pthread_once man page


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 (*init_routine)(void));
pthread_once_t 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.

Return Value

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.



Application Usage



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();

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 void 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>.

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.

Future Directions


See Also

The Base Definitions volume of POSIX.1‐2008, <pthread.h>


2013 IEEE/The Open Group POSIX Programmer's Manual