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, to get the best experience we recommend using a machine with as many cores as possible, allowing individual compilations to 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, we recommend a minimum of 4 cores (8 is preferred), with at least 2GB per core (so 8 to 16GB).

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

Using fast, local drives can make a significant difference in build and link times. You should avoid network drives such as NFS, SMB, or worse, configuration management filesystems (such as ClearCase dynamic views) as much as possible since these will produce very degraded performance (typically 2 to 3 times slower than on fast, local drives). If you cannot avoid using such slow drives for accessing source code, you should at least configure your project file so the result of the compilation is stored on a drive local to the machine performing the compilation. You can do this by setting the Object_Dir project file attribute.

2.2. Running GNAT

You need to take three steps to create an executable file from an Ada source file:

  • You must compile the source file(s).

  • You must bind the file(s) using the GNAT binder.

  • You must link all appropriate object files to produce an executable.

You most commonly perform all three steps by using the gnatmake utility program. You pass it the name of the main program and it automatically performs the necessary compilation, binding, and linking steps.

2.3. Running a Simple Ada Program

You may use any text editor to prepare an Ada program. (If you use Emacs, an 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 named hello.adb:

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

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 (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 you use can use the gnatchop utility 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. It is capable of compiling programs in several languages, including Ada and C. It assumes you have given it an Ada program if the file extension is either .ads or .adb, in which case it will 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 you must always specify the -c.)

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 perform 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, as well as 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 you can 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 with 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 compile the body. If you want to submit these files to the compiler for semantic checking and not code generation, use the -gnatc switch:

$ gcc -c greetings.ads -gnatc

Although you can do the compilation in separate steps, in practice it’s almost always more convenient to use the gnatmake or gprbuild tools:

$ gnatmake gmain.adb