Static functions declared in "C" header files

This is not an answer to the stated questions, but hopefully shows why one might implement a static (or static inline) function in a header file.

I can personally only think of two good reasons to declare some functions static in a header file:

  1. If the header file completely implements an interface that should only be visible in the current compilation unit

    This is extremely rare, but might be useful in e.g. an educational context, at some point during the development of some example library; or perhaps when interfacing to another programming language with minimal code.

    A developer might choose to do so if the library or interaface implementation is trivial and nearly so, and ease of use (to the developer using the header file) is more important than code size. In these cases, the declarations in the header file often use preprocessor macros, allowing the same header file to be included more than once, providing some sort of crude polymorphism in C.

    Here is a practical example: Shoot-yourself-in-the-foot playground for linear congruential pseudorandom number generators. Because the implementation is local to the compilation unit, each compilation unit will get their own copies of the PRNG. This example also shows how crude polymorphism can be implemented in C.


    #if defined(PRNG_NAME) && defined(PRNG_MULTIPLIER) && defined(PRNG_CONSTANT) && defined(PRNG_MODULUS)
    #define MERGE3_(a,b,c) a ## b ## c
    #define MERGE3(a,b,c) MERGE3_(a,b,c)
    #define NAME(name) MERGE3(PRNG_NAME, _, name)
    static uint32_t NAME(state) = 0U;
    static uint32_t NAME(next)(void)
        NAME(state) = ((uint64_t)PRNG_MULTIPLIER * (uint64_t)NAME(state) + (uint64_t)PRNG_CONSTANT) % (uint64_t)PRNG_MODULUS;
        return NAME(state);
    #undef NAME
    #undef MERGE3
    #undef PRNG_NAME
    #undef PRNG_CONSTANT
    #undef PRNG_MODULUS

    An example using the above, example-prng32.h:

    #include <stdlib.h>
    #include <stdint.h>
    #include <stdio.h>
    #define PRNG_NAME       glibc
    #define PRNG_MULTIPLIER 1103515245UL
    #define PRNG_CONSTANT   12345UL
    #define PRNG_MODULUS    2147483647UL
    #include "prng32.h"
    /* provides glibc_state and glibc_next() */
    #define PRNG_NAME       borland
    #define PRNG_MULTIPLIER 22695477UL
    #define PRNG_CONSTANT   1UL
    #define PRNG_MODULUS    2147483647UL
    #include "prng32.h"
    /* provides borland_state and borland_next() */
    int main(void)
        int i;
        glibc_state = 1U;
        printf("glibc lcg: Seed %u\n", (unsigned int)glibc_state);
        for (i = 0; i < 10; i++)
            printf("%u, ", (unsigned int)glibc_next());
        printf("%u\n", (unsigned int)glibc_next());
        borland_state = 1U;
        printf("Borland lcg: Seed %u\n", (unsigned int)borland_state);
        for (i = 0; i < 10; i++)
            printf("%u, ", (unsigned int)borland_next());
        printf("%u\n", (unsigned int)borland_next());
        return EXIT_SUCCESS;

    The reason for marking both the _state variable and the _next() function static is that this way each compilation unit that includes the header file has their own copy of the variables and the functions -- here, their own copy of the PRNG. Each must be separately seeded, of course; and if seeded to the same value, will yield the same sequence.

    One should generally shy away from such polymorphism attempts in C, because it leads to complicated preprocessor macro shenanigans, making the implementation much harder to understand, maintain, and modify than necessary.

    However, when exploring the parameter space of some algorithm -- like here, the types of 32-bit linear congruential generators, this lets us use a single implementation for each of the generators we examine, ensuring there are no implementation differences between them. Note that even this case is more like a development tool, and not something you ought to see in a implementation provided for others to use.

  1. If the header implements simple static inline accessor functions

    Preprocessor macros are commonly used to simplify code accessing complicated structure types. static inline functions are similar, except that they also provide type checking at compile time, and can refer to their parameters several times (with macros, that is problematic).

    One practical use case is a simple interface for reading files using low-level POSIX.1 I/O (using <unistd.h> and <fcntl.h> instead of <stdio.h>). I've done this myself when reading very large (dozens of megabytes to gigabytes range) text files containing real numbers (with a custom float/double parser), as the GNU C standard I/O is not particularly fast.

    For example, inbuffer.h:

    #ifndef   INBUFFER_H
    #define   INBUFFER_H
    typedef struct {
        unsigned char  *head;       /* Next buffered byte */
        unsigned char  *tail;       /* Next byte to be buffered */
        unsigned char  *ends;       /* data + size */
        unsigned char  *data;
        size_t          size;
        int             descriptor;
        unsigned int    status;     /* Bit mask */
    } inbuffer;
    #define INBUFFER_INIT { NULL, NULL, NULL, NULL, 0, -1, 0 }
    int inbuffer_open(inbuffer *, const char *);
    int inbuffer_close(inbuffer *);
    int inbuffer_skip_slow(inbuffer *, const size_t);
    int inbuffer_getc_slow(inbuffer *);
    static inline int inbuffer_skip(inbuffer *ib, const size_t n)
        if (ib->head + n <= ib->tail) {
            ib->head += n;
            return 0;
        } else
            return inbuffer_skip_slow(ib, n);
    static inline int inbuffer_getc(inbuffer *ib)
        if (ib->head < ib->tail)
            return *(ib->head++);
            return inbuffer_getc_slow(ib);
    #endif /* INBUFFER_H */

    Note that the above inbuffer_skip() and inbuffer_getc() do not check if ib is non-NULL; this is typical for such functions. These accessor functions are assumed to be "in the fast path", i.e. called very often. In such cases, even the function call overhead matters (and is avoided with static inline functions, since they are duplicated in the code at the call site).

    Trivial accessor functions, like the above inbuffer_skip() and inbuffer_getc(), may also let the compiler avoid the register moves involved in function calls, because functions expect their parameters to be located in specific registers or on the stack, whereas inlined functions can be adapted (wrt. register use) to the code surrounding the inlined function.

    Personally, I do recommend writing a couple of test programs using the non-inlined functions first, and compare the performance and results to the inlined versions. Comparing the results ensure the inlined versions do not have bugs (off by one type is common here!), and comparing the performance and generated binaries (size, at least) tells you whether inlining is worth it in general.

First I'd like to clarify my understanding of the situation you describe: The header contains (only) a static function declaration while the C file contains the definition, i.e. the function's source code. For example


static void f();
// potentially more declarations


#include "some.h"
static void f() { printf("Hello world\n"); }
// more code, some of it potentially using f()

If this is the situation you describe, I take issue with your remark

Since static functions have internal linkage we need to define it in every file we include the header file where the function is declared.

If you declare the function but do not use it in a given translation unit, I don't think you have to define it. gcc accepts that with a warning; the standard does not seem to forbid it, unless I missed something. This may be important in your scenario because translation units which do not use the function but include the header with its declaration don't have to provide an unused definition.

Now let's examine the questions:
  • What is the problem of declaring static functions in header files?
    It is somewhat unusual. Typically, static functions are functions needed in only one file. They are declared static to make that explicit by limiting their visibility. Declaring them in a header therefore is somewhat antithetical. If the function is indeed used in multiple files with identical definitions it should be made external, with a single definition. If only one translation unit actually uses it, the declaration does not belong in a header.

    One possible scenario therefore is to ensure a uniform function signature for different implementations in the respective translation units. The common header leads to a compile time error for different return types in C (and C++); different parameter types would cause a compile time error only in C (but not in C++' because of function overloading).
  • What are the risks?
    I do not see risks in your scenario. (As opposed to also including the function definition in a header which may violate the encapsulation principle.)
  • What the impact in compilation time?
    A function declaration is small and its complexity is low, so the overhead of having additional function declarations in a header is likely negligible. But if you create and include an additional header for the declaration in many translation units the file handling overhead can be significant (i.e. the compiler idles a lot while it waits for the header I/O)
  • Is there any risk in runtime?
    I cannot see any.

Why would you want a both global and static function? In c, functions are global by default. You only use static functions if you want to limit the access to a function to the file they are declared. So you actively restrict access by declaring it static...

The only requirement for implementations in the header file, is for c++ template functions and template class member functions.