How to link multiple implementation files in C
c, compilation, header, implementation
Solution
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
Problem
I have a number of `.c` files, i.e. the implementation files say - main.c - A.c - B.c Where functions from any of the files can call any function from a different files. My question being, do I need a `.h` i.e. header file for each of A and B's implementation where each header file has the definition of ALL the functions in A or B. Also, main.c will have both `A.h` and `B.h #included` in it? If someone can finally make it clear, also, how do I later compile and run the multiple files in the terminal. Thanks.