Pthreads: Main overwrites mutex lock

Question

I am fairly new in threads programming and I wanted to test the mutex functionality. So I programmed the following in order to test it.

**thread_test.h

...
extern int flags;
extern pthread_mutex my_mutex;
...

**thread_test.c

...
#include"thread_test.h"
...    
void * thread_test(void *thread_parameters)
{
long tid = (long) thread_parameters;

pthread_mutex_lock(&my_mutex);
++flags;
printf("**THREAD %d** started. Flag value is %d.\n",tid, flags);
sleep(6);
pthread_mutex_unlock(&my_mutex);

pthread_exit(NULL);
}
...

**main.c

...
#include"thread_test.h"
...
#define THREADS 5
pthread_t threads[THREADS];
pthread_mutex_t my_mutex;
int     flags = 0;
...
int main(){
int rct;
for(rct = 0; rct<THREADS; rct++)
if(pthread_create(&threads[rct],NULL, thread_test, (void *)rct))
   printf("ERROR!")
else
   {
   sleep(1);
   printf("Thread %d initialised in main and the flags value is %d.\n", rct,flags);
   }

pthread_mutex_destroy(&my_mutex);
...

It appears that even though I lock the mutex in the child threads, the main program somehow overwrites the mutex lock while a thread has it and assigns variable flags with a new value..

Does anyone have an idea why this is happening?

Answer 1

From what I see you have several errors in your code, part of which your compiler should have told you if you switch all warnings on.

• pthread_mutex_t variables must be initialized. For static initialization using = PTHREAD_MUTEX_INITIALIZER at the point of defininition would be enough. (And there is not much point in destroying a static mutex on the other end.)

• in the code snippet you gave there is no declaration of thread_test visible to main

• you exit main (and destroy the mutex) before the threads have terminated. You may do this, but then you'd have to use an explit pthread_exit in main (and definitively don't do the destroy then). The commonly used approach is not to do this, but to use pthread_join for all threads that have been created.

Also, you could indent your code before posting here, that would much help to make it more readable.