The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


pthread_once - dynamic package initialization


#include <pthread.h>

int pthread_once(pthread_once_t *
       void (*
once_control = PTHREAD_ONCE_INIT;


The first call to pthread_once() by any thread in a process, with a given once_control, shall call the init_routine with no arguments. Subsequent calls of pthread_once() with the same once_control shall not call the init_routine. On return from pthread_once(), init_routine shall have completed. The once_control parameter shall determine whether the associated initialization routine has been called.

The pthread_once() function is not a cancellation point. However, if init_routine is a cancellation point and is canceled, the effect on once_control shall be as if pthread_once() was never called.

If the call to init_routine is terminated by a call to longjmp(), _longjmp(), or siglongjmp(), the behavior is undefined.

The constant PTHREAD_ONCE_INIT is defined in the <pthread.h> header.

The behavior of pthread_once() is undefined if once_control has automatic storage duration or is not initialized by PTHREAD_ONCE_INIT.


Upon successful completion, pthread_once() shall return zero; otherwise, an error number shall be returned to indicate the error.


The pthread_once() function shall not return an error code of [EINTR].

The following sections are informative.




If init_routine recursively calls pthread_once() with the same once_control, the recursive call will not call the specified init_routine, and thus the specified init_routine will not complete, and thus the recursive call to pthread_once() will not return. Use of longjmp(), _longjmp(), or siglongjmp() within an init_routine to jump to a point outside of init_routine prevents init_routine from returning.


Some C libraries are designed for dynamic initialization. That is, the global initialization for the library is performed when the first procedure in the library is called. In a single-threaded program, this is normally implemented using a static variable whose value is checked on entry to a routine, as follows:

static int random_is_initialized = 0;
extern void initialize_random(void);

int random_function() { if (random_is_initialized == 0) { initialize_random(); random_is_initialized = 1; } ... /* Operations performed after initialization. */ }

To keep the same structure in a multi-threaded program, a new primitive is needed. Otherwise, library initialization has to be accomplished by an explicit call to a library-exported initialization function prior to any use of the library.

For dynamic library initialization in a multi-threaded process, if an initialization flag is used the flag needs to be protected against modification by multiple threads simultaneously calling into the library. This can be done by using a mutex (initialized by assigning PTHREAD_MUTEX_INITIALIZER). However, the better solution is to use pthread_once() which is designed for exactly this purpose, as follows:

#include <pthread.h>
static pthread_once_t random_is_initialized = PTHREAD_ONCE_INIT;
extern void initialize_random(void);

int random_function() { (void) pthread_once(&random_is_initialized, initialize_random); ... /* Operations performed after initialization. */ }

If an implementation detects that the value specified by the once_control argument to pthread_once() does not refer to a pthread_once_t object initialized by PTHREAD_ONCE_INIT, it is recommended that the function should fail and report an [EINVAL] error.




XBD <pthread.h>


First released in Issue 5. Included for alignment with the POSIX Threads Extension.

Issue 6

The pthread_once() function is marked as part of the Threads option.

The [EINVAL] error is added as a ``may fail'' case for if either argument is invalid.

Issue 7

The pthread_once() function is moved from the Threads option to the Base.

The [EINVAL] error for an uninitialized pthread_once_t object is removed; this condition results in undefined behavior.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0284 [863], XSH/TC2-2008/0285 [874], XSH/TC2-2008/0286 [874], and XSH/TC2-2008/0287 [747] are applied.

End of informative text.


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