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PTHREAD_MUTEXATTR_DESTROY

Section: POSIX Programmer's Manual (3P)
Updated: 2017
Index Return to Main Contents

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_mutexattr_destroy, pthread_mutexattr_init - destroy and initialize the mutex attributes object

SYNOPSIS

#include <pthread.h>
int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_init(pthread_mutexattr_t *attr);

DESCRIPTION

The pthread_mutexattr_destroy() function 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. 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. The pthread_mutexattr_init() function shall initialize a mutex attributes object attr with the default value for all of the attributes defined by the implementation. Results are undefined if pthread_mutexattr_init() is called specifying an already initialized attr attributes object. 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 behavior is undefined if the value specified by the attr argument to pthread_mutexattr_destroy() does not refer to an initialized mutex attributes object.

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.

ERRORS

The pthread_mutexattr_init() function shall fail if:

ENOMEM: Insufficient memory exists to initialize the mutex attributes object. 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. 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. 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 r-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.

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 tes-an-set instruction (plus possibly a few other basic instructions). There is at least one implementation method that can be used to reduce the cost of testing at loc-time if a mutex has no-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 pr-lock any mutexes that are initialized to no-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 no-default mutex. The underlying unlock operation is more complicated since the implementation never really wants to release the pr-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.

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). In order to permit such usage, while at the same time keeping the usual case (that is, usage within a single process) efficient, a proces-shared option has been defined. If an implementation supports the _POSIX_THREAD_PROCESS_SHARED option, then the proces-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 proces-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 proces-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 proces-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(&sema->lock, &psharedm);
    (void) pthread_cond_init(&sema->nonzero, &psharedc);
    sema->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(&sema->lock);
    if (sema->count == 0)
        pthread_cond_signal(&semap->nonzero);
    sema->count++;
    pthread_mutex_unlock(&sema->lock);
}
void
semaphore_wait(semaphore_t *semap)
{
    pthread_mutex_lock(&sema->lock);
    while (sema->count == 0)
        pthread_cond_wait(&sema->nonzero, &sema->lock);
    sema->coun-;
    pthread_mutex_unlock(&sema->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.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.-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/ma-pages/reporting_bugs.html .