pthread_create error

Question

I have

    client_sock = accept(server_sock, (struct sockaddr *)&client_name, &client_name_len);        
    if (pthread_create(&newthread , NULL, (void * ) accept_request, client_sock) != 0) {
            perror("pthread_create");
    }

That's just part of the entire script. Every time I try to compile it, I get warning: passing argument 4 of 'pthread_create' makes pointer from integer without a cast

Any idea why this is happening? Thanks in advance.

Answer 1

Argument 4 is client_sock, which is the argument that gets passed to accept_request. It is expected to be a pointer, but since it's just passed through to your function, it can be an integer without doing any harm. Just cast it to void* to remove the warning.

On the other hand, the third argument accept_request should be a function pointer accepting void* and returning void*. You shouldn't have to cast it to void*. It would be best to change the declaration of accept_request to match the specification.

void *accept_request( void *client_sock );

Answer 2

I assume client_sock is defined as "int client_sock". In this case you should write the following:

if (pthread_create(&newthread , NULL, (void * ) accept_request, &client_sock) != 0) {

Then in accept_request (which, btw, should be a function taking a pointer) you will do like:

void *accept_request( void *client_sock_addr ) {
   int client_sock = *((int*) client_sock_addr);
}

Converting an int to an (void*) might not be portable (as data sizes of int and void* are not necessarily the same). Of course, it might work on your current compiler...

Thanks jiles for pointing this: Depending on the rest of the code, the value at that address might change (for example if you have a loop around the accept construct and 2 accepts arrive before one thread is created). The nicest way to do it is indeed allocate memory with malloc like

int *client_sock_addr=malloc(sizeof(int)); 
*client_sock_addr = accept(server_sock, (struct sockaddr *)&client_name, &client_name_len);        
if (pthread_create(&newthread , NULL, (void * ) accept_request, client_sock_addr) != 0) {
        perror("pthread_create");
}

And then in the function do:

void *accept_request( void *param ) {
   int *client_sock_addr = (int*) client_sock_addr;
   int client_sock = *client_sock_addr;

   // Before exiting the thread
   free(client_sock_addr);
}

Answer 3

This is simply because you are not respecting the signature of pthread_create:

int pthread_create(pthread_t *restrict thread,
              const pthread_attr_t *restrict attr,
              void *(*start_routine)(void*), void *restrict arg);

As you can see the third argument should be a function pointer so the correct cast would be to (void *(*)(void*)) and for client_sock which will be the argument to start_routine you have to take the address like that &client_sock