12.1.2 Switches and Project Files

For each of the packages Builder, Compiler, Binder, and Linker, you can specify a Default_Switches attribute, a Switches attribute, or both; as their names imply, these switch-related attributes affect the switches that are used for each of these GNAT components when gnatmake is invoked. As will be explained below, these component-specific switches precede the switches provided on the gnatmake command line.

The Default_Switches attribute is an attribute indexed by language name (case insensitive) whose value is a string list. For example:

     package Compiler is
       for Default_Switches ("Ada")
           use ("-gnaty",
                "-v");
     end Compiler;

The Switches attribute is indexed on a file name (which may or may not be case sensitive, depending on the operating system) whose value is a string list. For example:

     package Builder is
        for Switches ("main1.adb")
            use ("-O2");
        for Switches ("main2.adb")
            use ("-g");
     end Builder;

For the Builder package, the file names must designate source files for main subprograms. For the Binder and Linker packages, the file names must designate ALI or source files for main subprograms. In each case just the file name without an explicit extension is acceptable.

For each tool used in a program build (gnatmake, the compiler, the binder, and the linker), the corresponding package contributes a set of switches for each file on which the tool is invoked, based on the switch-related attributes defined in the package. In particular, the switches that each of these packages contributes for a given file f comprise:

• the value of attribute Switches (f), if it is specified in the package for the given file,
• otherwise, the value of Default_Switches ("Ada"), if it is specified in the package.

If neither of these attributes is defined in the package, then the package does not contribute any switches for the given file.

When gnatmake is invoked on a file, the switches comprise two sets, in the following order: those contributed for the file by the Builder package; and the switches passed on the command line.

When gnatmake invokes a tool (compiler, binder, linker) on a file, the switches passed to the tool comprise three sets, in the following order:

1. the applicable switches contributed for the file by the Builder package in the project file supplied on the command line;
2. those contributed for the file by the package (in the relevant project file – see below) corresponding to the tool; and
3. the applicable switches passed on the command line.

The term applicable switches reflects the fact that gnatmake switches may or may not be passed to individual tools, depending on the individual switch.

gnatmake may invoke the compiler on source files from different projects. The Project Manager will use the appropriate project file to determine the Compiler package for each source file being compiled. Likewise for the Binder and Linker packages.

As an example, consider the following package in a project file:

     project Proj1 is
        package Compiler is
           for Default_Switches ("Ada")
               use ("-g");
           for Switches ("a.adb")
               use ("-O1");
           for Switches ("b.adb")
               use ("-O2",
                    "-gnaty");
        end Compiler;
     end Proj1;

If gnatmake is invoked with this project file, and it needs to compile, say, the files a.adb, b.adb, and c.adb, then a.adb will be compiled with the switch -O1, b.adb with switches -O2 and -gnaty, and c.adb with -g.

The following example illustrates the ordering of the switches contributed by different packages:

     project Proj2 is
        package Builder is
           for Switches ("main.adb")
               use ("-g",
                    "-O1",
                    "-f");
        end Builder;
     
        package Compiler is
           for Switches ("main.adb")
               use ("-O2");
        end Compiler;
     end Proj2;

If you issue the command:

         gnatmake -Pproj2 -O0 main

then the compiler will be invoked on main.adb with the following sequence of switches

        -g -O1 -O2 -O0

with the last -O switch having precedence over the earlier ones; several other switches (such as -c) are added implicitly.

The switches -g and -O1 are contributed by package Builder, -O2 is contributed by the package Compiler and -O0 comes from the command line.

The -g switch will also be passed in the invocation of Gnatlink.

A final example illustrates switch contributions from packages in different project files:

     project Proj3 is
        for Source_Files use ("pack.ads", "pack.adb");
        package Compiler is
           for Default_Switches ("Ada")
               use ("-gnata");
        end Compiler;
     end Proj3;
     
     with "Proj3";
     project Proj4 is
        for Source_Files use ("foo_main.adb", "bar_main.adb");
        package Builder is
           for Switches ("foo_main.adb")
               use ("-s",
                    "-g");
        end Builder;
     end Proj4;
     
     -- Ada source file:
     with Pack;
     procedure Foo_Main is
        ...
     end Foo_Main;

If the command is

     gnatmake -PProj4 foo_main.adb -cargs -gnato

then the switches passed to the compiler for foo_main.adb are -g (contributed by the package Proj4.Builder) and -gnato (passed on the command line). When the imported package Pack is compiled, the switches used are -g from Proj4.Builder, -gnata (contributed from package Proj3.Compiler, and -gnato from the command line.

When using gnatmake with project files, some switches or arguments may be expressed as relative paths. As the working directory where compilation occurs may change, these relative paths are converted to absolute paths. For the switches found in a project file, the relative paths are relative to the project file directory, for the switches on the command line, they are relative to the directory where gnatmake is invoked. The switches for which this occurs are: -I, -A, -L, -aO, -aL, -aI, as well as all arguments that are not switches (arguments to switch -o, object files specified in package Linker or after -largs on the command line). The exception to this rule is the switch –RTS= for which a relative path argument is never converted.