pthread_once()--Perform One-Time Initialization

 #include <pthread.h>
 int pthread_once(pthread_once_t *once_control, void (*init_routine)(void)); 

The pthread_once() function performs one time initialization based on a specific once_control variable. The init_routine is called only one time when multiple calls to pthread_once() use the same once_control.

The once_control variable is not set until the init_routine returns. If the init_routine is a cancellation point and the thread calling the init_routine by pthread_once() is cancelled, the once_control variable will not be set and a subsequent call to pthread_once() using that once_control variable will result in another call to the init_routine.

You must initialize the once_control variable to PTHREAD_ONCE_INIT prior to calling pthread_once() with it.

The function passed as init_routine must correspond to the following C function prototype:

void initRoutine(void);

Authorities and Locks

None.

Parameters

once_control

(Input) The control variable associated with this initialization.

init_routine

(Input) A function pointer to a routine that takes no parameters and returns no value.

Return Value

0

pthread_once() was successful.

value

pthread_once() was not successful. value is set to indicate the error condition.

Error Conditions

If pthread_once() was not successful, the error condition returned usually indicates one of the following errors. Under some conditions, the value returned could indicate an error other than those listed here.

[EINVAL]

The value specified for the argument is not correct.

Related Information

• The <pthread.h> header file. See Header files for Pthread functions.

Example

Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.

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

#define         NUMTHREADS    3
int             number      = 0;
int             okStatus    = 777;
pthread_once_t  onceControl = PTHREAD_ONCE_INIT;

void initRoutine(void)
{
  printf("In the initRoutine\n");
  number++;
}

void *threadfunc(void *parm)
{
  printf("Inside secondary thread\n");
  pthread_once(&onceControl, initRoutine);
  return __VOID(okStatus);
}

int main(int argc, char **argv)
{
  pthread_t             thread[NUMTHREADS];
  int                   rc=0;
  int                   i=NUMTHREADS;
  void                 *status;

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

  for (i=0; i < NUMTHREADS; ++i) {
    printf("Create thread %d\n",
           i);
    rc = pthread_create(&thread[i], NULL, threadfunc, NULL);
    checkResults("pthread_create()\n", rc);
  }


  for (i=0; i < NUMTHREADS; ++i) {
    printf("Wait for thread %d\n", i);
    rc = pthread_join(thread[i], &status);
    checkResults("pthread_join()\n", rc);
    if (__INT(status) != okStatus) {
      printf("Secondary thread failed\n");
      exit(1);
    }
  }

  if (number != 1) {
    printf("An incorrect number of 1 one-time init routine was called!\n");
    exit(1);
  }
  printf("One-time init routine called exactly once\n");
  printf("Main completed\n");
  return 0;
}

Output:

Enter Testcase - QP0WTEST/TPONCE0
Create thread 0
Create thread 1
Create thread 2
Wait for thread 0
Inside secondary thread
In the initRoutine
Inside secondary thread
Wait for thread 1
Wait for thread 2
Inside secondary thread
One-time init routine called exactly once
Main completed

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