SHM_OPEN(3C) Standard C Library Functions SHM_OPEN(3C)

NAME

shm_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
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. It is unspecified 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
memory 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
description 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 reading, 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 undefined.


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 references 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-
negative integer representing the lowest numbered unused file
descriptor. Otherwise, it returns -1 and sets errno to indicate the
error condition.

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
specified 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
system.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | MT-Safe |
+--------------------+-------------------+
|Standard | See standards(7). |
+--------------------+-------------------+

SEE ALSO

close(2), dup(2), exec(2), fcntl(2), mmap(2), umask(2),
shm_unlink(3C), sysconf(3C), fcntl.h(3HEAD), attributes(7),
standards(7)

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.

February 5, 2008 SHM_OPEN(3C)