NAME
shm_open — create/open a shared memory object
SYNOPSIS
#include <sys/mman.h>
int shm_open(const char *name, int oflag, mode_t mode);
DESCRIPTION
The
shm_open()
system call establishes a connection between a shared memory object and a
file descriptor. It creates an open file description that corresponds to the
shared memory object and returns a file descriptor that refers to that open
file description. This file descriptor (which is the lowest numbered file
descriptor not currently open for that process) is used by other functions to
refer to that shared memory object.
The
name
argument points to the shared memory object name, and must conform to the
general construction rules for a pathname.
The
oflag
argument is the bitwise inclusive OR of the flags listed under
Read-Write Flags
and
General Flags
(these flags are defined in the header file
<fcntl.h>).
The new file descriptor has the
FD_CLOEXEC
flag set, and consequently does not
remain open across
exec*()
system calls.
Read-Write Flags
The value of
oflag
must be composed by taking the inclusive OR of exactly one of the following
flags:
- O_RDONLY
Open for read access only.
- O_RDWR
Open for read and write access.
General Flags
Any combination of the following flags may also be used in setting the value of
oflag.
- O_CREAT
If the shared memory object exists, this flag will have no effect, except as
noted under
O_EXCL
below. Otherwise the shared memory object is created; the
shared memory object's user ID is set to the effective user ID of the process;
the shared memory object's group ID is set to the effective group ID of the
process. The shared memory object's permission bits is set to the value of
the
mode
argument except those set in the file mode creation mask of the process.
The new shared memory object will have a size of zero.
- O_EXCL
If
O_EXCL
and
O_CREAT
are set in
oflag
and the named shared memory object exists,
shm_open()
will fail. The
O_EXCL
flag is ignored if
O_CREAT
is not set in
oflag.
- O_TRUNC
If the shared memory object exists, and it is successfully opened for reading
and writing
(O_RDWR
set in
oflag),
the object will be truncated to zero length.
The mode and owner shall remain unchanged by this function call.
RETURN VALUE
shm_open()
returns the following values:
- n
Successful completion.
n
is the lowest numbered unused file descriptor for the process.
- -1
Failure.
errno
is set to indicate the error.
ERRORS
If
shm_open()
fails,
errno
is set to one of the following values:
- EACCES
The shared memory object exists and the permissions specified by
oflag
are denied, or the shared memory object does not exist and permission to create
the it is denied, or
O_TRUNC
is specified and write permission is denied.
- EEXIST
The
O_CREAT
and
O_EXCL
are set in
oflag
and the named shared memory object already exists.
- EINTR
The
shm_open()
operation was interrupted by a signal.
- EINVAL
The
shm_open()
operation is not supported for the given name.
- EMFILE
Too many file descriptors are currently in use by this process.
- ENAMETOOLONG
The length of the
name
string exceeds
PATH_MAX,
or the length of a (pathname) component of the
name
string exceeds
NAME_MAX
while
_POSIX_NO_TRUNC
is in effect.
- ENFILE
Too many shared memory objects are currently open in the system.
- ENOENT
The
O_CREAT
flag is not set in
oflag
and the named shared memory object does not exist.
- ENOSPC
There are insufficient resources for the creation of the new shared memory
object.
- ENOSYS
shm_open()
is not supported by the implementation.
STANDARDS CONFORMANCE
shm_open() in librt: POSIX 1003.1b
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