shm_open -- open or create a shared memory object shm_unlink -- remove a
shared memory object
Standard C Library (libc, -lc)
shm_open(const char *path, int flags, mode_t mode);
shm_unlink(const char *path);
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
The shm_unlink() function makes no effort to ensure that path refers to a
shared memory object.
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.
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.
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.
mmap(2), munmap(2), open(2), unlink(2)
The shm_open() and shm_unlink() functions are believed to conform to IEEE
Std 1003.1b-1993 (``POSIX.1'').
The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3.
Garrett A. Wollman <wollman@FreeBSD.org> (C library support and this manual
Matthew Dillon <dillon@FreeBSD.org> (MAP_NOSYNC)
FreeBSD 5.2.1 March 24, 2000 FreeBSD 5.2.1 [ Back ]