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 semaphore 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 semaphore value is not changed.
Instead, the calling thread will attempt to wake up a waiter.
If the semaphore has waiters having realtime priorities, the thread must wake
up the highest priority waiter.
Otherwise 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 asynchronously, 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.
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.
STANDARDS CONFORMANCE
sem_post(): POSIX
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