pthread_mutex_lock works only with sleep - c

Question

I pass a struct in pthread_create which contains a char* and I lock the main and the thread with mutexes so I can protect this string because when the second thread will be created the string will change and the first thread will use the second string and not the first. Here is the code:
main.c
while( th_num < th_size )
{
pthread_mutex_lock(&lock);
received = 0;
/* Read the desired readable size */
if( read(newsock, &size, sizeof(size)) < 0 )
{ perror("Read"); exit(1); }
/* Read all data */
while( received < size )
{
if( (nread = read(newsock, buffer + received, size - received)) < 0 )
{ perror("Read"); exit(1); }
received += nread;
}
printf("Received string: %s\n",buffer);
Q->receiver = (char*) malloc(sizeof(char)*strlen(buffer)+1);
strncpy(Q->receiver, buffer, strlen(buffer)+1);
if( (err = pthread_create(&thread_server[th_num], NULL, thread_start, (void*) Q)) == true )
{ show_error("pthread_create", err); }
/* -------------------------------------------------- */
th_num++;
pthread_mutex_unlock(&lock);
usleep(500);
}
pthread_server.c
pthread_mutex_lock(&lock);
/*
do some stuff here
*/
pthread_mutex_unlock(&lock);
The program works fine but the problem is that it only works if I put usleep(500). My guess is that the thread cant lock the mutex in time so it needs sleep to do this right. Is there a way to do it without usleep()?

Answer 1

Assuming I don't understand why you need to call pthread_create(); in a mutual exclusion portion of code, your problems is:
you use threads but the flow of your program is approaching to be sequential because of the large mutual exclusion portion of code.
Let X a generic thread in your program.
Without the usleep(500); when the X thread finish it releases the mutex with pthread_mutex_unlock(&lock); but afterwards the thread X reacquires the lock so no one else can access in the mutual exclusion portion of code.
Now I don't know what your shared data is, so I can only suggest you:
1) Reduce the mutual exclusion portion of code, only use it when you access to a shared data;
2) Rethink about your program structure.