Question

I have 4 files:

main.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "main.h"


int main() {
    struct Fun *fun = (struct Fun*)malloc(sizeof(struct Fun));
    fun->a = 2;
    fun->b = 12;
    fun->Func = Plus();
    int result = fun->Func(fun, 8);
    printf("%d\n", result);
    return 0; }

main.h

    #ifndef MAN_H_
    #define MAN_H_

    struct Fun {
        int   a;
        int   b;
        int (*Func)(struct Fun *x,int y);

    };

header.c

    #include "header.h"

    int Plus(struct Fun *x, int y) {
        return x->a * x->b + y; };

header.h

    #ifndef HEADER_H_
    #define HEADER_H_

    #include "man.h"

    #endif /* HEADER_H_ */

when I build, I get a warning:

../main.c:12:5: warning: implicit declaration of function ‘Plus’ [-Wimplicit-function-declaration]
../main.c:12:15: warning: assignment makes pointer from integer without a cast [enabled by default]

if I run, it has no result.

But when I put all the code to main.c and edit fun->Func = Plus(); to fun->Func = Plus; it works fine: no warning, and the result is 32.

Answer 1

You should provide a function signature for Plus in header.h, and also add an #include "header.h" in main.c, so that something is known about the Plus function when it’s used in main.c.

In header.h:

int Plus(struct Fun *, int);

Without such a signature, when compiling main.c the compiler makes assumptions about the function: an implicit declaration. This implicit declaration won’t match the actual function definition.

Also, changing fun->Func = Plus(); to fun->Func = Plus; is still necessary: the first form assigns to your function pointer the result of an attempted function call, the second form is the correct way to assign a function pointer.

Finally, this is likely just a cut/paste omission, but main.h is missing an #endif.