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SHM_OPEN(2)		  FreeBSD System Calls Manual		   SHM_OPEN(2)

NAME
     shm_open, shm_unlink -- shared memory object operations

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/mman.h>

     int
     shm_open(const char *path,	int flags, mode_t mode);

     int
     shm_unlink(const char *path);

DESCRIPTION
     The shm_open() system call	opens (or optionally creates) a	POSIX shared
     memory object named path.	The flags argument contains a subset of	the
     flags used	by open(2).  An	access mode of either O_RDONLY or O_RDWR must
     be	included in flags.  The	optional flags O_CREAT,	O_EXCL,	and O_TRUNC
     may also be specified.

     If	O_CREAT	is specified, then a new shared	memory object named path will
     be	created	if it does not exist.  In this case, the shared	memory object
     is	created	with mode mode subject to the process' umask value.  If	both
     the O_CREAT and O_EXCL flags are specified	and a shared memory object
     named path	already	exists,	then shm_open()	will fail with EEXIST.

     Newly created objects start off with a size of zero.  If an existing
     shared memory object is opened with O_RDWR	and the	O_TRUNC	flag is	speci-
     fied, then	the shared memory object will be truncated to a	size of	zero.
     The size of the object can	be adjusted via	ftruncate(2) and queried via
     fstat(2).

     The new descriptor	is set to close	during execve(2) system	calls; see
     close(2) and fcntl(2).

     As	a FreeBSD extension, the constant SHM_ANON may be used for the path
     argument to shm_open().  In this case, an anonymous, unnamed shared mem-
     ory object	is created.  Since the object has no name, it cannot be
     removed via a subsequent call to shm_unlink().  Instead, the shared mem-
     ory object	will be	garbage	collected when the last	reference to the
     shared memory object is removed.  The shared memory object	may be shared
     with other	processes by sharing the file descriptor via fork(2) or
     sendmsg(2).  Attempting to	open an	anonymous shared memory	object with
     O_RDONLY will fail	with EINVAL.  All other	flags are ignored.

     The shm_unlink() system call removes a shared memory object named path.

RETURN VALUES
     If	successful, shm_open() returns a non-negative integer, and
     shm_unlink() returns zero.	 Both functions	return -1 on failure, and set
     errno to indicate the error.

COMPATIBILITY
     The path argument does not	necessarily represent a	pathname (although it
     does in most other	implementations).  Two processes opening the same path
     are guaranteed to access the same shared memory object if and only	if
     path begins with a	slash (`/') character.

     Only the O_RDONLY,	O_RDWR,	O_CREAT, O_EXCL, and O_TRUNC flags may be used
     in	portable programs.

     The result	of using open(2), read(2), or write(2) on a shared memory
     object, or	on the descriptor returned by shm_open(), is undefined.	 It is
     also undefined whether the	shared memory object itself, or	its contents,
     persist across reboots.

     In	FreeBSD, read(2) and write(2) on a shared memory object	will fail with
     EOPNOTSUPP	and neither shared memory objects nor their contents persist
     across reboots.

ERRORS
     The following errors are defined for shm_open():

     [EINVAL]		A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL,
			or O_TRUNC was included	in flags.

     [EMFILE]		The process has	already	reached	its limit for open
			file descriptors.

     [ENFILE]		The system file	table is full.

     [EINVAL]		O_RDONLY was specified while creating an anonymous
			shared memory object via SHM_ANON.

     [EFAULT]		The path argument points outside the process' allo-
			cated address space.

     [ENAMETOOLONG]	The entire pathname exceeded 1023 characters.

     [EINVAL]		The path does not begin	with a slash (`/') character.

     [ENOENT]		O_CREAT	is specified and the named shared memory
			object does not	exist.

     [EEXIST]		O_CREAT	and O_EXCL are specified and the named shared
			memory object dies exist.

     [EACCES]		The required permissions (for reading or reading and
			writing) are denied.

     The following errors are defined for shm_unlink():

     [EFAULT]		The path argument points outside the process' allo-
			cated address space.

     [ENAMETOOLONG]	The entire pathname exceeded 1023 characters.

     [ENOENT]		The named shared memory	object does not	exist.

     [EACCES]		The required permissions are denied.  shm_unlink()
			requires write permission to the shared	memory object.

SEE ALSO
     close(2), ftruncate(2), fstat(2), mmap(2),	munmap(2)

STANDARDS
     The shm_open() and	shm_unlink() functions are believed to conform to IEEE
     Std 1003.1b-1993 (``POSIX.1'').

HISTORY
     The shm_open() and	shm_unlink() functions first appeared in FreeBSD 4.3.
     The functions were	reimplemented as system	calls using shared memory
     objects directly rather than files	in FreeBSD 7.0.

AUTHORS
     Garrett A.	Wollman	<wollman@FreeBSD.org> (C library support and this man-
     ual page)

     Matthew Dillon <dillon@FreeBSD.org> (MAP_NOSYNC)

FreeBSD	10.1			March 20, 2007			  FreeBSD 10.1

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

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