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gcc -M

Show dependencies of compilation units, including system header files. The printed output of dependencies is suitable to be copy-pasted into a Makefile.
In order to not include system header files, use gcc -MM.

Demonstration - Source files

The output of gcc -M is demonstrated with the following three trivial compilation units and corresponding header files:
main.c: 
#include "hello-world.h"

int main() {

  hello_world();

}
Github repository about-gcc, path: /options/M/main.c
hello-world.c: 
#include "hello-world.h"

void hello_world() {
  print_line("Hello world");
}
Github repository about-gcc, path: /options/M/hello-world.c
hello-world.h: 
#ifndef HELLO_WORLD
#define HELLO_WORLD

#include "print-line.h"

void hello_world();

#endif
Github repository about-gcc, path: /options/M/hello-world.h
print-line.c: 
#include "print-line.h"

void print_line(const char* line) {
  printf("%s\n", line);
}
Github repository about-gcc, path: /options/M/print-line.c
print-line.h: 
#ifndef PRINT_LINE_H
#define PRINT_LINE_H

#include <stdio.h>

void print_line(const char* line);

#endif
Github repository about-gcc, path: /options/M/print-line.h

Demonstration - output

gcc -M main.c prints 
main.o: main.c /usr/include/stdc-predef.h hello-world.h print-line.h \
 /usr/include/stdio.h /usr/include/bits/libc-header-start.h \
 /usr/include/features.h /usr/include/sys/cdefs.h \
 /usr/include/bits/wordsize.h /usr/include/bits/long-double.h \
 /usr/include/gnu/stubs.h /usr/include/gnu/stubs-64.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stddef.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stdarg.h \
 /usr/include/bits/types.h /usr/include/bits/typesizes.h \
 /usr/include/bits/types/__fpos_t.h /usr/include/bits/types/__mbstate_t.h \
 /usr/include/bits/types/__fpos64_t.h /usr/include/bits/types/__FILE.h \
 /usr/include/bits/types/FILE.h /usr/include/bits/types/struct_FILE.h \
 /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h
gcc -M hello-world.c prints 
hello-world.o: hello-world.c /usr/include/stdc-predef.h hello-world.h \
 print-line.h /usr/include/stdio.h /usr/include/bits/libc-header-start.h \
 /usr/include/features.h /usr/include/sys/cdefs.h \
 /usr/include/bits/wordsize.h /usr/include/bits/long-double.h \
 /usr/include/gnu/stubs.h /usr/include/gnu/stubs-64.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stddef.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stdarg.h \
 /usr/include/bits/types.h /usr/include/bits/typesizes.h \
 /usr/include/bits/types/__fpos_t.h /usr/include/bits/types/__mbstate_t.h \
 /usr/include/bits/types/__fpos64_t.h /usr/include/bits/types/__FILE.h \
 /usr/include/bits/types/FILE.h /usr/include/bits/types/struct_FILE.h \
 /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h
gcc -M print-line.c prints 
print-line.o: print-line.c /usr/include/stdc-predef.h print-line.h \
 /usr/include/stdio.h /usr/include/bits/libc-header-start.h \
 /usr/include/features.h /usr/include/sys/cdefs.h \
 /usr/include/bits/wordsize.h /usr/include/bits/long-double.h \
 /usr/include/gnu/stubs.h /usr/include/gnu/stubs-64.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stddef.h \
 /usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/include/stdarg.h \
 /usr/include/bits/types.h /usr/include/bits/typesizes.h \
 /usr/include/bits/types/__fpos_t.h /usr/include/bits/types/__mbstate_t.h \
 /usr/include/bits/types/__fpos64_t.h /usr/include/bits/types/__FILE.h \
 /usr/include/bits/types/FILE.h /usr/include/bits/types/struct_FILE.h \
 /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h

See also

GCC options

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