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); |
![[Note]](../stylesheet/note.png) |
Note |
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Link with |
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-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 definemodeis 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_CREATwas also specified, and a shared memory object with the givennamealready 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).
RETURN VALUE
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.
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()namein the specifiedmode, orO_TRUNCwas specified and the caller does not have write permission on the object. - EEXIST
-
Both
O_CREATandO_EXCLwere specified toshm_open() and the shared memory object specified bynamealready exists. - EINVAL
-
The
nameargument toshm_open() was invalid. - EMFILE
-
The per-process limit on the number of open file descriptors has been reached.
- ENAMETOOLONG
-
The length of
nameexceedsPATH_MAX. - ENFILE
-
The system-wide limit on the total number of open files has been reached.
- ENOENT
-
An attempt was made to
shm_open() anamethat did not exist, andO_CREATwas not specified. - ENOENT
-
An attempt was to made to
shm_unlink() anamethat does not exist.
ATTRIBUTES
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 |
CONFORMING TO
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.
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 filesystem, which is normally
mounted under /dev/shm.
SEE ALSO
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2), memfd_create(2), mmap(2), open(2), umask(2), shm_overview(7)
COLOPHON
This page is part of release 4.07 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/.
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