shm_open, shm_unlink - create/open or unlink POSIX shared memory objects
/* For mode constants */
/* For O_* constants */
int shm_open(const char *name, int oflag, mode_t
int shm_unlink(const char *name);
Link with -lrt
() 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)
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
; that is, a null-terminated string of up to
(i.e., 255) characters consisting of an initial slash,
followed by one or more characters, none of which are slashes.
is a bit mask created by ORing together exactly one of
and any of the other flags listed here:
- Open the object for read access. A shared memory object
opened in this way can be mmap(2)ed only for read
- Open the object for read-write access.
- 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
- 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.
- 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.
- If the shared memory object already exists, truncate it to
Definitions of these flag values can be obtained by including
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
The file descriptor is normally used in subsequent calls to ftruncate(2)
(for a newly created object) and mmap(2)
. After a call to
the file descriptor may be closed without affecting the memory
The operation of shm_unlink
() is analogous to unlink(2)
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
fail (unless O_CREAT
in which case a new, distinct object is created).
On success, shm_open
() returns a nonnegative file descriptor. On failure,
() returns -1. shm_unlink
() returns 0 on success, or -1
On failure, errno
is set to indicate the cause of the error. Values which
may appear in errno
include the following:
- Permission to shm_unlink() the shared memory object
- 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.
- Both O_CREAT and O_EXCL were specified to
shm_open() and the shared memory object specified by name
- The name argument to shm_open() was
- The per-process limit on the number of open file
descriptors has been reached.
- The length of name exceeds PATH_MAX.
- The system-wide limit on the total number of open files has
- An attempt was made to shm_open() a name that
did not exist, and O_CREAT was not specified.
- 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)
|shm_open (), shm_unlink ()
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
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
filesystem that is normally mounted under /dev/shm
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at