Section: Linux Programmer's Manual (3)
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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 mode);
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
the same region of shared memory.
function performs the converse operation,
removing an object previously created by
The operation of
is analogous to that of
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
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
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
except that those bits set in the process file mode
creation mask (see
are cleared for the new object.
A set of macro constants which can be used to define
is listed in
(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
The newly allocated bytes of a shared memory
object are automatically initialized to 0.
was also specified, and a shared memory object with the given
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 zero bytes.
Definitions of these flag values can be obtained by including
On successful completion
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.
is set for the file descriptor.
The file descriptor is normally used in subsequent calls
(for a newly created object) and
After a call to
the file descriptor may be closed without affecting the memory mapping.
is analogous to
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
an object with the same
will fail (unless
was specified, in which case a new, distinct object is created).
returns a nonnegative file descriptor.
returns 0 on success, or -1 on error.
is set to indicate the cause of the error.
Values which may appear in
include the following:
the shared memory object was denied.
Permission was denied to
in the specified
was specified and the caller does not have write permission on the object.
were specified to
and the shared memory object specified by
The process already has the maximum number of files open.
The length of
The limit on the total number of files open on the system has been
An attempt was made to
that did not exist, and
was not specified.
An attempt was to made to
that does not exist.
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
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
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- SEE ALSO