Make utilities use makefile dependency lists to determine dependencies between files, for example to determine header file dependencies.

The compiler names the makefile dependency list filename.d, where filename is the name of the source file. If you specify multiple source files, a dependency file is created for each source file.

If you want to produce makefile dependencies and preprocessor source file output in a single step, you can do so using the combination --md -E (or --md -P to suppress line number generation).