SEM_WAIT(3) BSD Library Functions Manual SEM_WAIT(3)NAME
sem_wait, sem_trywait -- decrement (lock) a semaphore
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <semaphore.h>
int
sem_wait(sem_t *sem);
int
sem_trywait(sem_t *sem);
DESCRIPTION
The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-
zero and the value can be decremented.
The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not
decremented and an error is returned.
RETURN VALUES
The sem_wait() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indi-
cate the error.
ERRORS
sem_wait() and sem_trywait() will fail if:
[EINVAL] sem points to an invalid semaphore.
Additionally, sem_trywait() will fail if:
[EAGAIN] The semaphore value was zero, and thus could not be decremented.
SEE ALSO sem_post(3)STANDARDS
sem_wait() and sem_trywait() conform to ISO/IEC 9945-1:1996 (``POSIX.1'').
BSD January 22, 2003 BSD
Check Out this Related Man Page
SEM_WAIT(2) BSD System Calls Manual SEM_WAIT(2)NAME
sem_trywait, sem_wait -- lock a semaphore
SYNOPSIS
#include <semaphore.h>
int
sem_trywait(sem_t *sem);
int
sem_wait(sem_t *sem);
DESCRIPTION
The semaphore referenced by sem is locked. When calling sem_wait(), if the semaphore's value is zero, the calling thread will block until
the lock is acquired or until the call is interrupted by a signal. Alternatively, the sem_trywait() function will fail if the semaphore is
already locked, rather than blocking on the semaphore.
If successful (the lock was acquired), sem_wait() and sem_trywait() will return 0. Otherwise, -1 is returned and errno is set, and the state
of the semaphore is unchanged.
ERRORS
sem_wait() and sem_trywait() succeed unless:
[EAGAIN] The semaphore is already locked.
[EDEADLK] A deadlock was detected.
[EINTR] The call was interrupted by a signal.
[EINVAL] sem is not a valid semaphore descriptor.
NOTES
Applications may encounter a priority inversion while using semaphores. When a thread is waiting on a semaphore which is about to be posted
by a lower-priority thread and the lower-priority thread is preempted by another thread (of medium priority), a priority inversion has
occured, and the higher-priority thread will be blocked for an unlimited time period. Programmers using the realtime functionality of the
system should take care to avoid priority inversions.
SEE ALSO sem_open(2), sem_post(2), semctl(2), semget(2), semop(2)HISTORY
sem_wait() and sem_trywait() are specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).
Darwin June 8, 2000 Darwin
Hi there,
Could someone please confirm which POSIX semaphore routines should be used for a multiprocess (and not multithreaded) environment?
sys/sem.h definitely works. but the routines, semget, semctl, semop are pretty unwieldy. So, I am looking for an easier way out.
From the man pages... (2 Replies)
Apparently there are three similar features in the linux API. Can someone explain the difference between the data structures that are initialized by these functions:
(1) pthread_mutex_init
(2) sem_init
(3) sem_get
Thanks,
Siegfried (5 Replies)
I thought one used the select function to wait on multiple semaphores but according to http://opengroup.org/onlinepubs/007908799/xsh/select.html, the select function is only good for I/O features (like pipes and fifos).
How can I block for multiple semaphores? What if I have a mixture of things... (1 Reply)
Hi all
How i can create thread in SUSPENDED mode, and resume when i want.
I have win code with CreateThread (parameter CREATE_SUSPEND), and Resume Thread, but on POSIX with pthread i cant do it.
Please help me.
Best regards
// Kolesar (5 Replies)
i'm using smitty to create user...what happen is it prompt me "failed" with error
3004-703 Check "/etc/security/login.cfg" file.
3004-691 Error changing "shell".
3004-703 Check "/usr/lib/security/mkuser.default" file.
3004-721 Could not create user.
3004-703 Check... (13 Replies)