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.\" $FreeBSD: src/lib/libc/gen/shm_open.3,v 1.3.2.5 2001/12/14 18:33:51 ru Exp $
.\" $DragonFly: src/lib/libc/gen/shm_open.3,v 1.3 2004/03/11 12:28:50 hmp Exp $
.\"
.Dd March 24, 2000
.Dt SHM_OPEN 3
.Os
.Sh NAME
.Nm shm_open
.Nd open or create a shared memory object
.Nm shm_unlink
.Nd remove a shared memory object
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/mman.h
.Ft int
.Fn shm_open "const char *path" "int flags" "mode_t mode"
.Ft int
.Fn shm_unlink "const char *path"
.Sh DESCRIPTION
The
.Fn shm_open
function opens (or optionally creates) a
.Tn POSIX
shared memory object named
.Fa path .
The
.Fn shm_unlink
function removes a shared memory object named
.Fa path .
.Pp
In the
.Dx
implementation,
.Tn POSIX
shared memory objects are implemented as ordinary files.
The
.Fn shm_open
and
.Fn shm_unlink
act as wrappers around the
.Xr open 2
and
.Xr unlink 2
routines, and
.Fa path ,
.Fa flags ,
and
.Fa mode
arguments are as specified for those functions.
The
.Fa flags
argument is checked to ensure that the access mode specified is not
.Dv O_WRONLY
(which is not defined for shared memory objects).
.Pp
In addition, the
.Dx
implementation causes
.Fn mmap
of a descriptor returned by
.Fn shm_open
to behave as if the
.Dv MAP_NOSYNC
flag had been specified to
.Xr mmap 2 .
(It does so by setting a special file flag using
.Xr fcntl 2 . )
.Pp
The
.Fn shm_unlink
function makes no effort to ensure that
.Fa path
refers to a shared memory object.
.Sh COMPATIBILITY
The
.Fa path
argument does not necessarily represent a pathname (although it does in this
and most other implementations).
Two processes opening the same
.Fa path
are guaranteed to access the same shared memory object if and only if
.Fa path
begins with a slash
.Pq Ql \&/
character.
.Pp
Only the
.Dv O_RDONLY ,
.Dv O_RDWR ,
.Dv O_CREAT ,
.Dv O_EXCL ,
and
.Dv O_TRUNC
flags may be used in portable programs.
.Pp
The result of using
.Xr open 2 ,
.Xr read 2 ,
or
.Xr write 2
on a shared memory object, or on the descriptor returned by
.Fn shm_open ,
is undefined.
It is also undefined whether the shared memory object itself, or its
contents, persist across reboots.
.Sh RETURN VALUES
If successful,
.Fn shm_open
returns a non-negative integer;
.Fn shm_unlink
returns zero.
Both functions return -1 on failure, and set
.Va errno
to indicate the error.
.Sh ERRORS
The
.Fn shm_open
and
.Fn shm_unlink
functions can fail with any error defined for
.Fn open
and
.Fn unlink ,
respectively.  In addition, the following errors are defined for
.Fn shm_open :
.Bl -tag -width Er
.It Bq Er EINVAL
The object named by
.Fa path
is not a shared memory object
(i.e., it is not a regular file).
.It Bq Er EINVAL
The
.Fa flags
argument to
.Fn shm_open
specifies an access mode of
.Dv O_WRONLY .
.El
.Sh SEE ALSO
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr open 2 ,
.Xr unlink 2
.Sh STANDARDS
The
.Fn shm_open
and
.Fn shm_unlink
functions are believed to conform to
.St -p1003.1b-93 .
.Sh HISTORY
The
.Fn shm_open
and
.Fn shm_unlink
functions first appeared in
.Fx 4.3 .
.Sh AUTHORS
.An Garrett A. Wollman Aq wollman@FreeBSD.org
(C library support and this manual page)
.Pp
.An Matthew Dillon Aq dillon@FreeBSD.org
.Pq Dv MAP_NOSYNC