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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 $
.\"
.Dd June 28, 2019
.Dt SHM_OPEN 3
.Os
.Sh NAME
.Nm shm_open ,
.Nm shm_unlink
.Nd POSIX shared memory object operations
.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 .
Using the same
.Fa path
allows unrelated processes to access the same object.
.Sh IMPLEMENTATION NOTES
.Dx
mounts a
.Xr tmpfs 5
file system at
.Pa /var/run/shm .
.Tn POSIX
shared memory objects are implemented as ordinary files under
that directory.
The
.Fn shm_open
and
.Fn shm_unlink
functions act as wrappers around
.Xr open 2
and
.Xr unlink 2 .
Any leading slash
.Pq Ql \&/
characters are removed from
.Fa path
to make it relative to
.Pa /var/run/shm .
The
.Fa flags
and
.Fa mode
arguments are passed through unaltered.
.Fa flags
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 . )
.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 COMPATIBILITY
On
.Dx
and many other operating systems the
.Fa path
argument is interpreted as a file system pathname under a special
directory where a memory-backed file system is mounted.
Most operating systems do some name mangling to
.Fa path .
Leading slashes are commonly removed to turn an absolute pathname into
a relative one.
Problematic characters may be escaped and there may be a length limit on
.Fa path .
On some systems the mangled pathname is completely different from the
given
.Fa path .
On a few systems, shared memory objects live outside the ordinary file
system in their own dedicated namespace.
.Pp
According to
.Tn POSIX
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.
The most portable form of pathname is probably
.Ql /foobar ,
i.e.\& one leading slash, no other slashes and no dots.
.Pp
The result of using
.Xr open 2
on the pathname of a shared memory object,
or using
.Xr read 2
or
.Xr write 2
on a file descriptor returned by
.Fn shm_open ,
is undefined by
.Tn POSIX .
It is also undefined whether the shared memory object itself, or its
contents, persist across reboots.
On
.Dx
and most other systems they do not.
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.
.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 ,
.Xr tmpfs 5
.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 Mt wollman@FreeBSD.org
(C library support and this manual page)
.An Matthew Dillon Aq Mt dillon@FreeBSD.org
.Pq Dv MAP_NOSYNC