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 (PROT_READ) access.
- 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 <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.
- 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
- If the shared memory object already exists, truncate it to zero
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
will fail (unless O_CREAT
specified, 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 was
- 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 already exists.
- The name argument to shm_open() was invalid.
- The process already has the maximum number of files open.
- The length of name exceeds PATH_MAX.
- The limit on the total number of files open on the system has been
- 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
These functions are provided in glibc 2.2 and later.
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
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
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