SHM_OPEN
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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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);
Link with -lrt.
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 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).
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()
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.
VERSIONS
These functions are provided in glibc 2.2 and later.
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 makes use
of a dedicated
tmpfs(5)
filesystem that 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 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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- Running on 
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