tialize 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.
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_destroy() function may fail if:
EINVAL The value specified by attr is invalid.
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
cussed 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 IEEE Std 1003.1-2001. Mutex
attributes objects, however, make it possible to test out these con-
cepts 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 implemen-
tation 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 com-
plicated 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
IEEE Std 1003.1-2001 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 multi-
ple 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 pro-
cesses.
The default setting of PTHREAD_PROCESS_PRIVATE has been chosen for the
process-shared attribute so that the most efficient forms of these syn-
chronization objects are created by default.
Synchronization variables that are initialized with the
PTHREAD_PROCESS_PRIVATE process-shared attribute may only be operated
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)
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 count-
ing 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);
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() ,
pthread_mutexattr_destroy , the Base Definitions volume of
IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
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