void * function pointer - c

Question

I have made a typedef for a function pointer that takes in an integer and returns a void *:
typedef void* (*fp)(int index);
I then made a struct that contains a fp and another struct of the same type:
typedef struct fp_holder {
fp function_pointer;
iterable *next;
} fp_holder;
I am trying to figure out how to call fp inside a fp_holder.
To test this out, I did the following:
void *test_fp(int index) {
if (index == 0) {
printf('H');
fflush(stdout);
return [something_that_works];
}
else if (index == 1) {
printf('e');
fflush(stdout);
return [something_that_works];
}
else if (index == 2) {
printf('l');
fflush(stdout);
return [something_that_works];
}
else if (index == 3) {
printf('l');
fflush(stdout);
return [something_that_works];
}
else if (index == 4) {
printf('o');
fflush(stdout);
return [something_that_works];
}
else {
return (void *) NULL;
}
}
fp_holder *a = (fp_holder *) malloc(sizeof(fp_holder));
a->function_pointer = test_fp;
a->next = NULL;
So with all that setup, I tried to call a's function_pointer by trying the following:
a->function_pointer(0);
(*a->function_pointer)(0);
((*)a->function_pointer)(0);
I just cannot figure out why those are not working.
Help would be appreciated! :)
EDIT
What I am trying to do:
Call a's function_pointer with an argument.
I will try out some answers right now and see what happens.
EDIT2
Answered! I was calling it right by doing a->function_pointer(0) but what was giving me a segmentation error [which is what my issue was - and maybe I should have clarified this] was the printf statement and NOT my call. printf needs a string not the char as I put in.

Answer 1

Does your original code actually do
printf('H');
instead of
printf("H");
?
Simplified version of the code you posted, with correct arguments to printf:
#include <stdio.h>
typedef void* (*function_pointer_t)(int index);
struct function_holder {
function_pointer_t callback;
};
void* testFn(int i)
{
printf("testFn %d\n", i);
}
int main(void) {
struct function_holder fh = { testFn };
struct function_holder* fhp = &fh;
fh.callback = testFn;
fh.callback(1);
fhp->callback(2);
return 0;
}
works as expected: http://ideone.com/1syLlG

Answer 2

your code has couple of errors. I am not sure what you are trying to achieve by this code, however here is your working code.
#include "stdio.h"
#include "string.h"
#include "stdlib.h"
typedef void* (*fp)(int index);
typedef struct fp_holder {
fp function_pointer;
struct fp_holder *next;
} fp_holder;
void *test_fp(int index) {
if (index == 0) {
printf("H");
fflush(stdout);
return "H";
}
else if (index == 1) {
printf("e");
fflush(stdout);
return "e";
}
else if (index == 2) {
printf("l");
fflush(stdout);
return "l";
}
else if (index == 3) {
printf("l");
fflush(stdout);
return "l";
}
else if (index == 4) {
printf("o");
fflush(stdout);
return "o";
}
else {
return (void *) NULL; }
}
int main() {
fp_holder *a = (fp_holder *) malloc(sizeof(fp_holder));
a->function_pointer = test_fp;
a->next = NULL;
a->function_pointer(0);
}