SHM_OPEN(3C)SHM_OPEN(3C)NAMEshm_open - 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() function establishes a connection between a shared mem‐
ory 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 argu‐
ment points to a string naming a shared memory object. It is unspeci‐
fied whether the name appears in the file system and is visible to
other functions that take pathnames as arguments. The name argument
conforms to the construction rules for a pathname. The first character
of name must be a slash (/) character and the remaining characters of
name cannot include any slash characters. For maximum portability,
name should include no more than 14 characters, but this limit is not
enforced.
If successful, shm_open() returns a file descriptor for the shared mem‐
ory object that is the lowest numbered file descriptor not currently
open for that process. The open file description is new, and therefore
the file descriptor does not share it with any other processes. It is
unspecified whether the file offset is set. The FD_CLOEXEC file
descriptor flag associated with the new file descriptor is set.
The file status flags and file access modes of the open file descrip‐
tion are according to the value of oflag. The oflag argument is the
bitwise inclusive OR of the following flags defined in the header
<fcntl.h>. Applications specify exactly one of the first two values
(access modes) below in the value of oflag:
O_RDONLY
Open for read access only.
O_RDWR
Open for read or write access.
Any combination of the remaining flags may be specified in the value of
oflag:
O_CREAT
If the shared memory object exists, this flag has no
effect, except as noted under O_EXCL below. Otherwise the
shared memory object is created; the user ID of the shared
memory object will be set to the effective user ID of the
process; the group ID of the shared memory object will be
set to a system default group ID or to the effective group
ID of the process. The permission bits of the shared memory
object will be set to the value of the mode argument except
those set in the file mode creation mask of the process.
When bits in mode other than the file permission bits are
set, the effect is unspecified. The mode argument does not
affect whether the shared memory object is opened for read‐
ing, for writing, or for both. The shared memory object has
a size of zero.
O_EXCL
If O_EXCL and O_CREAT are set, shm_open() fails if the
shared memory object exists. The check for the existence of
the shared memory object and the creation of the object if
it does not exist is atomic with respect to other processes
executing shm_open() naming the same shared memory object
with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT
is not set, the result is undefined.
O_TRUNC
If the shared memory object exists, and it is successfully
opened O_RDWR, the object will be truncated to zero length
and the mode and owner will be unchanged by this function
call. The result of using O_TRUNC with O_RDONLY is unde‐
fined.
When a shared memory object is created, the state of the shared memory
object, including all data associated with the shared memory object,
persists until the shared memory object is unlinked and all other ref‐
erences are gone. It is unspecified whether the name and shared memory
object state remain valid after a system reboot.
RETURN VALUES
Upon successful completion, the shm_open() function returns a non-nega‐
tive integer representing the lowest numbered unused file descriptor.
Otherwise, it returns −1 and sets errno to indicate the error condi‐
tion.
ERRORS
The shm_open() function will fail if:
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
shared memory object is denied, or O_TRUNC is speci‐
fied and write permission is denied.
EEXIST
O_CREAT and O_EXCL are set 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 a
pathname component is longer than NAME_MAX while
_POSIX_NO_TRUNC is in effect.
ENFILE
Too many shared memory objects are currently open in
the system.
ENOENT
O_CREAT is not set and the named shared memory object
does not exist.
ENOSPC
There is insufficient space for the creation of the
new shared memory object.
ENOSYS
The shm_open() function is not supported by the sys‐
tem.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬───────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼───────────────────┤
│Interface Stability │ Committed │
├────────────────────┼───────────────────┤
│MT-Level │ MT-Safe │
├────────────────────┼───────────────────┤
│Standard │ See standards(5). │
└────────────────────┴───────────────────┘
SEE ALSOclose(2), dup(2), exec(2), fcntl(2), mmap(2), umask(2), shm_unlink(3C),
sysconf(3C), fcntl.h(3HEAD), attributes(5), standards(5)NOTES
Solaris 2.6 was the first release to support the Asynchronous Input and
Output option. Prior to this release, this function always returned −1
and set errno to ENOSYS.
Feb 5, 2008 SHM_OPEN(3C)
