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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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 be mmap(2)ed only 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 per-process limit on the number of open file descriptors
has been reached.
ENAMETOOLONG
The length of name exceeds PATH_MAX.
ENFILE The system-wide limit on the total number of open files 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.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────┬───────────────┬────────────────┐
│Interface │ Attribute │ Value │
├─────────────────────────┼───────────────┼────────────────┤
│shm_open(), shm_unlink() │ Thread safety │ MT-Safe locale │
└─────────────────────────┴───────────────┴────────────────┘
POSIX.1-2001, POSIX.1-2008.
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.1-2008 says that the group
ownership may be set to either the calling process's effective group
ID or, if the object is visible in the filesystem, the group ID of
the parent directory.
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 makes use of a
dedicated tmpfs(5) filesystem that is normally mounted under
/dev/shm.
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2),
memfd_create(2), mmap(2), open(2), umask(2), shm_overview(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 SHM_OPEN(3)
Pages that refer to this page: execve(2), memfd_create(2), mmap2(2), mmap(2), umask(2), sem_init(3), tmpfs(5), file-hierarchy(7), sem_overview(7), shm_overview(7)