shm_open - Opens a shared memory object, creating the
object if necessary (P1003.1b)
#include <sys/mman.h>
int shm_open ( const char *name, int ooption, mode_t
mode);
Realtime Library (librt.so, librt.a)
Points to a string naming the shared memory object. Specifies
options that define file status and file access
modes. This argument is constructed from the bitwise
inclusive OR of zero or more of the options defined in the
<fcntl.h> header file. Specifies the shared memory
object's permission bits. This argument is used only when
the shared memory object is being created.
The shm_open function establishes a connection between a
shared memory object and a file descriptor. It creates an
open file descriptor that refers to the shared memory
object and a file descriptor that refers to that open file
descriptor. This file descriptor is used by other functions
to refer to the shared memory object. The name
points to a string naming a shared memory object. The name
can be a pathname, in which case other processes referring
to the same pathname refer to the same shared memory
object.
When a shared memory object is created, its state and all
data associated with it persist until the shared memory is
unlinked.
The shm_open function returns a file descriptor that is
the lowest numbered file descriptor not currently open for
that process. File status options and access modes are
set according to the ooption argument. These options are
defined in the <fcntl.h> header file and can have zero or
more of the following values: O_RDONLY - Open for read
access only. O_RDWR - Open for read or write access.
O_CREAT - Create the shared memory if the memory object
does not exist already. The user ID and group ID of the
shared memory object are identical to those of the calling
process. The shared memory object's permission bits are
set to the value of mode, except those set in the file
mode creation mask of the process. O_EXCL - Prevent the
opening of a shared memory object if O_CREAT is set and
the shared memory object already exists. Use this option
only in combination with O_CREAT. O_TRUNC - Truncate the
shared memory object to zero length if it is successfully
opened for read or write access (O_RDWR).
The initial contents of the shared memory object are
binary zeros.
On a successful call to shm_open, a nonnegative integer is
returned that represents the lowest-numbered unused file
descriptor. The file descriptor points to the shared memory
object. Otherwise, -1 is returned and errno is set to
indicate the error.
The shm_open function fails under the following conditions:
Permission to create the shared memory object is
denied. The shared memory object exists and the permissions
specified by ooption are denied. O_TRUNC is specified
and write permission is denied. O_CREAT and O_EXCL
are set, but the named shared memory object already
exists. A signal has interrupted the shm_open operation.
The shm_open operation is not supported for the given
name. Too many file descriptors are currently in use by
this process. The length of the path argument exceeds
PATH_MAX, or a pathname component is longer than NAME_MAX
while _POSIX_NO_TRUNC is in effect. Too many shared memory
objects are currently open in the system. O_CREAT is
not set and the named shared memory object does not exist.
Memory space for creation of the new shared memory object
is insufficient.
Functions: close(2), dup(2), exec(2), fcntl(2), fstat(2),
mmap(2), umask(2), shm_unlink(3)
Guide to Realtime Programming
shm_open(3)
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