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NAME

     mq_open - open a message queue (REALTIME)

LIBRARY

     library “librt”

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 .

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.