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SEM_OPEN(3) Library Functions Manual SEM_OPEN(3)

sem_open, sem_close, sem_unlink — named semaphore operations

library “libc”

#include <semaphore.h>

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

int
sem_close(sem_t *sem);

int
sem_unlink(const char *name);

The () 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 () 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, () will fail. This flag is ignored unless O_CREAT is also specified.

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

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.
[]
The semaphore does not exist, but permission to create it is denied.
[]
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.
[]
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 ISO/IEC 9945-1:1996 (“POSIX.1”).

Support for named semaphores first appeared in FreeBSD 5.0.

January 9, 2010 FreeBSD-12.0