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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 int *msg_prio);

       #include <time.h>
       #include <mqueue.h>

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

       Link with -lrt.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L


       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  or
       equal to the mq_msgsize attribute of the queue (see mq_getattr(3)).  If
       msg_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 how long the call will block.   This  value  is  an  absolute
       timeout in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00
       +0000 (UTC), 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.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue   │
       │mq_receive(), mq_timedreceive() │ Thread safety │ MT-Safe │


       POSIX.1-2001, POSIX.1-2008.


       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), mq_overview(7), time(7)


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