Issues regarding mutexes on POSIX threads - linux

Question

I'm having some issues with the following code. I just can't seem to find the bug:
1 #include <pthread.h>
2 #include <stdio.h>
3 #include <stdlib.h>
4 #include <unistd.h>
5
6 struct bla
7 {
8 pthread_mutex_t mut;
9 int x;
10 };
11
12
13 pthread_t tid;
14
15 void *thr_fn1 (struct bla *fp);
16
17 int main()
18 {
19 struct bla *fp;
20 fp = (struct bla *) malloc (sizeof(struct bla));
21 fp->x=3;
22 printf ("Initializing mutex_init\n");
23 pthread_mutex_init (&fp->mut,NULL);
24 pthread_create (&tid,NULL,thr_fn1,fp);
25 sleep (2);
26 printf ("Main thread ended sleep. Incrementing.\n");
27 fp->x++;
28 pthread_join (tid,NULL);
29 printf ("x=%d\n",fp->x);
30 pthread_mutex_destroy(&fp->mut);
31 return 0;
32
33 }
34
35 void *thr_fn1 (struct bla *fp)
36 {
37 printf ("Locking new thread!\n");
38 pthread_mutex_lock (&fp->mut);
39 printf ("Sleeping.\n");
40 sleep (5);
41 pthread_mutex_unlock (&fp->mut);
42 printf ("Thread unlocked.\n");
43 return ((void *) 0);
44 }
Why does the value still get incremented at line 27? Shouldn't it be protected by the mutex in the second thread by the lock (line 38)?
Thanks!

Answer 1

There is no automatic association between mutexes and data. If you want a mutex to protect some particular set of data, you are responsible for locking and unlocking the mutex around accesses to that data:
sleep (2);
pthread_mutex_lock(&fp->mut);
printf ("Main thread ended sleep. Incrementing.\n");
fp->x++;
pthread_mutex_unlock(&fp->mut);