How to link multiple implementation files in C

Header contents

The header A.h for A.c should only contain the information that is necessary for external code that uses the facilities defined in A.c. It should not declare static functions; it should not declare static variables; it should not declare internal types (types used only in A.c). It should ensure that a file can use just #include "A.h" and then make full use of the facilities published by A.c. It should be self-contained, idempotent (so you can include it twice without any compilation errors) and minimal. You can simply check that the header is self-contained by writing #include "A.h" as the first #include line in A.c; you can check that it is idempotent by including it twice (but that's better done as a separate test). If it doesn't compile, it is not self-contained. Similarly for B.h and B.c.

For more information on headers and standards, see 'Should I use #include in headers?', which references a NASA coding standard, and 'Linking against a static library', which includes a script chkhdr that I use for testing self-containment and idempotency.

Linking

Note that main.o depends on main.c, A.h and B.h, but main.c itself does not depend on the headers.

When it comes to compilation, you can use:

gcc -o program main.c A.c B.c

If you need other options, add them (most flags at the start; libraries at the end, after the source code). You can also compile each file to object code separately and then link the object files together:

gcc -c main.c
gcc -c A.c
gcc -c B.c
gcc -o program main.o A.o B.o

You must provide an header file just if what is declared in a .c file is required in another .c file.

Generally speaking you can have a header file for every source file in which you export all the functions declared or extern symbols.

In practice you won't alway need to export every function or every variable, just the one that are required by another source file, and you will need to include it just in the required file (and in the source paired with the specific header file).

When trying to understand how it works just think about the fact that every source file is compiled on its own, so if it's going to use something that is not declared directly in its source file, then it must be declared through an header file. In this way the compiler can know that everything exists and it is correctly typed.