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NAME

     mq_receive, mq_timedreceive - receive a message from message queue
     (REALTIME)

LIBRARY

     library “librt”

SYNOPSIS

     #include <mqueue.h>

     ssize_t
     mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
             unsigned *msg_prio);

     ssize_t
     mq_timedreceive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
             unsigned *msg_prio, const struct timespec *abs_timeout);

DESCRIPTION

     The mq_receive() system call receives oldest of the highest priority
     message(s) from the message queue specified by mqdes.  If the size of the
     buffer in bytes, specified by the msg_len argument, is less than the
     mq_msgsize attribute of the message queue, the system call will fail and
     return an error.  Otherwise, the selected message will be removed from
     the queue and copied to the buffer pointed to by the msg_ptr argument.

     If the argument msg_prio is not NULL, the priority of the selected
     message will be stored in the location referenced by msg_prio.  If the
     specified message queue is empty and O_NONBLOCK is not set in the message
     queue description associated with mqdes, mq_receive() will block until a
     message is enqueued on the message queue or until mq_receive() is
     interrupted by a signal.  If more than one thread is waiting to receive a
     message when a message arrives at an empty queue and the Priority
     Scheduling option is supported, then the thread of highest priority that
     has been waiting the longest will be selected to receive the message.
     Otherwise, it is unspecified which waiting thread receives the message.
     If the specified message queue is empty and O_NONBLOCK is set in the
     message queue description associated with mqdes, no message will be
     removed from the queue, and mq_receive() will return an error.

     The mq_timedreceive() system call will receive the oldest of the highest
     priority messages from the message queue specified by mqdes as described
     for the mq_receive() system call.  However, if O_NONBLOCK was not
     specified when the message queue was opened via the mq_open() system
     call, and no message exists on the queue to satisfy the receive, the wait
     for such a message will be terminated when the specified timeout expires.
     If O_NONBLOCK is set, this system call is equivalent to mq_receive().

     The timeout expires when the absolute time specified by abs_timeout
     passes, as measured by the clock on which timeouts are based (that is,
     when the value of that clock equals or exceeds abs_timeout), or if the
     absolute time specified by abs_timeout has already been passed at the
     time of the call.

     The timeout is based on the CLOCK_REALTIME clock.

RETURN VALUES

     Upon successful completion, the mq_receive() and mq_timedreceive() system
     calls return the length of the selected message in bytes and the message
     is removed from the queue.  Otherwise, no message is removed from the
     queue, the system call returns a value of -1, and the global variable
     errno is set to indicate the error.

ERRORS

     The mq_receive() and mq_timedreceive() system calls will fail if:

     [EAGAIN]           O_NONBLOCK flag is set in the message queue
                        description associated with mqdes, and the specified
                        message queue is empty.

     [EBADF]            The mqdes argument is not a valid message queue
                        descriptor open for reading.

     [EMSGSIZE]         The specified message buffer size, msg_len, is less
                        than the message size attribute of the message queue.

     [EINTR]            The mq_receive() or mq_timedreceive() operation was
                        interrupted by a signal.

     [EINVAL]           The process or thread would have blocked, and the
                        abs_timeout parameter specified a nanoseconds field
                        value less than zero or greater than or equal to 1000
                        million.

     [ETIMEDOUT]        The O_NONBLOCK flag was not set when the message queue
                        was opened, but no message arrived on the queue before
                        the specified timeout expired.

SEE ALSO

     mq_open(2), mq_send(2), mq_timedsend(2)

STANDARDS

     The mq_receive() and mq_timedreceive() system calls conform to .

HISTORY

     Support for POSIX message queues first appeared in FreeBSD 7.0.