Originální popis anglicky:
shm_open, shm_unlink - Create/open or unlink POSIX shared memory objects
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t
mode);
int shm_unlink(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
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
embedded slashes.
oflag is a bit mask created by ORing together exactly one of
O_RDONLY or
O_RWDR 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 (
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 set as for open(2), 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).)
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 initialised to 0.)
- O_EXCL
- If O_CREAT was also specified, and a share 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.
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).
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.
- ENAMETOOLONG
- The length of name exceeds PATH_MAX.
- ENFILE
- The limit on the total number of files open on the system
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.
These functions are provided in glibc 2.2 and later. Programs using these
functions must specify the
-lrt flag to
cc in order to link
against the required ("realtime") library.
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 Unices.
The POSIX shared memory object implementation on Linux 2.4 makes use of a
dedicated file system, which is normally mounted under
/dev/shm.
POSIX 1003.1 (2001).
close(2),
fchmod(2),
fchown(2),
fcntl(2),
fstat(2),
ftruncate(2),
mmap(2),
open(2),
umask(2)
