Question

I have a basic question on mutext handling.
I have a file which appears at 2 other threads(out of 3). I need to mutually exclude access to it by a thread. So I do the following in a function called from the thread1 handler:

int sub_routine_thread1{

pthread_mutex_lock(&mut)
FILE *fp;
fp = fopen("myfile", "r");
if(fp == NULL){
return -1;
}
pthread_mutex_unlock(&mut)
return 0;
}

As you can see I already know that if the file pointer return a NULL, then my mutex gets locked here and the unlocking never happens. So I changed it to the following:

int sub_routine_thread1{

pthread_mutex_lock(&mut)
FILE *fp;
fp = fopen("myfile", "r");
if(fp == NULL){
   pthread_mutex_unlock(&mut)
   return -1;
}
pthread_mutex_unlock(&mut)
return 0;
}

But after this though I have a bad feeling this is not the way mutex locking should be done. I chanced upon something about cleanup handlers and perhaps thought this is how I must code:

int sub_routine_thread1{
     pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
     pthread_mutex_lock(&mut);
     FILE *fp;
     fp = fopen("myfile", "r");
     if(fp == NULL){
     return -1;
     }
     pthread_cleanup_pop(1);
     return 0;
}

Is this the right way? Can you please clarify?

Answer 1

But after this though I have a bad feeling this is not the way mutex loxking should be done.

Yes it is. Your second example is perfectly fine. pthread_cleanup_push is used to run functions when the thread is cancelled, that’s not what you should use here.

Allthough, I’d probably prefer to do something like

int sub_routine_thread1() {
  FILE *fp;
  int ret = -1;
  pthread_mutex_lock(&mut)
  fp = fopen("myfile", "r");
  if(fp != NULL){
     //do_stuff_unlocked(fp);
     ret = 0;
  }
  pthread_mutex_unlock(&mut)
  return ret;
}