shm_open(3)shm_open(3)NAMEshm_open - Opens a shared memory object, creating the object if neces‐
sary (P1003.1b)
SYNOPSIS
#include <sys/mman.h>
int shm_open ( const char *name, int ooption, mode_t mode);
LIBRARY
Realtime Library (librt.so, librt.a)
PARAMETERS
Points to a string naming the shared memory object. Specifies options
that define file status and file access modes. This argument is con‐
structed 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.
DESCRIPTION
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 func‐
tions 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 associ‐
ated with it persist until the shared memory is unlinked.
The shm_open function returns a file descriptor that is the lowest num‐
bered 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.
RETURN VALUES
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.
ERRORS
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 inter‐
rupted 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 sys‐
tem. 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.
SEE ALSO
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)
