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sem_trywait(3p) [centos man page]

SEM_TRYWAIT(3P) 					     POSIX Programmer's Manual						   SEM_TRYWAIT(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond- ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. NAME
sem_trywait, sem_wait - lock a semaphore (REALTIME) SYNOPSIS
#include <semaphore.h> int sem_trywait(sem_t *sem); int sem_wait(sem_t *sem); DESCRIPTION
The sem_trywait() function shall lock the semaphore referenced by sem only if the semaphore is currently not locked; that is, if the sema- phore value is currently positive. Otherwise, it shall not lock the semaphore. The sem_wait() function shall lock the semaphore referenced by sem by performing a semaphore lock operation on that semaphore. If the sema- phore value is currently zero, then the calling thread shall not return from the call to sem_wait() until it either locks the semaphore or the call is interrupted by a signal. Upon successful return, the state of the semaphore shall be locked and shall remain locked until the sem_post() function is executed and returns successfully. The sem_wait() function is interruptible by the delivery of a signal. RETURN VALUE
The sem_trywait() and sem_wait() functions shall return zero if the calling process successfully performed the semaphore lock operation on the semaphore designated by sem. If the call was unsuccessful, the state of the semaphore shall be unchanged, and the function shall return a value of -1 and set errno to indicate the error. ERRORS
The sem_trywait() and sem_wait() functions shall fail if: EAGAIN The semaphore was already locked, so it cannot be immediately locked by the sem_trywait() operation ( sem_trywait() only). EINVAL The sem argument does not refer to a valid semaphore. The sem_trywait() and sem_wait() functions may fail if: EDEADLK A deadlock condition was detected. EINTR A signal interrupted this function. The following sections are informative. EXAMPLES
None. APPLICATION USAGE
Applications using these functions may be subject to priority inversion, as discussed in the Base Definitions volume of IEEE Std 1003.1-2001, Section 3.285, Priority Inversion. The sem_trywait() and sem_wait() functions are part of the Semaphores option and need not be provided on all implementations. RATIONALE
None. FUTURE DIRECTIONS
None. SEE ALSO
semctl(), semget(), semop(), sem_post(), sem_timedwait(), the Base Definitions volume of IEEE Std 1003.1-2001, <semaphore.h> COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol- ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . IEEE
/The Open Group 2003 SEM_TRYWAIT(3P)

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) LIBRARY
Realtime Library (librt.so, 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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