SEM_OPEN(3)						   BSD Library Functions Manual 					       SEM_OPEN(3)

NAME

     sem_open, sem_close, sem_unlink -- named semaphore operations

LIBRARY

     Standard C Library (libc, -lc)

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.

     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.

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 ISO/IEC 9945-1:1996 (``POSIX.1'').

HISTORY

     Support for named semaphores first appeared in FreeBSD 5.0.

BSD
								  January 9, 2010							       BSD