SEM_OPEN(3)

NAME
sem_open, sem_close, sem_unlink - named semaphore operations

LIBRARY
libc

SYNOPSIS
#include <semaphore.h>
sem_t *
sem_open(const char *name, int oflag, ...);
int
sem_close(sem_t *sem);
int
sem_unlink(const char *name);

DESCRIPTION
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:

O_CREAT
    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.

RETURN VALUES
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.

ERRORS
The sem_open() function will fail if:

[EACCES]
    The semaphore exists and the permissions specified by oflag at the time it was created deny access to this process.

[EACCES]
    The semaphore does not exist, but permission to create it is denied.

[EEXIST]
    O_CREAT and O_EXCL are set but the semaphore already exists.

[EINTR]
    The call was interrupted by a signal.

[EINVAL]
    The sem_open() operation is not supported for the given name.

[EINVAL]
    The value argument is greater than SEM_VALUE_MAX.

[ENAMETOOLONG]
    The name argument is too long.

[ENFILE]
    The system limit on semaphores has been reached.

[ENOENT]
    O_CREAT is not set but the named semaphore does not exist.

[ENOSPC]
    There is not enough space to create the semaphore.

The sem_close() function will fail if:

[EINVAL]
    The sem argument is not a valid semaphore.

The sem_unlink() function will fail if:

[EACCES]
    Permission is denied to unlink the semaphore.

[ENAMETOOLONG]
    The specified name is too long.

[ENOENT]
    The named semaphore does not exist.

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

STANDARDS
The sem_open(), sem_close(), and sem_unlink() functions conform to p1003.1-96.

HISTORY
Support for named semaphores first appeared in FreeBSD 5.0.