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sem_post.2 (2010 09)

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sem_post(2) sem_post(2)
NAME
sem_post - unlock a POSIX semaphore
SYNOPSIS
#include <sys/semaphore.h>
int sem_post(sem_t *sem);
DESCRIPTION
sem_post() is used to post the semaphore referenced by sem . The calling thread will not return from
its call to
sem_post() unless it can either: increment the semaphore value, if there are no blocked
threads on this semaphore; give the semaphore to a blocked thread, if there are any blocked threads on
this semaphore; or have an error condition.
If the semaphore value is < 0, the semaphore has blocked threads, waiting for it to become available (the
absolute value of the semaphore’s value indicates the number of waiters at that moment). If the sema-
phore value is >= 0, the semaphore has no waiters.
If the semaphore has no waiters at the time its value is checked, the semaphore’s value will be atomically
incremented, with respect to the checking of its value, up to its maximum value as specified by
{_POSIX_SEM_VALUE_MAX}
. If the semaphore has waiters at the time its value is checked, the sema-
phore value is not changed. Instead, the calling thread will attempt to wake up a waiter. If the sema-
phore has waiters having realtime priorities, the thread must wake up the highest priority waiter. Other-
wise the thread at the head of the channel queue is woken up.
When a waiter is successfully woken, the semaphore being posted will be given to the woken waiter. In
other words, the state of the semaphore remains unchanged. Instead, the semaphore being posted will be
inherited by the waiter being woken from this call to
sem_post().
If the specified semaphore referred to by sem is a named semaphore, then this semaphore must have been
opened by the calling process with
sem_open(). The calling process must have both read and write
permissions on the semaphore to perform this operation. The sem_post() routine may be called asyn-
chronously, i.e. from a signal handler.
To use this function, link in the realtime library by specifying
-lrt on the compiler or linker command
line.
EXAMPLES
The following call to
sem_post() will post the semaphore sem .
sem_post(sem);
RETURN VALUE
A successful call to sem_post() will return 0 and the calling thread would have posted the semaphore.
Otherwise, the call to sem_post() will return -1 with errno set to the appropriate value of the error
condition.
ERRORS
sem_post() fails and does not perform the requested operation if any of the following conditions are
encountered:
[EPERM] The calling process does not have the privileges necessary to post the semaphore.
[EINVAL] The argument sem does not refer to a valid semaphore.
SEE ALSO
<semaphore.h>.
STANDARDS CONFORMANCE
sem_post(): POSIX
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

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