
   pragma Export (Var1, C, External_Name => "var1_name");
   Var2 : Integer;
   pragma Export (Var2, C, Link_Name => "var2_link_name");
end Exports;

In this case, the link name for Var1 is whatever link name the C compiler would assign for the C function var1_name. This typically would be either var1_name or _var1_name, depending on operating system conventions, but other possibilities exist. The link name for Var2 is var2_link_name, and this is not operating system dependent.

One exception occurs for library level procedures. A potential ambiguity arises between the required name _main for the C main program, and the name we would otherwise assign to an Ada library level procedure called Main (which might well not be the main program).

To avoid this ambiguity, we attach the prefix _ada_ to such names. So if we have a library level procedure such as:

procedure Hello (S : String);

the external name of this procedure will be _ada_hello.