#include <mqueue.h> mqd_t
mq_open (const char *name int oflag ...);
DESCRIPTION
The
mq_open ();
system call establishes the connection between a process and a message queue
with a message queue descriptor.
It creates an open message queue
description that refers to the message queue, and a message queue descriptor
that refers to that open message queue description.
The message queue
descriptor is used by other functions to refer to that message queue.
The
Fa name
argument points to a string naming a message queue.
The
Fa name
argument should conform to the construction rules for a pathname.
The
Fa name
should begin with a slash character.
Processes calling
mq_open ();
with the same value of
Fa name
refers to the same message queue object, as long as that name has not been
removed.
If the
Fa name
argument is not the name of an existing message queue and creation is not
requested,
mq_open ();
will fail and return an error.
The
Fa oflag
argument requests the desired receive and/or send access to the message
queue.
The requested access permission to receive messages or send messages
would be granted if the calling process would be granted read or write access,
respectively, to an equivalently protected file.
The value of
Fa oflag
is the bitwise-inclusive OR of values from the following list.
Applications should specify exactly one of the first three values (access
modes) below in the value of
Fa oflag :
O_RDONLY
Open the message queue for receiving messages.
The process can use the
returned message queue descriptor with
mq_receive (,);
but not
mq_send (.);
A message queue may be open multiple times in the same or different processes
for receiving messages.
O_WRONLY
Open the queue for sending messages.
The process can use the returned
message queue descriptor with
mq_send ();
but not
mq_receive (.);
A message queue may be open multiple times in the same or different processes
for sending messages.
O_RDWR
Open the queue for both receiving and sending messages.
The process can use
any of the functions allowed for
O_RDONLY
and
O_WRONLY
A message queue may be open multiple times in the same or different processes
for sending messages.
Any combination of the remaining flags may be specified in the value of
Fa oflag :
O_CREAT
Create a message queue.
It requires two additional arguments:
Fa mode ,
which is of type
Vt mode_t ,
and
Fa attr ,
which is a pointer to an
Vt mq_attr
structure.
If the pathname
Fa name
has already been used to create a message queue that still exists, then
this flag has no effect, except as noted under
O_EXCL
Otherwise, a message queue will be created without any messages
in it.
The user ID of the message queue will be set to the effective user ID
of the process, and the group ID of the message queue will be set to the
effective group ID of the process.
The permission bits of the message queue
will be set to the value of the
Fa mode
argument, except those set in the file mode creation mask of the process.
When bits in
Fa mode
other than the file permission bits are specified, the effect is
unspecified.
If
Fa attr
is
NULL
the message queue is created with implementation-defined default message
queue attributes.
If attr is
non- NULL
and the calling process has the
appropriate privilege on name, the message queue
mq_maxmsg
and
mq_msgsize
attributes will be set to the values of the corresponding members in the
Vt mq_attr
structure referred to by
Fa attr .
If
Fa attr
is
non- NULL
but the calling process does not have the appropriate privilege
on name, the
mq_open ();
function will fail and return an error without creating the message queue.
O_EXCL
If
O_EXCL
and
O_CREAT
are set,
mq_open ();
will fail if the message queue name exists.
O_NONBLOCK
Determines whether an
mq_send ();
or
mq_receive ();
waits for resources or messages that are not currently available, or fails
with
errno
set to
Er EAGAIN ;
see
mq_send2
and
mq_receive2
for details.
The
mq_open ();
system call does not add or remove messages from the queue.
NOTES
Fx implements message queue based on file descriptor.
The descriptor
is inherited by child after
fork(2).
The descriptor is closed in a new image after
exec(3).
The
select(2)
and
kevent(2)
system calls are supported for message queue descriptor.
RETURN VALUES
Upon successful completion, the function returns a message queue
descriptor; otherwise, the function returns
Po Vt mqd_t Pc Ns -1
and sets the global variable
errno
to indicate the error.
ERRORS
The
mq_open ();
system call
will fail if:
Bq Er EACCES
The message queue exists and the permissions specified by
Fa oflag
are denied, or the message queue does not exist and permission to create the
message queue is denied.
Bq Er EEXIST
O_CREAT
and
O_EXCL
are set and the named message queue already exists.
Bq Er EINTR
The
mq_open ();
function was interrupted by a signal.
Bq Er EINVAL
The
mq_open ();
function is not supported for the given name.
Bq Er EINVAL
O_CREAT
was specified in
Fa oflag ,
the value of
Fa attr
is not
NULL
and either
mq_maxmsg
or
mq_msgsize
was less than or equal to zero.
Bq Er EMFILE
Too many message queue descriptors or file descriptors are currently in use
by this process.
Bq Er ENAMETOOLONG
The length of the
Fa name
argument exceeds
Br q Dv PATH_MAX
or a pathname component
is longer than
Br q Dv NAME_MAX .
Bq Er ENFILE
Too many message queues are currently open in the system.
Bq Er ENOENT
O_CREAT
is not set and the named message queue does not exist.
Bq Er ENOSPC
There is insufficient space for the creation of the new message queue.
The
mq_open ();
system call conforms to
St -p1003.1-2004 .
HISTORY
Support for
POSIX
message queues first appeared in
Fx 7.0 .
BUGS
This implementation places strict requirements on the value of
Fa name :
it must begin with a slash
(`/'
)
and contain no other slash characters.
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard is
the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html.
