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.