These options drive how GCC emits Makefile-style dependency rules. They’re useful for incremental builds that only recompile what chenged.

Example source

Create a tiny program to reference in the examples:

// deps.c
#include <stdio.h>

int main(void) {
    int value = 21 * 2;
    printf("%d\n", value);
    return 0;
}

-M

Emit dependency information including system headers to stdout. No object files or executables are produced.

$ gcc -M deps.c
deps.o: deps.c /usr/include/stdio.h \
  /usr/include/features.h \
  /usr/include/x86_64-linux-gnu/bits/libc-header-start.h \
  /usr/lib/gcc/x86_64-linux-gnu/12/include/stddef.h \
  ...

-MM

Like -M, but omit system headers from the dependency list.

$ gcc -MM deps.c
deps.o: deps.c

-MMD

Compile as usual and write dependencies (excluding system headers) to a .d file named after the output. If you don’t specify -o and link, GCC still builds the default executable (a.out) and writes the .d file.

$ gcc -MMD deps.c -o app
$ ls
app  deps.c  deps.d

$ cat deps.d
deps.o: deps.c

$ rm -f app deps.d
$ gcc -MMD deps.c
$ ls
a.out  deps.c  deps.d
$ ./a.out
42

-MD

Same as -MMD but includes system headers in the dependency list. The target in the .d file matches the final output name: if you link to an executable with -o, that name is used; otherwise the object name is used.

$ gcc -MD deps.c -o app
$ cat deps.d
app: deps.c /usr/include/stdio.h \
  /usr/include/features.h \
  ...

$ rm -f app deps.d
$ gcc -MD deps.c
$ ls
a.out  deps.c  deps.d
$ cat deps.d
deps.o: deps.c /usr/include/stdio.h \
  /usr/include/features.h \
  ...

-MF

Select the file to receive dependency rules. With -M or -MM it redirects stdout to the file; with -MD/-MMD it overrides the default .d path.

• Using -MF without -M/-MM/-MD/-MMD is an error:

$ gcc -MF deps.mk deps.c -o app
cc1: error: to generate dependencies you must specify either -M or -MM

• Proper use with -MM:

$ gcc -MM -MF deps.mk deps.c
$ cat deps.mk
deps.o: deps.c

• With -MMD (and compiling only):

$ gcc -MMD -MF deps.mk -c deps.c -o deps.o
$ ls
deps.c  deps.mk  deps.o

Note: -M or -MM suppress compilation/linking. If you also pass -o, GCC may create an empty file at that path because no code is produced.

$ gcc -MM -MF deps.mk deps.c -o placeholder
$ ls -l placeholder
-rw-r--r-- 1 user user 0 ... placeholder

-MT

Ovreride the target name(s) written in the dependency rule. You can supply -MT multiple times too emit multiple targets for the same prerequisite list.

• Requires -M/-MM or -MD/-MMD:

$ gcc deps.c -MT out.o
cc1: error: to generate dependencies you must specify either -M or -MM

• Basic use:

$ gcc -MM -MT mytarget deps.c
mytarget: deps.c

• Multiple targets and writing to a file:

$ gcc -MM -MF deps.mk -MT deps.d -MT deps.o deps.c
$ cat deps.mk
deps.d deps.o: deps.c