NAME

sem_init - initialize an unnamed semaphore (REALTIME)

SYNOPSIS

[SEM] #include <semaphore.h>

int sem_init(sem_t *sem, int pshared, unsigned value);

DESCRIPTION

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 may be used in subsequent calls to sem_wait(), ^[TMO] sem_timedwait(), sem_trywait(), sem_post(), and sem_destroy(). This semaphore shall remain usable until the semaphore is destroyed.

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_wait(), ^[TMO] sem_timedwait(), sem_trywait(), sem_post(), and sem_destroy() operations.

Only sem itself may be used for performing synchronization. The result of referring to copies of sem in calls to sem_wait(), ^[TMO] sem_timedwait(), sem_trywait(), sem_post(), and sem_destroy() is undefined.

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_wait(), ^[TMO] sem_timedwait(), sem_trywait(), sem_post(), and sem_destroy() operations. The use of the semaphore by threads other than those created in the same process is undefined.

Attempting to initialize an already initialized semaphore results in undefined behavior.

RETURN VALUE

Upon successful completion, the sem_init() function shall initialize the semaphore in sem. Otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

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 the appropriate privileges to initialize the semaphore.

EXAMPLES

None.

APPLICATION USAGE

The sem_init() function is part of the Semaphores option and need not be available on all implementations.

RATIONALE

Although this volume of IEEE Std 1003.1-2001 fails to specify a successful return value, it is likely that a later version may require the implementation to return a value of zero if the call to sem_init() is successful.

FUTURE DIRECTIONS

None.

SEE ALSO

sem_destroy(), sem_post(), sem_timedwait(), sem_trywait(), sem_wait(), the Base Definitions volume of IEEE Std 1003.1-2001, <semaphore.h>

CHANGE HISTORY

First released in Issue 5. Included for alignment with the POSIX Realtime Extension.

Issue 6

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.