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.

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)

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.

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.