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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 EAGAIN.

       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 queue.

              The call timed out before a message could be transferred.


       For an explanation of the terms used in this section, see attributes(7).

       │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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