Question

I am trying to teach myself pthreads threading. I have the following source, which compiles and runs properly:

#include <stdio.h>
#include <pthread.h>

#define PTHREAD_COUNT 10
#define FREQ 5

void *thread_function(void *arg) {
  int *incoming = (int *)arg;
  int freqIdx;

  for (freqIdx = 0; freqIdx < FREQ; freqIdx++)
    fprintf(stdout, "Hello, world (thread %d)\n", *incoming);

  return NULL;
}

int main(int argc, char **argv) {
  pthread_t thread_IDs[PTHREAD_COUNT];
  void *exit_status;
  int threadIdx;

  for (threadIdx = 0; threadIdx < PTHREAD_COUNT; threadIdx++) {
    pthread_create(&thread_IDs[threadIdx], NULL, thread_function, &threadIdx);
    pthread_join(thread_IDs[threadIdx], &exit_status);
  }

  return 0;
}

I get the following result:

Hello, world (thread 0)
Hello, world (thread 0)
Hello, world (thread 0)
Hello, world (thread 0)
Hello, world (thread 0)
Hello, world (thread 1)
...
Hello, world (thread 9)

If I pthread_create an array of pthread_t types over a loop, and then pthread_join in a separate loop, then things fail:

#include <stdio.h>
#include <pthread.h>

#define PTHREAD_COUNT 10
#define FREQ 5

void *thread_function(void *arg) {
  int *incoming = (int *)arg;
  int freqIdx;

  for (freqIdx = 0; freqIdx < FREQ; freqIdx++)
    fprintf(stdout, "Hello, world (thread %d)\n", *incoming);

  return NULL;
}

int main(int argc, char **argv) {
  pthread_t thread_IDs[PTHREAD_COUNT];
  void *exit_status;
  int threadIdx;

  /* here I split the thread _create and _join steps into separate loops */

  for (threadIdx = 0; threadIdx < PTHREAD_COUNT; threadIdx++)
    pthread_create(&thread_IDs[threadIdx], NULL, thread_function, &threadIdx);

  for (threadIdx = 0; threadIdx < PTHREAD_COUNT; threadIdx++)
    pthread_join(thread_IDs[threadIdx], &exit_status);

  return 0;
}

The output from this is quite wrong. Instead of getting five fprintf statements from each thread, I get one or two from, say, thread 2 and 3, and about 20 to 25 Hello, world statements from thread 0.

Why does this fail?

Answer 1

As others have stated, your problem is sharing the one threadIdx variable between all the threads. One way to fix this is to create one variable per thread:

int threadIdx;
int indexes[PTHREAD_COUNT];

for (threadIdx = 0; threadIdx < PTHREAD_COUNT; threadIdx++) {
    indexes[threadIdx] = threadIdx;
    pthread_create(&thread_IDs[threadIdx], NULL, thread_function, &indexes[threadIdx]);
}