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When using a project file, you can invoke `gnatmake' with one or several main subprograms, by specifying their source files on the command line.
$ gnatmake -Pprj main1.adb main2.adb main3.adb
Each of these needs to be a source file of the same project, except when the switch -u is used.
When -u is not used, all the mains need to be sources of the same project, one of the project in the tree rooted at the project specified on the command line. The package Builder of this common project, the "main project" is the one that is considered by `gnatmake'.
When -u is used, the specified source files may be in projects imported directly or indirectly by the project specified on the command line. Note that if such a source file is not part of the project specified on the command line, the switches found in package Builder of the project specified on the command line, if any, that are transmitted to the compiler will still be used, not those found in the project file of the source file.
When using a project file, you can also invoke `gnatmake' without explicitly specifying any main, and the effect depends on whether you have defined the Main attribute. This attribute has a string list value, where each element in the list is the name of a source file (the file extension is optional) that contains a unit that can be a main subprogram.
If the Main attribute is defined in a project file as a non-empty string list and the switch `-u' is not used on the command line, then invoking `gnatmake' with this project file but without any main on the command line is equivalent to invoking `gnatmake' with all the file names in the Main attribute on the command line.
Example:
project Prj is
for Main use ("main1.adb", "main2.adb", "main3.adb");
end Prj;
With this project file, "gnatmake -Pprj" is equivalent to "gnatmake -Pprj main1.adb main2.adb main3.adb".
When the project attribute Main is not specified, or is specified as an empty string list, or when the switch `-u' is used on the command line, then invoking `gnatmake' with no main on the command line will result in all immediate sources of the project file being checked, and potentially recompiled. Depending on the presence of the switch `-u', sources from other project files on which the immediate sources of the main project file depend are also checked and potentially recompiled. In other words, the `-u' switch is applied to all of the immediate sources of the main project file.
When no main is specified on the command line and attribute Main exists and includes several mains, or when several mains are specified on the command line, the default switches in package Builder will be used for all mains, even if there are specific switches specified for one or several mains.
But the switches from package Binder or Linker will be the specific switches for each main, if they are specified.