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       mq_receive, mq_timedreceive - receive a message from a message queue


       #include <mqueue.h>

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

       #define _XOPEN_SOURCE 600
       #include <time.h>
       #include <mqueue.h>

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

       Link with -lrt.


       mq_receive()  removes the oldest message with the highest priority from
       the message queue referred to by the descriptor mqdes, and places it in
       the  buffer  pointed to by msg_ptr.  The msg_len argument specifies the
       size of the buffer pointed to by msg_ptr; this must be greater than the
       mq_msgsize  attribute of the queue (see mq_getattr(3)).  If prio is not
       NULL, then the buffer to which it points is used to return the priority
       associated with the received message.

       If  the  queue  is empty, then, by default, mq_receive() blocks until a
       message becomes available, or the  call  is  interrupted  by  a  signal
       handler.   If  the  O_NONBLOCK  flag  is  enabled for the message queue
       description, then the call instead fails  immediately  with  the  error

       mq_timedreceive()  behaves  just  like mq_receive(), except that if the
       queue is empty and the O_NONBLOCK flag is not enabled for  the  message
       queue  description,  then  abs_timeout  points  to  a  structure  which
       specifies a ceiling on the time for which the call  will  block.   This
       ceiling  is  an  absolute  timeout in seconds and nanoseconds since the
       Epoch, 1970-01-01 00:00:00 +0000 (UTC), and  it  is  specified  in  the
       following structure:

           struct timespec {
               time_t tv_sec;        /* seconds */
               long   tv_nsec;       /* nanoseconds */

       If  no message is available, and the timeout has already expired by the
       time of the call, mq_timedreceive() returns immediately.


       On success, mq_receive() and mq_timedreceive()  return  the  number  of
       bytes in the received message; on error, -1 is returned, with errno set
       to indicate the error.


       EAGAIN The queue was empty, and the O_NONBLOCK flag  was  set  for  the
              message queue description referred to by mqdes.

       EBADF  The descriptor specified in mqdes was invalid.

       EINTR  The call was interrupted by a signal handler; see signal(7).

       EINVAL The call would have blocked, and abs_timeout was invalid, either
              because tv_sec was less than zero, or because tv_nsec  was  less
              than zero or greater than 1000 million.

              msg_len  was  less  than the mq_msgsize attribute of the message

              The call timed out before a message could be transferred.




       On Linux, mq_timedreceive() is a system call,  and  mq_receive()  is  a
       library function layered on top of that system call.


       mq_close(3),   mq_getattr(3),   mq_notify(3),  mq_open(3),  mq_send(3),
       mq_unlink(3), feature_test_macros(7), mq_overview(7), time(7)


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