SEM_WAIT(3) BSD Library Functions Manual SEM_WAIT(3)NAME
sem_wait, sem_trywait -- decrement (lock) a semaphore
POSIX Real-time Library (librt, -lrt)
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.
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.
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
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.
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.
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
I have written this small program using the concept of semaphores. I am a bit confused, hope u can help me with it.
The idea is that
1. Two threads are created
2. First will be displaying 0(zero) on the screen
3. Second will be displaing 1(one) on the screen
4. This process... (2 Replies)
I am using semaphores in my program, but when I compile the program, it gives the following error
$ gcc sem.c -o sem -lpthread
Undefined first referenced
symbol in file
sem_init ... (1 Reply)
if there are many processes running and I initialize the semaphore like this:
my_sem = sem_open(SEM_NAME, O_CREAT | O_RDWR,
S_IRUSR | S_IWUSR, 10);
(the last argument is 10)
and then i use
Will 10 processes be able to access the... (1 Reply)