sem_init — initialize an unnamed semaphore
#include <semaphore.h>
int sem_init(sem_t *sem, int pshared, unsigned value);
The sem_init() function shall initialize the unnamed semaphore referred to by sem. The value of the initialized semaphore shall be value. Following a successful call to sem_init(), the semaphore can be used in subsequent calls to sem_clockwait(), sem_destroy(), sem_post(), sem_timedwait(), sem_trywait(), and sem_wait(). This semaphore shall remain usable until the semaphore is destroyed. An unnamed semaphore may be implemented using a file descriptor.
If the pshared argument has a non-zero value, then the semaphore is shared between processes; in this case, any process that can access the semaphore sem can use sem for performing sem_clockwait(), sem_destroy(), sem_post(), sem_timedwait(), sem_trywait(), and sem_wait() operations.
If the pshared argument is zero, then the semaphore is shared between threads of the process; any thread in this process can use sem for performing sem_clockwait(), sem_destroy(), sem_post(), sem_timedwait(), sem_trywait(), and sem_wait() operations.
See 2.9.9 Synchronization Object Copies and Alternative Mappings for further requirements.
Attempting to initialize an already initialized semaphore results in undefined behavior.
Upon successful completion, the sem_init() function shall initialize the semaphore in sem and return 0. Otherwise, it shall return -1 and set errno to indicate the error.
The sem_init() function shall fail if:
- [EINVAL]
- The value argument exceeds {SEM_VALUE_MAX}.
- [ENOSPC]
- A resource required to initialize the semaphore has been exhausted, or the limit on semaphores ({SEM_NSEMS_MAX}) has been reached.
- [EPERM]
- The process lacks appropriate privileges to initialize the semaphore.
The sem_init() function may fail if:
- [EMFILE]
- All file descriptors available to the process are currently open.
- [ENFILE]
- The maximum allowable number of files is currently open in the system.
None.
None.
None.
None.
sem_clockwait , sem_destroy , sem_post , sem_trywait
XBD <semaphore.h>
First released in Issue 5. Included for alignment with the POSIX Realtime Extension.
The sem_init() function is marked as part of the Semaphores option.
The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Semaphores option.
The sem_timedwait() function is added to the SEE ALSO section for alignment with IEEE Std 1003.1d-1999.
IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/116 is applied, updating the DESCRIPTION to add the sem_timedwait() function for alignment with IEEE Std 1003.1d-1999.
SD5-XSH-ERN-176 is applied.
The sem_init() function is moved from the Semaphores option to the Base.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0526 [37] is applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0318 [972] is applied.
Austin Group Defect 368 is applied, adding a statement that an unnamed semaphore may be implemented using a file descriptor and adding the [EMFILE] and [ENFILE] errors.
Austin Group Defect 1216 is applied, adding sem_clockwait().
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