相关代码

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

void *print_message_function( void *ptr );

main()
{
     pthread_t thread1, thread2;
     const char *message1 = "Thread 1";
     const char *message2 = "Thread 2";
     int  iret1, iret2;

     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
     if(iret1)
     {
         fprintf(stderr,"Error - pthread_create() return code: %d\n",iret1);
         exit(EXIT_FAILURE);
     }

     iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
     if(iret2)
     {
         fprintf(stderr,"Error - pthread_create() return code: %d\n",iret2);
         exit(EXIT_FAILURE);
     }

     printf("pthread_create() for thread 1 returns: %d\n",iret1);
     printf("pthread_create() for thread 2 returns: %d\n",iret2);

     pthread_join( thread1, NULL);
     pthread_join( thread2, NULL); 

     exit(EXIT_SUCCESS);
}

void *print_message_function( void *ptr )
{
     char *message;
     message = (char *) ptr;
     printf("%s \n", message);
}

疑惑地方

在函数 pthread_create(pthread_t * thread, const pthread_attr_t * attr,void * (*start_routine)(void *), void *arg);
其中 void * (*start_routine)(void *) 这个参数是 "函数指针"
而根据函数 void *print_message_function( void *ptr );原型来看这是一个 返回任何指针类型的函数,不是函数指针，而且这个函数的实现中并没有返回值. 但是可以使用gcc编译通过，没有任何异常, 有哪位知道这是为什么吗？ 或者给出一个参考链接？

我所知道的类似void *print_message_function( void *ptr ); 函数的用法是这样的

 #include<stdio.h>

 void *test_f(int *n);
 
 int main(void) 
{
         int test_num = 32;
         int *test_p = &test_num;
         int *ret_v = test_f(test_p);
         printf("%d \n",*ret_v);
         return 0;
 }

void *test_f(int *n) 
{
         *n = *n + 1;
         return n;
}