The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

#### 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.

The following sections are informative.

#### 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.

End of informative text.

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