2.4 Using the `gnatmake' Utility

If you work on a program by compiling single components at a time using `gcc', you typically keep track of the units you modify. In order to build a consistent system, you compile not only these units, but also any units that depend on the units you have modified. For example, in the preceding case, if you edit gmain.adb, you only need to recompile that file. But if you edit greetings.ads, you must recompile both greetings.adb and gmain.adb, because both files contain units that depend on greetings.ads.

`gnatbind' will warn you if you forget one of these compilation steps, so that it is impossible to generate an inconsistent program as a result of forgetting to do a compilation. Nevertheless it is tedious and error-prone to keep track of dependencies among units. One approach to handle the dependency-bookkeeping is to use a makefile. However, makefiles present maintenance problems of their own: if the dependencies change as you change the program, you must make sure that the makefile is kept up-to-date manually, which is also an error-prone process.

The `gnatmake' utility takes care of these details automatically. Invoke it using either one of the following forms:

$ gnatmake gmain.adb
$ gnatmake gmain

The argument is the name of the file containing the main program; you may omit the extension. `gnatmake' examines the environment, automatically recompiles any files that need recompiling, and binds and links the resulting set of object files, generating the executable file, gmain. In a large program, it can be extremely helpful to use `gnatmake', because working out by hand what needs to be recompiled can be difficult.

Note that `gnatmake' takes into account all the Ada rules that establish dependencies among units. These include dependencies that result from inlining subprogram bodies, and from generic instantiation. Unlike some other Ada make tools, `gnatmake' does not rely on the dependencies that were found by the compiler on a previous compilation, which may possibly be wrong when sources change. `gnatmake' determines the exact set of dependencies from scratch each time it is run.