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MQ_OPEN(3)                 Library Functions Manual                 MQ_OPEN(3)

     mq_open - open a message queue (REALTIME)

     POSIX Real-time Library (librt, -lrt)

     #include <mqueue.h>

     mq_open(const char *name, int oflag);

     mq_open(const char *name, int oflag, mode_t mode, struct mq_attr *attr);

     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(3), but not mq_send(3).

     O_WRONLY      Open the queue for sending messages.  The process can use
                   the returned message queue descriptor with mq_send(3) but
                   not mq_receive(3).

     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

     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(3) or mq_receive(3) 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.

     The select(2) and poll(2) system calls to the message queue descriptor
     are supported by NetBSD, however, it is not portable.

     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.

     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

     [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

     [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.

     mq(3), mq_close(3), mq_unlink(3)

     This function conforms to the IEEE Std 1003.1-2001 ("POSIX.1") standard.

     This function first appeared in NetBSD 5.0.

     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

NetBSD 9.99                      June 7, 2010                      NetBSD 9.99