MQ_OPEN(2) FreeBSD 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, ...);
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 name argument points to a string naming a message
queue. The name argument should conform to the construction rules for a
pathname. The name should begin with a slash character. Processes
calling mq_open() with the same value of name refers 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() will fail and return 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 would be 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 should 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(), 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
oflag:
O_CREAT Create a message queue. It requires two additional
arguments: mode, which is of type mode_t, and attr, which
is a pointer to an mq_attr structure. If the pathname
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 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 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 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() 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 EAGAIN; see mq_send(2) and
mq_receive(2) for details.
The mq_open() system call does not add or remove messages from the queue.
NOTES
FreeBSD 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.
Please see the mqueuefs(5) man page for instructions on loading the
module or compiling the service into the kernel.
RETURN VALUES
Upon successful completion, the function 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() system call will fail 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.
[EINVAL] 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_receive(2), mq_send(2), mq_setattr(2),
mq_timedreceive(3), mq_timedsend(3), mq_unlink(3), mqueuefs(5)
STANDARDS
The mq_open() system call conforms to IEEE Std 1003.1-2004 ("POSIX.1").
HISTORY
Support for POSIX message queues first appeared in FreeBSD 7.0.
BUGS
This implementation places strict requirements on the value of name: it
must begin with a slash (`/') and contain no other slash characters.
The mode and attr arguments are variadic and may result in different
calling conventions than might otherwise be expected.
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.
FreeBSD 13.1-RELEASE-p6 September 28, 2019 FreeBSD 13.1-RELEASE-p6
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