The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


sem_unlink - remove a named semaphore


#include <semaphore.h>

int sem_unlink(const char *


The sem_unlink() function shall remove the semaphore named by the string name. If the semaphore named by name is currently referenced by other processes, then sem_unlink() shall have no effect on the state of the semaphore. If one or more processes have the semaphore open when sem_unlink() is called, destruction of the semaphore is postponed until all references to the semaphore have been destroyed by calls to sem_close(), _exit(), or exec. Calls to sem_open() to recreate or reconnect to the semaphore refer to a new semaphore after sem_unlink() is called. The sem_unlink() call shall not block until all references have been destroyed; it shall return immediately.


Upon successful completion, the sem_unlink() function shall return a value of 0. Otherwise, the semaphore shall not be changed and the function shall return a value of -1 and set errno to indicate the error.


The sem_unlink() function shall fail if:

Permission is denied to unlink the named semaphore.
The named semaphore does not exist.

The sem_unlink() function may fail if:

The length of the name argument exceeds {_POSIX_PATH_MAX} on systems that do not support the XSI option [XSI] [Option Start]  or exceeds {_XOPEN_PATH_MAX} on XSI systems, [Option End]  or has a pathname component that is longer than {_POSIX_NAME_MAX} on systems that do not support the XSI option [XSI] [Option Start]  or longer than {_XOPEN_NAME_MAX} on XSI systems. [Option End]  A call to sem_unlink() with a name argument that contains the same semaphore name as was previously used in a successful sem_open() call shall not give an [ENAMETOOLONG] error.

The following sections are informative.








A future version might require the sem_open() and sem_unlink() functions to have semantics similar to normal file system operations.


semctl, semget, semop, sem_close, sem_open

XBD <semaphore.h>


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

Issue 6

The sem_unlink() 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.

Issue 7

Austin Group Interpretation 1003.1-2001 #077 is applied, changing [ENAMETOOLONG] from a ``shall fail'' to a ``may fail'' error.

Austin Group Interpretation 1003.1-2001 #141 is applied, adding FUTURE DIRECTIONS.

The sem_unlink() function is moved from the Semaphores option to the Base.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0531 [37] is applied.

End of informative text.


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