| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
SHM_OPEN(3) Linux Programmer's Manual SHM_OPEN(3)
shm_open, shm_unlink - Create/open or unlink POSIX shared memory objects
#include <sys/mman.h>
#include <sys/stat.h> /* For mode constants */
#include <fcntl.h> /* For O_* constants */
int shm_open(const char *name, int oflag, mode_t mode);
int shm_unlink(const char *name);
Link with -lrt.
shm_open() creates and opens a new, or opens an existing, POSIX shared memory
object. A POSIX shared memory object is in effect a handle which can be used
by unrelated processes to mmap(2) the same region of shared memory. The
shm_unlink() function performs the converse operation, removing an object
previously created by shm_open().
The operation of shm_open() is analogous to that of open(2). name specifies
the shared memory object to be created or opened. For portable use, a shared
memory object should be identified by a name of the form /somename; that is, a
null-terminated string of up to NAME_MAX (i.e., 255) characters consisting of
an initial slash, followed by one or more characters, none of which are
slashes.
oflag is a bit mask created by ORing together exactly one of O_RDONLY or
O_RDWR and any of the other flags listed here:
O_RDONLY Open the object for read access. A shared memory object opened in
this way can only be mmap(2)ed for read (PROT_READ) access.
O_RDWR Open the object for read-write access.
O_CREAT Create the shared memory object if it does not exist. The user and
group ownership of the object are taken from the corresponding
effective IDs of the calling process, and the object's permission
bits are set according to the low-order 9 bits of mode, except that
those bits set in the process file mode creation mask (see
umask(2)) are cleared for the new object. A set of macro constants
which can be used to define mode is listed in open(2). (Symbolic
definitions of these constants can be obtained by including
<sys/stat.h>.)
A new shared memory object initially has zero length -- the size of
the object can be set using ftruncate(2). The newly allocated
bytes of a shared memory object are automatically initialized to 0.
O_EXCL If O_CREAT was also specified, and a shared memory object with the
given name already exists, return an error. The check for the
existence of the object, and its creation if it does not exist, are
performed atomically.
O_TRUNC If the shared memory object already exists, truncate it to zero
bytes.
Definitions of these flag values can be obtained by including <fcntl.h>.
On successful completion shm_open() returns a new file descriptor referring to
the shared memory object. This file descriptor is guaranteed to be the
lowest-numbered file descriptor not previously opened within the process. The
FD_CLOEXEC flag (see fcntl(2)) is set for the file descriptor.
The file descriptor is normally used in subsequent calls to ftruncate(2) (for
a newly created object) and mmap(2). After a call to mmap(2) the file
descriptor may be closed without affecting the memory mapping.
The operation of shm_unlink() is analogous to unlink(2): it removes a shared
memory object name, and, once all processes have unmapped the object, de-
allocates and destroys the contents of the associated memory region. After a
successful shm_unlink(), attempts to shm_open() an object with the same name
will fail (unless O_CREAT was specified, in which case a new, distinct object
is created).
On success, shm_open() returns a nonnegative file descriptor. On failure,
shm_open() returns -1. shm_unlink() returns 0 on success, or -1 on error.
On failure, errno is set to indicate the cause of the error. Values which may
appear in errno include the following:
EACCES Permission to shm_unlink() the shared memory object was denied.
EACCES Permission was denied to shm_open() name in the specified mode, or
O_TRUNC was specified and the caller does not have write permission on
the object.
EEXIST Both O_CREAT and O_EXCL were specified to shm_open() and the shared
memory object specified by name already exists.
EINVAL The name argument to shm_open() was invalid.
EMFILE The process already has the maximum number of files open.
ENAMETOOLONG
The length of name exceeds PATH_MAX.
ENFILE The limit on the total number of files open on the system has been
reached.
ENOENT An attempt was made to shm_open() a name that did not exist, and
O_CREAT was not specified.
ENOENT An attempt was to made to shm_unlink() a name that does not exist.
These functions are provided in glibc 2.2 and later.
POSIX.1-2001.
POSIX.1-2001 says that the group ownership of a newly created shared memory
object is set to either the calling process's effective group ID or "a system
default group ID".
POSIX leaves the behavior of the combination of O_RDONLY and O_TRUNC
unspecified. On Linux, this will successfully truncate an existing shared
memory object -- this may not be so on other UNIX systems.
The POSIX shared memory object implementation on Linux 2.4 makes use of a
dedicated file system, which is normally mounted under /dev/shm.
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2), mmap(2),
open(2), umask(2), shm_overview(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-02-25 SHM_OPEN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface