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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PTHREAD..._DESTROY(3P) POSIX Programmer's Manual PTHREAD..._DESTROY(3P)
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_mutexattr_destroy, pthread_mutexattr_init — destroy and
initialize the mutex attributes object
#include <pthread.h>
int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_init(pthread_mutexattr_t *attr);
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.
Upon successful completion, pthread_mutexattr_destroy() and
pthread_mutexattr_init() shall return zero; otherwise, an error
number shall be returned to indicate the error.
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.
None.
None.
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(3p) 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 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.
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).
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.
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
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);
}
None.
pthread_cond_destroy(3p), pthread_create(3p),
pthread_mutex_destroy(3p)
The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)
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 .
IEEE/The Open Group 2017 PTHREAD..._DESTROY(3P)
Pages that refer to this page: pthread.h(0p), pthread_mutexattr_getpshared(3p), pthread_mutexattr_init(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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