Section (3) shm_open
shm_open, shm_unlink — create/open or unlink POSIX shared memory objects
#include <sys/mman.h> #include <sys/stat.h> /* For mode constants */ #include <fcntl.h> /* For O_* constants */
||const char *name,|
||const char *name
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
function performs the converse operation, removing an object
previously created by
The operation of
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
/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_RDWR and any of the other flags listed
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_zsingle_quotesz_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
modeis 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.
O_CREATwas also specified, and a shared memory object with the given
namealready 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
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
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
O_CREAT was specified,
in which case a new, distinct object is created).
returns a nonnegative file descriptor. On failure,
shm_open() returns −1.
shm_unlink() returns 0 on
success, or −1 on error.
errno is set to
indicate the cause of the error. Values which may appear in
errno include the following:
shm_unlink() the shared memory object was denied.
Permission was denied to
namein the specified
O_TRUNCwas specified and the caller does not have write permission on the object.
O_EXCLwere specified to
shm_open() and the shared memory object specified by
shm_open() was invalid.
The per-process limit on the number of open file descriptors has been reached.
The length of
The system-wide limit on the total number of open files has been reached.
An attempt was made to
namethat did not exist, and
O_CREATwas not specified.
An attempt was to made to
namethat does not exist.
For an explanation of the terms used in this section, see attributes(7).
||Thread safety||MT-Safe locale|
POSIX.1-2001 says that the group ownership of a newly created shared memory object is set to either the calling process_zsingle_quotesz_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_zsingle_quotesz_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_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
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