Probably very easy question about pthread mutexes
I am working on a multi-threaded app that converts files. I want it to launch a new thread for each conversion, up to the available number of CPU cores. To keep track of how many threads are already running, I keep a global variable that is decremented by each thread once it's finished. To make sure there aren't any problems by more than one thread trying to update this variable simultaneously, I use a mutex. However, I'm not exactly sure how it works.
I assume that when creating a mutex, the common pthread_mutex_t variable must be seen by any part of the code that attempts to read/write the variable in question. So, what I've done is initialized the mutex on the main thread, then passed a pointer to it to each of the threads. The code isn't comprehensive, but hopefully you'll get what I'm trying to do:
Code:
#define MAX_THREADS 4
int counter; //thread counter
void *threadFunction(void *mutex){
pthread_mutex_t *mutex_t = mutex;
//do thread stuff
//decrement counter
pthread_mutex_lock(&(*mutex_t));
counter--;
pthread_mutex_unlock(&(*mutex_t));
}
int main(){
counter = 0;
pthread_mutex_t mutex;
pthread_t thread_id = 1;
pthread_mutex_init(&mutex, NULL);
pthread_create(&thread_id, NULL, threadFunction, (void*)&mutex);
counter++;
//check counter to see if more threads can start
pthread_mutex_lock(&mutex);
if(counter>MAX_THREADS){
//wait code
}
pthread_mutex_destroy(&mutex);
return 0;
}
Does this make sense?
