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Question

Many C++ projects (eg many Boost libraries) are "header-only linked".

Is this possible also in plain C ? How to put the source code into headers? Are there any sites about it?

Answer 1

Executive summary: You can, but you shouldn't.

C and C++ code is preprocessed before it's compiled: all headers are "pasted" into the source files that include them, recursively. If you define a function in a header and it is included by two C files, you will end up with two copies in each object file ( One Definition Rule violation).

You can create "header-only" C libraries if all your functions are marked as static , that is, not visible outside the translation unit. But it also means you will get a copy of all the static functions in each translation unit that includes the header file.

It is a bit different in C++: inline functions are not static , symbols emitted by the compiler are still visible by the linker, but the linker can discard duplicates, rather than giving up ("weak" symbols).

It's not idiomatic to write C code in the headers, unless it's based on macros (eg queue(3) ). In C++, the main reason to keep code in the headers are templates, which may result in code instantiation for different template parameters, which is not applicable to C.

Answer 2

You do not link headers.

In C++ it's slightly easier to write code that's already better-off in headers than in separately-compiled modules because templates require it. ¹

But you can also use the inline keyword for functions, which exists in C as well as C++. ²

// Won't cause redefinition link errors, because of 6.7.4/5
inline void foo(void) {
   // ...
}

[c99: 6.7.4/5:] A function declared with an inline function specifier is an inline function . The function specifier may appear more than once; the behavior is the same as if it appeared only once. Making a function an inline function suggests that calls to the function be as fast as possible. The extent to which such suggestions are effective is implementation-defined.

You're a bit stuck when it comes to data objects, though.

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¹ - Sort of.
² - C99 for sure. C89/C90 I'd have to check.

Answer 3

Boost makes heavy use templates and template meta-programming which you cannot emulate (all that easily) in C.

But you can of course cheat by having declarations and code in C headers which you #include but that is not the same thing. I'd say "When in Rome..." and program C as per C conventions with libraries.

Answer 4

Yes, it is quite possible. Declare all functions in headers and either all as static or just use a single compilation unit (ie only a single c file) in your projects.

As a personal anecdote, I know quite a number of physicists who insist that this technique is the only true way to program C. It is beneficial because it's the poor man's version of -fwhole-program , ie makes optimizations based on the knowledge of function behaviour possible. It is practical because you don't need to learn about using the linker flags. It is a bad idea because your whole program must be compiled as a whole and recompiled with each minor change.

Personally, I'd recommend to let it be or at least go with static for only a few functions.