NAME

       mq_receive, mq_timedreceive - receive a message from a message queue

SYNOPSIS

       #include <mqueue.h>

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

       #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.

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

       mq_timedreceive():
           _XOPEN_SOURCE >= 600 || _POSIX_C_SOURCE >= 200112L

DESCRIPTION

       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 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  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.

RETURN VALUE

       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.

ERRORS

       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.

       EMSGSIZE
              msg_len was less than the mq_msgsize attribute of the message queue.

       ETIMEDOUT
              The call timed out before a message could be transferred.

CONFORMING TO

       POSIX.1-2001.

NOTES

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

SEE ALSO

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.