Single Header Libs

C/C++

Last updated on 2019-10-20

Single file header and impl libs have become popular, from a users point of view they tend to look like this.

/* some src.c file */

#define SOME_LIB_INCLUDE_IMPL
#include "some_lib.h"

#include <stdio.h>

int main() {
        printf("%d\n", some_lib_function());

        return 0;
}

From the point of view of the library maintainer the code is structured something like this (more or less).

/* some_lib.h */

/* interface */

#ifndef USUAL_INC_GUARD
#define USUAL_INC_GUARD

int
some_lib_function();

#endif

/* impl */

#ifdef SOME_LIB_INCLUDE_IMPL
#ifndef SOME_LIB_IMPL_GUARD
#define SOME_LIB_IMPL_GUARD

int
some_lib_function() {
        return 42;
}

#endif
#endif

So now in one of your source files you define SOME_LIB_INCLUDE_IMPL before including the library header.

Compile and run,

gcc src.c && ./a.out

outputs …

This can make distributing small libs nice. Its a single file, and can have its source compiled into a source file. Which is preferable than inline functions or (shudder) C++ templates.

Some cool examples …