将成员从struct初始化为外部函数的方向

Question

Let's say I have this

typedef struct qwerty {
    void *f;
} qwerty_t;

and I want to do this

static qwerty_t x    = {
    .f = (void *)some_external_function
};

I'm getting error: initializer element is not constant because (if I understand correctly) the direction of some_external_function is unknown when I'm compiling.

If that is not the correct explanation of the error, please explain what is.

Also, how can I initialize .f to the direction of that function?

Answer 1

From the comments to the question:

1. You don't show the declaration of some_external_function ? If, for example, it is itself a function pointer, not an actual function, you would get that error. — Jonathan Leffler

2. Indeed, you're right. I was doing exactly that. — alexandernst

I'm not sure it's definitive, but... GCC 4.8.1 on Mac OS X 10.8.4 accepts this code OK:

extern int some_external_function(int, int);

typedef struct qwerty {
    void *f;
} qwerty_t;

static qwerty_t x    = {
    .f = (void *)some_external_function
};

int main(void)
{
    int i = 1;
    int j = 2;
    int (*f)(int, int) = (int (*)(int, int))x.f;
    return f(i, j);
}

int some_external_function(int i, int j)
{
    return i + j;
}

When compiled 'extra fussy' like this, you get some warnings:

$ gcc -std=c99   -Wall -Wextra -pedantic fp.c -o fp  
fp.c:8:14: warning: ISO C forbids conversion of function pointer to object pointer type [-Wpedantic]
         .f = (void *)some_external_function
              ^
fp.c: In function ‘main’:
fp.c:15:26: warning: ISO C forbids conversion of object pointer to function pointer type [-Wpedantic]
     int (*f)(int, int) = (int (*)(int, int))x.f;
                          ^
$

The warnings are pedantically accurate — but POSIX states that the size of a function pointer must be the same as the size of a data pointer (whereas the C standard permits them to differ). Without -pedantic , there are no warnings.

Splitting the code into two files, one with definitions of x and main() and the other with the definition of some_external_function() , and compiling and linking yields no errors and no new warnings.

Answer 2

This is likely because you are defining the static keyword for the x. Basically, C requires you to associate a static storage with a constant expression or value. Even something as simple as below would give you the same error:

int main() {
   int y = 10;
   static int z = y;
}