Mutex in shared memory when one user crashes?

Question

Suppose that a process is creating a mutex in shared memory and locking it and dumps core while the mutex is locked.

Now in another process how do I detect that mute

Answer 1

How about file-based locking (using flock(2))? These are automatically released when the process holding it dies.

Demo program:

#include 
#include 
#include 

void main() {
  FILE * f = fopen("testfile", "w+");

  printf("pid=%u time=%u Getting lock\n", getpid(), time(NULL));
  flock(fileno(f), LOCK_EX);
  printf("pid=%u time=%u Got lock\n", getpid(), time(NULL));

  sleep(5);
  printf("pid=%u time=%u Crashing\n", getpid(), time(NULL));
  *(int *)NULL = 1;
}

Output (I've truncated the PIDs and times a bit for clarity):

$ ./a.out & sleep 2 ; ./a.out 
[1] 15
pid=15 time=137 Getting lock
pid=15 time=137 Got lock
pid=17 time=139 Getting lock
pid=15 time=142 Crashing
pid=17 time=142 Got lock
pid=17 time=147 Crashing
[1]+  Segmentation fault      ./a.out
Segmentation fault

What happens is that the first program acquires the lock and starts to sleep for 5 seconds. After 2 seconds, a second instance of the program is started which blocks while trying to acquire the lock. 3 seconds later, the first program segfaults (bash doesn't tell you this until later though) and immediately, the second program gets the lock and continues.