C language callback function

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

#include <curl/curl.h>
#include <pthread.h>


#ifdef WIN32
#  include <io.h>
#  define READ_3RD_ARG unsigned int
#else
#  include <unistd.h>
#  define READ_3RD_ARG size_t
#endif

#if LIBCURL_VERSION_NUM < 0x070c03
#error "upgrade your libcurl to no less than 7.12.3"
#endif

void* cb(void *arg)
{
    printf("call back function, pid=%ld\n", pthread_self());
    return NULL;
}

void fun1(void)
{
    printf("fun1 call\n");

    return;
}

void fun2(int a)
{
    printf("fun2 call a=%d\n", a);

    return;
}

void fun3(int a, double b)
{
    printf("fun3 call a=%d,b=%lf\n", a, b);
    return;
}

int fun4(void)
{
    printf("fun4 call back\n");
    return 666;
}

int* fun5(int a)
{
    int* ptr = NULL;
    ptr = &a;
    printf("fun5 call bacl a=%d\n", a);
    return ptr;
}

int main(void)
{
    pthread_t therad1;
    pthread_create(&therad1, NULL, cb, NULL);
    pthread_join(therad1, NULL);

    pthread_t thread2;
    pthread_create(&thread2, NULL, cb, NULL);
    pthread_join(thread2, NULL);

    void (*ptr)(void);              // ptr Is a function pointer with no return value and no parameters 
    ptr = fun1;
    ptr();
    (*ptr)();

    void (*ptr2)(int);              // ptr2 Is a function pointer with no return and one parameter 
    ptr2 = fun2;
    ptr2(666);

    void (*ptr3)(int, double);
    ptr3 = fun3;
    ptr3(12, 234.321);

    int (*ptr4)(void);
    ptr4 = fun4;
    int rtn = ptr4();
    printf("rtn=%d\n", rtn);

    int* (*ptr5)(int);
    ptr5 = fun5;
    rtn = *ptr5(888);               // ptr5 Express int* Function pointer of type , Assign to a int Type variables need to be dereferenced before they can be used 
    printf("rtn=%d\n", rtn);

    return 0;
}