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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().

     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 pos‐
	      sible 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 success‐
     ful; 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), sem(4)

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.

BUGS
     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.

BSD			       January 15, 2003				   BSD
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