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 .