Opens a shared memory object.
This function is supported on OpenVMS Integrity servers and
Alpha only.
Format
#include <sys/mman.h>
int shm_open (const char *name, int oflag, mode_t mode);
1 – Argument
name
Pointer to a string naming a shared memory object.
oflag
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.
mode
The shared memory object's permission bits. This argument is used
only when the shared memory object is being created.
2 – Description
The shm_open function establishes a connection between a shared
memory object and a file descriptor. It creates an open file
description that refers to the shared memory object and a file
descriptor that refers to that open file description. The file
descriptor is used by other functions to refer to that shared
memory object. The name argument 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 for the shared
memory object that is the lowest numbered file descriptor not
currently open for that process.
The file status flags and file access modes of the open file
description are set according to the value of oflag, 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.
3 – Return Values
n Upon success, a nonnegative integer
representing the lowest numbered unused file
descriptor. The file descriptor points to the
shared memory object.
-1 Indicates failure. errno is set to indicate
the error:
o EACCES - Permission to create the shared
memory object is denied, or the shared
memory object exists and the permissions
specified by oflag are denied, or O_TRUNC
is specified and write permission is
denied.
o EEXIST - O_CREAT and O_EXCL are set, but
the named shared memory object already
exists.
o EINTR - A signal has interrupted the shm_
open operation.
o EINVAL - The shm_open operation is not
supported for the given name.
o EMFILE - Too many file descriptors are
currently in use by this process.
o ENAMETOOLONG - The length of the name
argument exceeds PATH_MAX or a pathname
component is longer than NAME_MAX.
o ENFILE - Too many shared memory objects are
currently open in the system.
o ENOENT - O_CREAT is not set and the named
shared memory object does not exist.
o ENOSPC - Memory space for creation of the
new shared memory object is insufficient.