Manualshelf

sem_wait.2 (2010 09)

ManualsBrandsHP ManualsSoftwareHP-UX Manpages
12
s
sem_wait(2) sem_wait(2)
NAME
sem_wait, sem_trywait - lock a POSIX semaphore
SYNOPSIS
#include <sys/semaphore.h>
int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
DESCRIPTION
sem_wait() is used to lock a semaphore. The calling thread will not return from its call to
sem_wait() until one of the following events occur: it successfully obtains a lock on the semaphore; it
is interrupted by a signal or an error condition occurs.
sem_trywait()
is used to lock a semaphore, if it is available. The value of the semaphore sem is
checked at some unspecified time during the call. If the semaphore is available at the time its value is
checked, the calling thread will atomically, with respect to the checking of the value, lock the semaphore.
The thread will now own a lock on the semaphore; the call will return successfully. If the semaphore is
unavailable at the time its value is checked, then the call returns -1 with errno set to EAGAIN.
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 these operations. The semaphore will be locked upon success-
ful return and will stay locked until it is explicitly released by a call to sem_post().
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_wait() will lock the semaphore sem .
sem_wait(sem);
The following call to sem_trywait()
will lock the semaphore sem , if it is available.
sem_trywait(sem);
RETURN VALUE
A successful call to sem_wait() will return 0 and the calling thread will then own a lock on the sema-
phore. Otherwise, the call to sem_wait() will return -1 with errno set to the appropriate value of the
error condition.
A successful call to
sem_trywait()
will return 0, if the semaphore was available and the calling
thread was able to lock the semaphore. Otherwise, the call to
sem_trywait()
will return -1 with errno
set to the appropriate value of the error condition.
ERRORS
sem_wait() and sem_trywait() fail and do not perform the requested operation if any of the follow-
ing conditions are encountered:
[EPERM] The calling process does not have the privileges necessary to lock the semaphore.
[EAGAIN] The semaphore was not available and hence could not be locked by
sem_trywait(). This error condition only occurs in sem_trywait().
[EINVAL] The argument sem does not refer to a valid semaphore.
[EINTR] The function was interrupted by a signal
SEE ALSO
sem_post(2), <semaphore.h>.
STANDARDS CONFORMANCE
sem_wait(),sem_trywait(): POSIX
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)