PTHREAD_ONCE(3P)

PROLOG
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.

NAME
pthread_once — dynamic package initialization

SYNOPSIS

#include <pthread.h>

int pthread_once(pthread_once_t *once_control,
    void (*init_routine)(void));
pthread_once_t once_control = PTHREAD_ONCE_INIT;

DESCRIPTION
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.

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

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

The following sections are informative.

EXAMPLES
None.

APPLICATION USAGE
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.

RATIONALE
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.

FUTURE DIRECTIONS
None.

SEE ALSO
The Base Definitions volume of POSIX.1-2017, <pthread.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .