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sem_trywait(2) [osx man page]

SEM_WAIT(2)						      BSD System Calls Manual						       SEM_WAIT(2)

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

Check Out this Related Man Page

sem_wait(3)						     Library Functions Manual						       sem_wait(3)

sem_wait, sem_trywait - Performs (or conditionally performs) a semaphore lock (P1003.1b) LIBRARY
Realtime Library (, librt.a) SYNOPSIS
#include <semaphore.h> int sem_wait ( sem_t *sem); int sem_trywait ( sem_t *sem); PARAMETERS
sem Specifies a pointer to the semaphore to be locked. DESCRIPTION
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. RETURN VALUES
If the sem_wait or sem_trywait function returns successfully, a value of 0 (zero) is returned and the function executes the semaphore lock operation. 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 unchanged. ERRORS
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. RELATED INFORMATION
Functions: sem_post(3) Guide to Realtime Programming delim off sem_wait(3)

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