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shm_open, shm_unlink - Create/open or unlink POSIX shared memory
#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.
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, name should have an initial slash (/) and contain no
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 only be mmap(2)ed for read
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
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 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
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).
On success, shm_open() returns a non-negative file descriptor. On
failure, shm_open() returns -1. shm_unlink() returns 0 on success, or
-1 on error.
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 process already has the maximum number of files open.
The length of name exceeds PATH_MAX.
ENFILE The limit on the total number of files open on the system has
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
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 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 file system, which is normally mounted under /dev/shm.
close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2),
mmap(2), open(2), umask(2), shm_overview(7)
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