pthread cond var test, block on child thread

condp2.c

/* build with `-O -pthread -D_GNU_SOURCE=1` */
#include <unistd.h>
#include <pthread.h>
#include <assert.h>

static pthread_cond_t run_first = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t cond_mutex = PTHREAD_MUTEX_INITIALIZER;

static void* second_thread(void* param) {
  assert(pthread_cond_wait(&run_first, &cond_mutex) == 0);
  assert(write(STDOUT_FILENO, "second\n", 7) == 7);
  assert(pthread_mutex_destroy(&cond_mutex) == 0);
  return NULL;
}

static void* first_thread(void* param) {
  assert(pthread_cond_signal(&run_first) == 0);
  assert(write(STDOUT_FILENO, "first\n", 6) == 6);
  return NULL;
}

int main(int argc, char* argv[])
{
  pthread_t first, second;

  assert(pthread_create(&second, NULL, second_thread, NULL) == 0);
  assert(pthread_create(&first, NULL, first_thread, NULL) == 0);
  assert(pthread_join(first, NULL) == 0);
  assert(pthread_join(second, NULL) == 0);
  return 0;
}