PTHREAD_ONCE(3POSIX)

💖 PTHREAD_ONCE(3POSIX) — POSIX Programmer's Manual 💖

okay bestie, welcome to the POSIX Programmer's Manual page for pthread_once. this one is lowkey iconic, it's the "run this exactly one time no matter how many threads roll up" girlie ✨

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🚨 PROLOG 🚨

heads up before we go any further: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go consult the corresponding Linux manual page for the deets on Linux behavior. or, plot twist, the interface might not even be implemented on Linux at all. 🙈

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✨ NAME ✨

pthread_once — dynamic package initialization 💅

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🎀 SYNOPSIS 🎀

#include <pthread.h>

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

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🦋 DESCRIPTION 🦋

so here's the tea, fr fr:

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. (yes, it fully pretends it never happened. gaslight gatekeep girlboss 😭)

If the call to init_routine is terminated by a call to longjmp(), _longjmp(), or siglongjmp(), the behavior is undefined. do not do this. i'm not joking. 💀

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. so like... initialize her properly or don't come crying to me. 🫶

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🌈 RETURN VALUE 🌈

Upon successful completion, pthread_once() shall return zero; otherwise, an error number shall be returned to indicate the error. no cap.

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💀 ERRORS 💀

The pthread_once() function shall not return an error code of [EINTR]. that's it. that's the whole section. iconic brevity honestly.

The following sections are informative.

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🍓 EXAMPLES 🍓

None.

(she said "no examples, figure it out bestie" 💅 — but keep reading, the RATIONALE has you covered fr fr)

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🧚 APPLICATION USAGE 🧚

okay this part is genuinely important so lock in 🔥

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. yes bestie that's a deadlock, it's giving "stuck forever" energy. 💀

Use of longjmp(), _longjmp(), or siglongjmp() within an init_routine to jump to a point outside of init_routine prevents init_routine from returning. 🚩

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👑 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. (imagine. so much admin. no thank you. 💅)

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. 🚨

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🔮 FUTURE DIRECTIONS 🔮

None.

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🌟 SEE ALSO 🌟

The Base Definitions volume of POSIX.1‐2017, <pthread.h>

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📜 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 .

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and that's the whole page bestie!! 🫶 this one dropped way back in 2017, courtesy of IEEE/The Open Group — so like, respect the elders. now go initialize your globals exactly once, mwah 💖✨🎀🦋🌸💅