2. Getting Started with GNAT

This chapter describes how to use GNAT’s command line interface to build executable Ada programs. On most platforms a visually oriented Integrated Development Environment is also available: GNAT Studio. GNAT Studio offers a graphical “look and feel”, support for development in other programming languages, comprehensive browsing features, and many other capabilities. For information on GNAT Studio please refer to the GNAT Studio documentation.

2.1. System Requirements

Even though any machine can run the GNAT toolset and GNAT Studio IDE, in order to get the best experience, we recommend using a machine with as many cores as possible since all individual compilations can run in parallel. A comfortable setup for a compiler server is a machine with 24 physical cores or more, with at least 48 GB of memory (2 GB per core).

For a desktop machine, a minimum of 4 cores is recommended (8 preferred), with at least 2GB per core (so 8 to 16GB).

In addition, for running and navigating sources in GNAT Studio smoothly, we recommend at least 1.5 GB plus 3 GB of RAM per 1 million source line of code. In other words, we recommend at least 3 GB for for 500K lines of code and 7.5 GB for 2 million lines of code.

Note that using local and fast drives will also make a difference in terms of build and link time. Network drives such as NFS, SMB, or worse, configuration management filesystems (such as ClearCase dynamic views) should be avoided as much as possible and will produce very degraded performance (typically 2 to 3 times slower than on local fast drives). If such slow drives cannot be avoided for accessing the source code, then you should at least configure your project file so that the result of the compilation is stored on a drive local to the machine performing the run. This can be achieved by setting the Object_Dir project file attribute.

2.2. Running GNAT

Three steps are needed to create an executable file from an Ada source file:

  • The source file(s) must be compiled.
  • The file(s) must be bound using the GNAT binder.
  • All appropriate object files must be linked to produce an executable.

All three steps are most commonly handled by using the gnatmake utility program that, given the name of the main program, automatically performs the necessary compilation, binding and linking steps.

2.3. Running a Simple Ada Program

Any text editor may be used to prepare an Ada program. (If Emacs is used, the optional Ada mode may be helpful in laying out the program.) The program text is a normal text file. We will assume in our initial example that you have used your editor to prepare the following standard format text file:

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

This file should be named hello.adb. With the normal default file naming conventions, GNAT requires that each file contain a single compilation unit whose file name is the unit name, with periods replaced by hyphens; the extension is ads for a spec and adb for a body. You can override this default file naming convention by use of the special pragma Source_File_Name (for further information please see Using Other File Names). Alternatively, if you want to rename your files according to this default convention, which is probably more convenient if you will be using GNAT for all your compilations, then the gnatchop utility can be used to generate correctly-named source files (see Renaming Files with gnatchop).

You can compile the program using the following command ($ is used as the command prompt in the examples in this document):

$ gcc -c hello.adb

gcc is the command used to run the compiler. This compiler is capable of compiling programs in several languages, including Ada and C. It assumes that you have given it an Ada program if the file extension is either .ads or .adb, and it will then call the GNAT compiler to compile the specified file.

The -c switch is required. It tells gcc to only do a compilation. (For C programs, gcc can also do linking, but this capability is not used directly for Ada programs, so the -c switch must always be present.)

This compile command generates a file hello.o, which is the object file corresponding to your Ada program. It also generates an ‘Ada Library Information’ file hello.ali, which contains additional information used to check that an Ada program is consistent.

To build an executable file, use either gnatmake or gprbuild with the name of the main file: these tools are builders that will take care of all the necessary build steps in the correct order. In particular, these builders automatically recompile any sources that have been modified since they were last compiled, or sources that depend on such modified sources, so that ‘version skew’ is avoided.

$ gnatmake hello.adb

The result is an executable program called hello, which can be run by entering:

$ hello

assuming that the current directory is on the search path for executable programs.

and, if all has gone well, you will see:

Hello WORLD!

appear in response to this command.

2.4. Running a Program with Multiple Units

Consider a slightly more complicated example that has three files: a main program, and the spec and body of a package:

package Greetings is
   procedure Hello;
   procedure Goodbye;
end Greetings;

with Ada.Text_IO; use Ada.Text_IO;
package body Greetings is
   procedure Hello is
   begin
      Put_Line ("Hello WORLD!");
   end Hello;

   procedure Goodbye is
   begin
      Put_Line ("Goodbye WORLD!");
   end Goodbye;
end Greetings;

with Greetings;
procedure Gmain is
begin
   Greetings.Hello;
   Greetings.Goodbye;
end Gmain;

Following the one-unit-per-file rule, place this program in the following three separate files:

greetings.ads
spec of package Greetings
greetings.adb
body of package Greetings
gmain.adb
body of main program

Note that there is no required order of compilation when using GNAT. In particular it is perfectly fine to compile the main program first. Also, it is not necessary to compile package specs in the case where there is an accompanying body; you only need to compile the body. If you want to submit these files to the compiler for semantic checking and not code generation, then use the -gnatc switch:

$ gcc -c greetings.ads -gnatc

Although the compilation can be done in separate steps, in practice it is almost always more convenient to use the gnatmake or gprbuild tools:

$ gnatmake gmain.adb