pthread failure to join with unknown error - macos

Question

I'm planning using pthreads and mach semaphores to try to basically farm out a parallel computation to a limited number of CPUs, and I can't quite get a test program to work. Right now I have something that just goes through threads and prints out some identifier so that I could verify that it works. The code is pretty simple, except that I'm on OSX so I have to use mach semaphores instead of POSIX. My code is below
#include <iostream>
#include <pthread.h>
#include <semaphore.h>
#include <errno.h>
#include <mach/semaphore.h>
#include <mach/mach.h>
#define MAX_THREADS 256
semaphore_t free_CPU = 0;
void* t_function(void *arg) {
int* cur_number;
cur_number = (int*) arg;
kern_return_t test = semaphore_wait(free_CPU);
std::cout << "I am thread # " << *cur_number << ". Kernel return is " << test << std::endl;
semaphore_signal(free_CPU);
std::cout << "I am thread # " << *cur_number << ". I just signaled the semaphore." << std::endl;
pthread_exit(NULL);
}
int main (int argc, char * const argv[]) {
int num_reps = 10;
int n_threads = 1;
if (n_threads < MAX_THREADS) {
n_threads += 0;
} else {
n_threads = MAX_THREADS;
}
pthread_t threads[n_threads];
semaphore_create(mach_task_self(), &free_CPU, SYNC_POLICY_FIFO, 1);
// Loop over a bunch of things, feeding out to only nthreads threads at a time!
int i;
int* numbers = new int[num_reps];
for (i = 0; i < num_reps; i++) {
numbers[i] = i;
std::cout << "Throwing thread " << numbers[i] << std::endl;
int rc = pthread_create(&threads[i], NULL, &t_function, &numbers[i]);
if (rc) {
std::cout << "Failed to throw thread " << i << " Error: " << strerror(errno) << std::endl;
exit(1);
}
}
std::cout << "Threw all threads" << std::endl;
// Loop over threads to join
for (i = 0; i < num_reps; i++) {
std::cout << "Joining thread " << i << std::endl;
int rc = pthread_join(threads[i],NULL);
if (rc) {
std::cout << "Failed to join thread " << i << ". Error: " << strerror(errno) << std::endl;
exit(1);
}
}
semaphore_destroy(mach_task_self(), free_CPU);
delete[] numbers;
return 0;
}
Running this code gives me:
Throwing thread 0
Throwing thread 1
Throwing thread 2
Throwing thread 3
Throwing thread 4
Throwing thread 5
Throwing thread 6
Throwing thread 7
Throwing thread 8
Throwing thread 9
Threw all threads
Joining thread 0
I am thread # 0. Kernel return is 0
I am thread # 0. I just signaled the semaphore.
I am thread # 1. Kernel return is 0
I am thread # 1. I just signaled the semaphore.
I am thread # 2. Kernel return is 0
I am thread # 2. I just signaled the semaphore.
I am thread # 3. Kernel return is 0
I am thread # 3. I just signaled the semaphore.
I am thread # 4. Kernel return is 0
I am thread # 4. I just signaled the semaphore.
I am thread # 5. Kernel return is 0
I am thread # 5. I just signaled the semaphore.
I am thread # 6. Kernel return is 0
I am thread # 6. I just signaled the semaphore.
I am thread # 7. Kernel return is 0
I am thread # 7. I just signaled the semaphore.
I am thread # 8. Kernel return is 0
I am thread # 8. I just signaled the semaphore.
I am thread # 9. Kernel return is 0
I am thread # 9. I just signaled the semaphore.
Joining thread 1
Joining thread 2
Joining thread 3
Joining thread 4
Joining thread 5
Joining thread 6
Joining thread 7
Joining thread 8
Failed to join thread 8. Error: Unknown error: 0
To me, it looks like everything is totally fine, except it just bites the dust when it tries to join thread 8. I have no clue what's going on.

Answer 1

Your problem lies here:
#define MAX_THREADS 256
:
int n_threads = 1;
if (n_threads < MAX_THREADS) {
n_threads += 0;
} else {
n_threads = MAX_THREADS;
}
pthread_t threads[n_threads];
This is giving you an array of one thread ID. You're then trying to populate ten of them.
I'm not entirely certain what you're trying to acheive with that. It seems to me that, if you just used num_reps to dimension your array, it would work fine (you'd get an array of ten elements).