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Manual Pages  — SEM_OPEN


sem_open, sem_close, sem_unlink – named semaphore operations



Standard C Library (libc, -lc)


#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_wait(3), sem_trywait(3), sem_post(3), and sem_close().

This implementation places strict requirements on the value of name: it must begin with a slash (‘/’) and contain no other slash characters.

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.

O_EXCL 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_closeand 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.
  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.
  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 but 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.


close(2), open(2), umask(2), unlink(2), sem_getvalue(3), sem_post(3), sem_trywait(3), sem_wait(3)


The sem_open(), sem_close(), and sem_unlink() functions conform to IEEE Std 1003.1-96 ("POSIX.1").


Support for named semaphores first appeared in FreeBSD 5.0 .

SEM_OPEN (3) January 9, 2010

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