Threads do not necessarily start before pthread_create() returns

A thread may or may not start running before the return from pthread_create(). Depending on the amount of time left in the creating threads, time slice, and the other activity on the system, the creating thread may return before the new thread runs.

The thread implementations of some systems guarantee a certain ordered behavior for thread creation versus the execution of the first statement in the new thread. On the iSeries, it is unknown which happens first, the execution of the first instruction in the new thread or the return from pthread_create().

The following example shows an incorrectly written application.

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#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>
#include "check.h"

pthread_t      thread

void *threadfunc(void *parm)
{
  pthread_id_np_t tid;
  #error "This is an ERROR."
  #error "The 'thread' variable is shared between threads"
  #error "and must be protected by a mutex."
  pthread_getunique_np(&thread, &tid);
  printf("Thread 0x%.8x %.8x started\n", tid);
  return NULL;
}

int main(int argc, char **argv)
{
  int                   rc=0;

  printf("Enter Testcase - %s\n", argv[0]);

  #error "This is an ERROR."
  #error "The order of thread thread startup, and return from"
  #error "the pthread_create() API is NOT deterministic."
  rc = pthread_create(&thread, NULL, threadfunc, NULL);
  checkResults("pthread_create(NULL)\n", rc);

  /* sleep() isn't a very robust way to wait for the thread */
  sleep(5);

  printf("Main completed\n");
  return 0;
}