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(3) Library Functions Manual sem_wait(3)NAME
sem_wait, sem_trywait - Performs (or conditionally performs) a semaphore lock (P1003.1b)
Realtime Library (librt.so, librt.a)
int sem_wait (
int sem_trywait (
sem Specifies a pointer to the semaphore to be locked.
The sem_wait function locks the semaphore referenced by sem by performing a semaphore lock operation on it. If the semaphore value is zero,
the sem_wait function blocks until it either locks the semaphore or is interrupted by a signal.
The sem_trywait function locks a semaphore only if the semaphore is currently not locked. If the semaphore value is zero, the sem_trywait
function returns without locking the semaphore.
These functions help ensure that the resource associated with the semaphore cannot be accessed by other processes. The semaphore remains
locked until the process unlocks it with a call to the sem_post function.
Use the sem_wait function instead of the sem_trywait function if the process should wait for access to the semaphore.
If the sem_wait or sem_trywait function returns successfully, a value of 0 (zero) is returned and the function executes the semaphore lock
On an unsuccessful call, a value of -1 is returned and errno is set to indicate that an error occurred. The state of the semaphore remains
The sem_wait and sem_trywait functions fail under the following conditions:
[EAGAIN] The semaphore was already locked and cannot be locked by the sem_trywait operation (sem_trywait only).
[EINTR] A signal interrupted this function.
[EINVAL] The sem argument does not refer to a valid semaphore.
Guide to Realtime Programming delim off
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:
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)
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.
// 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)