DragonFly On-Line Manual Pages
MQ_OPEN(2) DragonFly System Calls Manual MQ_OPEN(2)
NAME
mq_open - open a message queue (REALTIME)
LIBRARY
POSIX Real-time Library (librt, -lrt)
SYNOPSIS
#include <mqueue.h>
mqd_t
mq_open(const char *name, int oflag);
#include <sys/types.h>
mqd_t
mq_open(const char *name, int oflag, mode_t mode, struct mq_attr *attr);
DESCRIPTION
The mq_open() function 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 name argument points to a string naming a message
queue, which should conform to the construction rules for a pathname.
The name should begin with a slash character. The processes calling
mq_open() with the same value of name will refer to the same message
queue object, as long as that name has not been removed. If the name
argument is not the name of an existing message queue and creation is not
requested, mq_open() fails and returns an error.
The oflag argument requests the desired receive and/or send access to the
message queue. The requested access permission to receive messages or
send messages are granted if the calling process would be granted read or
write access, respectively, to an equivalently protected file.
The value of oflag is the bitwise-inclusive OR of values from the
following list. Applications must specify exactly one of the first three
values (access modes) below in the value of oflag:
O_RDONLY Open the message queue for receiving messages. The process
can use the returned message queue descriptor with
mq_receive(2), but not mq_send(2).
O_WRONLY Open the queue for sending messages. The process can use
the returned message queue descriptor with mq_send(2) but
not mq_receive(2).
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.
In all cases, a message queue may be open multiple times in the same or
different processes for sending/receiving messages.
Any combination of the remaining flags may be specified in the value of
oflag:
O_CREAT Create a message queue. It requires two additional
arguments: mode and attr. If the pathname name has already
been used to create a message queue that still exists, then
this flag will have 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 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 specified, the effect is unspecified.
If attr is NULL, the message queue will be 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 mq_attr structure referred to
by attr. If 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() fails if the
message queue name exists. The check for the existence of
the message queue and the creation of the message queue if
it does not exist will be atomic with respect to other
threads executing mq_open() naming the same name with
O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is
not set, the result is undefined.
O_NONBLOCK Determines whether an mq_send(2) or mq_receive(2) waits for
resources or messages that are not currently available, or
fails with errno set to EAGAIN.
The mq_open() function does not add or remove messages from the queue.
NOTES
The select(2) and poll(2) system calls to the message queue descriptor
are supported by DragonFly, however, it is not portable.
RETURN VALUES
Upon successful completion, mq_open() returns a message queue descriptor.
Otherwise, the function returns (mqd_t) -1 and sets the global variable
errno to indicate the error.
ERRORS
The mq_open() function fails if:
[EACCES] The message queue exists and the permissions specified
by oflag are denied, or the message queue does not
exist and permission to create the message queue is
denied.
[EEXIST] O_CREAT and O_EXCL are set and the named message queue
already exists.
[EINTR] The mq_open() function was interrupted by a signal.
[EINVAL] The mq_open() function is not supported for the given
name, or O_CREAT was specified in oflag, the value of
attr is not NULL, and either mq_maxmsg or mq_msgsize
was less than or equal to zero.
[EMFILE] Too many message queue descriptors or file descriptors
are currently in use by this process.
[ENAMETOOLONG] The length of the name argument exceeds {PATH_MAX} or
a pathname component is longer than {NAME_MAX}.
[ENFILE] Too many message queues are currently open in the
system.
[ENOENT] O_CREAT is not set and the named message queue does
not exist.
[ENOSPC] There is insufficient space for the creation of the
new message queue.
SEE ALSO
mq_close(2), mq_getattr(2), mq_notify(2), mq_receive(2), mq_send(2),
mq_setattr(2), mq_unlink(2)
STANDARDS
This function conforms to the IEEE Std 1003.1-2001 ("POSIX.1") standard.
HISTORY
This function first appeared in DragonFly 2.5.
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
DragonFly 5.7-DEVELOPMENT October 19, 2019 DragonFly 5.7-DEVELOPMENT