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SHM_OPEN(3) Linux Programmer's Manual SHM_OPEN(3)
NAME
shm_open, shm_unlink - Create/open or unlink POSIX shared memory objects
SYNOPSIS
#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.
DESCRIPTION
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-termi-
nated 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 atomi-
cally.
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).
RETURN VALUE
On success, shm_open() returns a non-negative file descriptor. On failure,
shm_open() returns -1. shm_unlink() returns 0 on success, or -1 on error.
ERRORS
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.
VERSIONS
These functions are provided in glibc 2.2 and later.
CONFORMING TO
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".
NOTES
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.
SEE ALSO
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2), mmap(2), open(2),
umask(2), shm_overview(7)
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2009-02-25 SHM_OPEN(3)
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