C: pointer to a function in Struct

Question

I am practicing to build a struct data structure in C, and the struct has a field pointer to function. I have written up the following code, but when I tried to compile with gcc, I got the following error:

In function ‘main’: custhreadsTest1.c:27:16: warning: assignment from incompatible pointer type [enabled by default]

Whats the correct way of assigning a function pointer to the pointer field in my struct? Thanks a lot!

#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>

typedef struct
{
  ucontext_t (*gt_context);
  void (*function)(void *);
  int status_flag;
} custhread_t;

void *add(void *)
{
//  do something here
}

int main(void)
{
  ucontext_t ctx_main;
  getcontext(&ctx_main);

  custhread_t th1;
  th1.gt_context = &ctx_main;
  th1.function = &add;
  th1.status_flag = 1;

  return 0;

}

Answer 1

You're assigning it using the correct syntax (the & is optional), but the type of function pointer you are assigning is incompatible with the function pointer type in the struct (your add function takes void *, but your function pointer in the struct doesn't take any arguments).

Your add function should simply be:

void add(void)
{
    // do something
}

Alternatively, if your add function does take a pointer to void as an argument, then:

typedef struct
{
    ucontext_t (*gt_context);
    // pointer to a function that takes a void pointer, no return value
    void (*function)(void *); 
    int status_flag;
} custhread_t;


void add(void *somePointer)
{
    // do something with somePointer
}

And so therefore, if your add function takes and returns a void pointer, then:

typedef struct
{
    ucontext_t (*gt_context);
    // pointer to a function that takes and returns a void pointer
    void *(*function)(void *);
    int status_flag;
} custhread_t;


void *add(void *somePointer)
{
    // do something with somePointer
    // return a void pointer
}

Answer 2

You're doing it right. Just make sure the function pointer types match. For example, remove the '*' from the struct so that the argument is just void rather than a void pointer. Like this.

typedef struct
{
    ucontext_t (*gt_context);
    void (*function)(void);
    int status_flag;
} custhread_t;