Manual browser: sem_open(3)

SEM_OPEN(3) Library Functions Manual SEM_OPEN(3)


sem_open, sem_close, sem_unlinknamed semaphore operations


POSIX Real-time Library (librt, -lrt)


#include <semaphore.h>

sem_t *
sem_open(const char *name, int oflag, ...);

sem_close(sem_t *sem);

sem_unlink(const char *name);


The sem_open() function creates or opens the named semaphore specified by name. The returned semaphore may be used in subsequent calls to sem_getvalue(3), sem_timedwait(3), sem_trywait(3), sem_wait(3), sem_post(3), and sem_close().

The following bits may be set in the oflag argument:

Create the semaphore if it does not already exist.

The third argument to the call to sem_open() must be of type mode_t and specifies the mode for the semaphore. Only the S_IWUSR, S_IWGRP, and S_IWOTH bits are examined; it is not possible to grant only “read” permission on a semaphore. The mode is modified according to the process's file creation mask; see umask(2).

The fourth argument must be an unsigned int and specifies the initial value for the semaphore, and must be no greater than SEM_VALUE_MAX.

Create the semaphore if it does not exist. If the semaphore already exists, sem_open() will fail. This flag is ignored unless O_CREAT is also specified.

The sem_close() function closes a named semaphore that was opened by a call to sem_open().

The sem_unlink() function removes the semaphore named name. Resources allocated to the semaphore are only deallocated when all processes that have the semaphore open close it.


If successful, the sem_open() function returns the address of the opened semaphore. If the same name argument is given to multiple calls to sem_open() by the same process without an intervening call to sem_close(), the same address is returned each time. If the semaphore cannot be opened, sem_open() returns SEM_FAILED and the global variable errno is set to indicate the error.

The sem_close() and sem_unlink() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The sem_open() function will fail if:
The semaphore exists and the permissions specified by oflag at the time it was created deny access to this process; or the semaphore does not exist, but permission to create it is denied.
O_CREAT and O_EXCL are set but the semaphore already exists.
The call was interrupted by a signal.
The sem_open() operation is not supported for the given name; or the value argument is greater than SEM_VALUE_MAX.
The name argument is too long.
The system limit on semaphores has been reached.
O_CREAT is not set and the named semaphore does not exist.
There is not enough space to create the semaphore.

The sem_close() function will fail if:

The sem argument is not a valid semaphore.

The sem_unlink() function will fail if:

Permission is denied to unlink the semaphore.
The specified name is too long.
The named semaphore does not exist.


The sem_open(), sem_close(), and sem_unlink() functions conform to ISO/IEC 9945-1:1996 (“POSIX.1”).


Support for named semaphores first appeared in NetBSD 2.0.


This implementation places strict requirements on the value of name: it must begin with a slash (‘/’), contain no other slash characters, and be less than 14 characters in length not including the terminating null character.
February 29, 2012 NetBSD 7.0