A mapping file is a sequence of sets of three lines. In each set, the first line is the unit name, in lower case, with "%s" appended for specifications and "%b" appended for bodies; the second line is the file name; and the third line is the path name.
main%b main.2.ada /gnat/project1/sources/main.2.ada
When the switch -gnatem is specified, the compiler will create in memory the two mappings from the specified file. If there is any problem (non existent file, truncated file or duplicate entries), no mapping will be created.
Several -gnatem switches may be specified; however, only the last one on the command line will be taken into account.
When using a project file,
gnatmake create a temporary mapping file
and communicates it to the compiler using this switch.