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shm_open - Opens a shared memory object, creating the object if necessary
int shm_open (
const char *name,
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
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
· 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
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
Functions: close(2), dup(2), exec(2), fcntl(2), fstat(2), mmap(2),
Guide to Realtime Programming
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