header - How to structure #includes in C

Question

Say I have a C program which is broken to a set of *.c and *.h files. If code from one file uses functions from another file, where should I include the header file? Inside the *.c file that used the function, or inside the header of that file?

E.g. file foo.c includes foo.h, which contains all declarations for foo.c; same for bar.c and bar.h. Function foo1() inside foo.c calls bar1(), which is declared in bar.h and defined in bar.c. Now the question is, should I include bar.h inside foo.h, or inside foo.c?

What would be a good set of rules-of-thumb for such issues?

Answer 1

You should include foo.h inside foo.c. This way other c files that include foo.h won't carry bar.h unnecessarily. This is my advice for including header files:

• Add include definitions in the c files - this way the file dependencies are more obvious when reading the code.
• Split the foo.h in two separate files, say foo_int.h and foo.h. The first one carries the type and forward declarations needed only by foo.c. The foo.h includes the functions and types needed by external modules. This is something like the private and public section of foo.
• Avoid cross references, i.e. foo references bar and bar references foo. This may cause linking problems and is also a sign of bad design