PTHREAD_M...TR_DESTROY(3P)

okay bestie, we're reading PTHREAD_M...TR_DESTROY(3P) from the POSIX Programmer's Manual, and it's lowkey a whole journey. 💅

✨ PROLOG ✨

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. Not me being dramatic, that's just the facts. 💀

🌸 NAME 🌸

pthread_mutexattr_destroy, pthread_mutexattr_init — destroy and initialize the mutex attributes object. Iconic duo behavior, fr fr. 🫶

🦋 SYNOPSIS 🦋

#include <pthread.h>

int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_init(pthread_mutexattr_t *attr);

💖 DESCRIPTION 💖

pthread_mutexattr_destroy() shall destroy a mutex attributes object; the object becomes, in effect, uninitialized. An implementation may cause pthread_mutexattr_destroy() to set the object referenced by attr to an invalid value. So like, after destroy, that attr is not your bestie anymore. It's invalid-coded. 😭

A destroyed attr attributes object can be reinitialized using pthread_mutexattr_init(); the results of otherwise referencing the object after it has been destroyed are undefined. Undefined behavior is not cute, okay? Do not touch it unless you reinit. 🚫

pthread_mutexattr_init() shall initialize a mutex attributes object attr with the default value for all of the attributes defined by the implementation. Defaults are serving base-level, but make it work. ✨

Results are undefined if pthread_mutexattr_init() is called specifying an already initialized attr attributes object. Double-init is a no-no, bestie. Don't do it. 💀

After a mutex attributes object has been used to initialize one or more mutexes, any function affecting the attributes object (including destruction) shall not affect any previously initialized mutexes. The mutexes are independent. They said no attachment issues. 👑

The behavior is undefined if the value specified by the attr argument to pthread_mutexattr_destroy() does not refer to an initialized mutex attributes object. So if it's not initialized, don't destroy it. That's the vibe check. 🛑

⭐ RETURN VALUE ⭐

Upon successful completion, pthread_mutexattr_destroy() and pthread_mutexattr_init() shall return zero; otherwise, an error number shall be returned to indicate the error. Zero means slay. Nonzero means read the error. Periodt. 💅

🔥 ERRORS 🔥

The pthread_mutexattr_init() function shall fail if:

• ENOMEM — Insufficient memory exists to initialize the mutex attributes object. Memory said no. It happens. 😭

These functions shall not return an error code of [EINTR].

The following sections are informative.

🎀 EXAMPLES 🎀

None.

🌈 APPLICATION USAGE 🌈

None.

🧠 RATIONALE 🧠

If an implementation detects that the value specified by the attr argument to pthread_mutexattr_destroy() does not refer to an initialized mutex attributes object, it is recommended that the function should fail and report an [EINVAL] error. Recommended, not mandatory. Still, listen. 👂

See pthread_attr_destroy() for a general explanation of attributes. Attributes objects allow implementations to experiment with useful extensions and permit extension of this volume of POSIX.1‐2017 without changing the existing functions. Thus, they provide for future extensibility of this volume of POSIX.1‐2017 and reduce the temptation to standardize prematurely on semantics that are not yet widely implemented or understood. Basically: attributes objects are the experimental sandbox, but make it standards-friendly. 🧪

Examples of possible additional mutex attributes that have been discussed are spin_only, limited_spin, no_spin, recursive, and metered. (To explain what the latter attributes might mean: recursive mutexes would allow for multiple re-locking by the current owner; metered mutexes would transparently keep records of queue length, wait time, and so on.) Since there is not yet wide agreement on the usefulness of these resulting from shared implementation and usage experience, they are not yet specified in this volume of POSIX.1‐2017. Mutex attributes objects, however, make it possible to test out these concepts for possible standardization at a later time. So they're like the group chat ideas that haven't been promoted to the official plan yet. 💬

🌀 Mutex Attributes and Performance 🌀

Care has been taken to ensure that the default values of the mutex attributes have been defined such that mutexes initialized with the defaults have simple enough semantics so that the locking and unlocking can be done with the equivalent of a test-and-set instruction (plus possibly a few other basic instructions). Defaults are optimized. That's the point. ⚡

There is at least one implementation method that can be used to reduce the cost of testing at lock-time if a mutex has non-default attributes. One such method that an implementation can employ (and this can be made fully transparent to fully conforming POSIX applications) is to secretly pre-lock any mutexes that are initialized to non-default attributes. Any later attempt to lock such a mutex causes the implementation to branch to the slow path'' as if the mutex were unavailable; then, on the slow path, the implementation can do the real work'' to lock a non-default mutex. The underlying unlock operation is more complicated since the implementation never really wants to release the pre-lock on this kind of mutex. This illustrates that, depending on the hardware, there may be certain optimizations that can be used so that whatever mutex attributes are considered ``most frequently used'' can be processed most efficiently. Sneaky implementation behavior, but it's allowed. 🤫

🌊 Process Shared Memory and Synchronization 🌊

The existence of memory mapping functions in this volume of POSIX.1‐2017 leads to the possibility that an application may allocate the synchronization objects from this section in memory that is accessed by multiple processes (and therefore, by threads of multiple processes). Multiverse of processes, basically. 🌌

In order to permit such usage, while at the same time keeping the usual case (that is, usage within a single process) efficient, a process-shared option has been defined.

If an implementation supports the _POSIX_THREAD_PROCESS_SHARED option, then the process-shared attribute can be used to indicate that mutexes or condition variables may be accessed by threads of multiple processes.

The default setting of PTHREAD_PROCESS_PRIVATE has been chosen for the process-shared attribute so that the most efficient forms of these synchronization objects are created by default.

Synchronization variables that are initialized with the PTHREAD_PROCESS_PRIVATE process-shared attribute may only be operated on by threads in the process that initialized them. Synchronization variables that are initialized with the PTHREAD_PROCESS_SHARED process-shared attribute may be operated on by any thread in any process that has access to it. In particular, these processes may exist beyond the lifetime of the initializing process. For example, the following code implements a simple counting semaphore in a mapped file that may be used by many processes.

/* sem.h */
struct semaphore {
    pthread_mutex_t lock;
    pthread_cond_t nonzero;
    unsigned count;
};
typedef struct semaphore semaphore_t;

semaphore_t *semaphore_create(char *semaphore_name);
semaphore_t *semaphore_open(char *semaphore_name);
void semaphore_post(semaphore_t *semap);
void semaphore_wait(semaphore_t *semap);
void semaphore_close(semaphore_t *semap);

/* sem.c */
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <pthread.h>
#include "sem.h"

semaphore_t *
semaphore_create(char *semaphore_name)
{
int fd;
    semaphore_t *semap;
    pthread_mutexattr_t psharedm;
    pthread_condattr_t psharedc;

    fd = open(semaphore_name, O_RDWR | O_CREAT | O_EXCL, 0666);
    if (fd < 0)
        return (NULL);
    (void) ftruncate(fd, sizeof(semaphore_t));
    (void) pthread_mutexattr_init(&psharedm);
    (void) pthread_mutexattr_setpshared(&psharedm,
        PTHREAD_PROCESS_SHARED);
    (void) pthread_condattr_init(&psharedc);
    (void) pthread_condattr_setpshared(&psharedc,
        PTHREAD_PROCESS_SHARED);
    semap = (semaphore_t *) mmap(NULL, sizeof(semaphore_t),
            PROT_READ | PROT_WRITE, MAP_SHARED,
            fd, 0);
    close (fd);
    (void) pthread_mutex_init(&semap->lock, &psharedm);
    (void) pthread_cond_init(&semap->nonzero, &psharedc);
    semap->count = 0;
    return (semap);
}

semaphore_t *
semaphore_open(char *semaphore_name)
{
    int fd;
    semaphore_t *semap;

    fd = open(semaphore_name, O_RDWR, 0666);
    if (fd < 0)
        return (NULL);
    semap = (semaphore_t *) mmap(NULL, sizeof(semaphore_t),
            PROT_READ | PROT_WRITE, MAP_SHARED,
            fd, 0);
    close (fd);
    return (semap);
}

void
semaphore_post(semaphore_t *semap)
{
    pthread_mutex_lock(&semap->lock);
    if (semap->count == 0)
        pthread_cond_signal(&semapx->nonzero);
    semap->count++;
    pthread_mutex_unlock(&semap->lock);
}

void
semaphore_wait(semaphore_t *semap)
{
    pthread_mutex_lock(&semap->lock);
    while (semap->count == 0)
        pthread_cond_wait(&semap->nonzero, &semap->lock);
    semap->count--;
    pthread_mutex_unlock(&semap->lock);
}

void
semaphore_close(semaphore_t *semap)
{
    munmap((void *) semap, sizeof(semaphore_t));
}

The following code is for three separate processes that create, post, and wait on a semaphore in the file /tmp/semaphore. Once the file is created, the post and wait programs increment and decrement the counting semaphore (waiting and waking as required) even though they did not initialize the semaphore.

/* create.c */
#include "pthread.h"
#include "sem.h"

int
main()
{
    semaphore_t *semap;

    semap = semaphore_create("/tmp/semaphore");
    if (semap == NULL)
        exit(1);
    semaphore_close(semap);
    return (0);
}

/* post */
#include "pthread.h"
#include "sem.h"

int
main()
{
    semaphore_t *semap;

    semap = semaphore_open("/tmp/semaphore");
    if (semap == NULL)
        exit(1);
    semaphore_post(semap);
    semaphore_close(semap);
    return (0);
}

/* wait */
#include "pthread.h"
#include "sem.h"

int
main()
{
    semaphore_t *semap;

    semap = semaphore_open("/tmp/semaphore");
    if (semap == NULL)
        exit(1);
    semaphore_wait(semap);
    semaphore_close(semap);
    return (0);
}

🔮 FUTURE DIRECTIONS 🔮

None.

👀 SEE ALSO 👀

pthread_cond_destroy(), pthread_create(), pthread_mutex_destroy()

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

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✨ COPYRIGHT ✨

okay bestie, legal bit, but it's actually important so stay with me 🫶

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, and the copyright on that is Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. like. both of them. iconic joint custody fr 💅

🌸 real talk though: if this version and the original IEEE and The Open Group Standard ever disagree about anything, the original IEEE and The Open Group Standard is the referee document. not this page. this page is the vibe, that one is the verdict. no cap.

and the original Standard can be obtained online at http://www.opengroup.org/unix/online.html — go get the real one if you need it, it's right there 🦋

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🎀 about the typos tho

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. so like… not the original authors' fault, it's a glow-up-gone-slightly-wrong situation 😭

To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html — tell them, they'll fix it, everybody wins 💖

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and that's the whole page, bestie. every last crumb of it. This one came from the IEEE/The Open Group and it's dated 2017 🌈 Thanks for reading all the way to the copyright section, that's genuinely main character behavior 👑 mwah 💋

bye pookie, go write some correct threaded code 🧚✨