pthread_once (3p) - Linux Manuals

pthread_once: dynamic package initialization


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


pthread_once - dynamic package initialization


#include <pthread.h>

int pthread_once(pthread_once_t *once_control,
       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.

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 may fail if:

If either once_control or init_routine is invalid.

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

The following sections are informative.






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 int initialize_random();

int random_function()
    if (random_is_initialized == 0) {
        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, a simple initialization flag is not sufficient; the flag needs to be protected against modification by multiple threads simultaneously calling into the library. Protecting the flag requires the use of a mutex; however, mutexes have to be initialized before they are used. Ensuring that the mutex is only initialized once requires a recursive solution to this problem.

The use of pthread_once() not only supplies an implementation-guaranteed means of dynamic initialization, it provides an aid to the reliable construction of multi-threaded and realtime systems. The preceding example then becomes:

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

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

Note that a pthread_once_t cannot be an array because some compilers do not accept the construct &<array_name>.




Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .


The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>