The compiler will not know that the function int fun() exists. So we have to define the function prototype of int fun();
To overcome this error, see the below program

#include<stdio.h>
int fun();

int main()
{
    int (*p)() = fun;
    (*p)();
    return 0;
}
int fun()
{
    printf("Pak-mcqs.net\n");
    return 0;
}