SHM_OPEN(3)

NAME
shm_open - open or create a shared memory object shm_unlink - remove a shared memory object

LIBRARY
libc

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 function opens (or optionally creates) a POSIX shared memory object named path. The shm_unlink function removes a shared memory object named path.

In the FreeBSD implementation, POSIX shared memory objects are implemented as ordinary files. The shm_open and shm_unlink act as wrappers around the open(2) and unlink(2) routines, and path, flags, and mode arguments are as specified for those functions. The flags argument is checked to ensure that the access mode specified is not O_WRONLY (which is not defined for shared memory objects).

In addition, the FreeBSD implementation causes mmap() of a descriptor returned by shm_open to behave as if the MAP_NOSYNC flag had been specified to mmap(2). (It does so by setting a special file flag using fcntl(2).)

The shm_unlink function makes no effort to ensure that path refers to a shared memory object.

PORTABILITY
The path argument does not necessarily represent a pathname (although it does in this and 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.

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

ERRORS
The shm_open() and shm_unlink() functions can fail with any error defined for open() and unlink(), respectively. In addition, the following errors are defined for shm_open():

[EINVAL]
    The object named by path is not a shared memory object (i.e., it is not a regular file).

[EINVAL]
    The flags argument to shm_open() specifies an access mode of O_WRONLY.

SEE ALSO
mmap(2), munmap(2), open(2), unlink(2)

STANDARDS
The shm_open and shm_unlink functions are believed to conform to p1003.1b.

HISTORY
The shm_open and shm_unlink functions first appeared in FreeBSD 5.0.

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

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