NAME

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

SYNOPSIS

       #include <mqueue.h>

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

       #include <mqueue.h>
       #include <time.h>
       ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
              size_t msg_len, unsigned *restrict msg_prio,
              const struct timespec *restrict abs_timeout);

DESCRIPTION

       The  mq_receive()  function  shall receive the 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  function shall fail and return an error.
       Otherwise, the selected message shall be removed from the queue and copied to the buffer  pointed  to  by
       the msg_ptr argument.

       If the value of msg_len is greater than {SSIZE_MAX}, the result is implementation-defined.

       If  the  argument  msg_prio  is  not  NULL,  the  priority of the selected message shall 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() shall 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 shall 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 shall be removed
       from the queue, and mq_receive() shall return an error.

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

       If the Timers option is supported, the timeout shall be based on the CLOCK_REALTIME clock; if the  Timers
       option  is  not  supported,  the  timeout  shall  be  based on the system clock as returned by the time()
       function.

       The resolution of the timeout shall be the resolution of the clock on which it  is  based.  The  timespec
       argument is defined in the <time.h> header.

       Under  no  circumstance  shall  the  operation  fail  with a timeout if a message can be removed from the
       message queue immediately.  The validity of the abs_timeout parameter need not be checked  if  a  message
       can be removed from the message queue immediately.

RETURN VALUE

       Upon  successful  completion, the mq_receive()    and mq_timedreceive() functions shall return the length
       of the selected message in bytes and the message shall be removed from the queue. Otherwise,  no  message
       shall  be removed from the queue, the functions shall return a value of -1, and set errno to indicate the
       error.

ERRORS

       The mq_receive()    and mq_timedreceive() functions shall fail if:

       EAGAIN O_NONBLOCK was set in the message 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.

       The mq_receive()    and mq_timedreceive() functions may fail if:

       EBADMSG
              The implementation has detected a data corruption problem with the message.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       mq_open()  ,  mq_send()  , mq_timedsend() , msgctl() , msgget() , msgrcv() , msgsnd() , time() , the Base
       Definitions volume of IEEE Std 1003.1-2001, <mqueue.h>, <time.h>

COPYRIGHT

       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 http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                         MQ_RECEIVE(P)