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RedHat 9 (Linux i386) - man page for sem_wait (redhat section 3)

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       sem_init, sem_wait, sem_trywait, sem_post, sem_getvalue, sem_destroy - operations on sema-

       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

       int sem_wait(sem_t * sem);

       int sem_trywait(sem_t * sem);

       int sem_post(sem_t * sem);

       int sem_getvalue(sem_t * sem, int * sval);

       int sem_destroy(sem_t * sem);

       This manual page documents POSIX 1003.1b semaphores, not to be confused with SystemV sema-
       phores as described in ipc(5), semctl(2) and semop(2).

       Semaphores are counters for resources shared between threads. The basic operations on sem-
       aphores are: increment the counter atomically, and wait until the counter is non-null  and
       decrement it atomically.

       sem_init  initializes  the  semaphore object pointed to by sem.	The count associated with
       the semaphore is set initially to value.  The pshared argument indicates whether the sema-
       phore  is  local to the current process ( pshared is zero) or is to be shared between sev-
       eral processes ( pshared is not zero). LinuxThreads currently does  not	support  process-
       shared semaphores, thus sem_init always returns with error ENOSYS if pshared is not zero.

       sem_wait  suspends  the	calling thread until the semaphore pointed to by sem has non-zero
       count. It then atomically decreases the semaphore count.

       sem_trywait is a non-blocking variant of sem_wait.  If the semaphore pointed to by sem has
       non-zero  count,  the count is atomically decreased and sem_trywait immediately returns 0.
       If the semaphore count is zero, sem_trywait immediately returns with error EAGAIN.

       sem_post atomically increases the count of the semaphore pointed to by sem.  This function
       never blocks and can safely be used in asynchronous signal handlers.

       sem_getvalue  stores in the location pointed to by sval the current count of the semaphore

       sem_destroy destroys a semaphore object, freeing the resources it might hold.  No  threads
       should  be waiting on the semaphore at the time sem_destroy is called. In the LinuxThreads
       implementation, no resources are associated with semaphore objects, thus sem_destroy actu-
       ally does nothing except checking that no thread is waiting on the semaphore.

       sem_wait is a cancellation point.

       On  processors  supporting  atomic  compare-and-swap (Intel 486, Pentium and later, Alpha,
       PowerPC, MIPS II, Motorola 68k), the sem_post function is async-signal safe and can there-
       fore be called from signal handlers. This is the only thread synchronization function pro-
       vided by POSIX threads that is async-signal safe.

       On the Intel 386 and the Sparc, the current LinuxThreads implementation of sem_post is not
       async-signal safe by lack of the required atomic operations.

       The  sem_wait  and  sem_getvalue functions always return 0.  All other semaphore functions
       return 0 on success and -1 on error, in addition to writing an error code in errno.

       The sem_init function sets errno to the following codes on error:

	      EINVAL value exceeds the maximal counter value SEM_VALUE_MAX

	      ENOSYS pshared is not zero

       The sem_trywait function sets errno to the following error code on error:

	      EAGAIN the semaphore count is currently 0

       The sem_post function sets errno to the following error code on error:

	      ERANGE after incrementation, the semaphore value would  exceed  SEM_VALUE_MAX  (the
		     semaphore count is left unchanged in this case)

       The sem_destroy function sets errno to the following error code on error:

	      EBUSY  some threads are currently blocked waiting on the semaphore.

       Xavier Leroy <Xavier.Leroy@inria.fr>

       pthread_mutex_init(3), pthread_cond_init(3), pthread_cancel(3), ipc(5).

					   LinuxThreads 			    SEMAPHORES(3)
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