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SEM_OPEN(3)	       FreeBSD 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 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.

FreeBSD	10.1			January	9, 2010			  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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