mq_receive(3)



MQ_RECEIVE(3)              Linux Programmer's Manual             MQ_RECEIVE(3)

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 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)):

       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  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  han-
       dler.  If the O_NONBLOCK flag is enabled for the message queue descrip-
       tion, 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  speci-
       fies 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.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The mq_receive() and mq_timedreceive() functions are thread-safe.

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 3.71 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux                             2014-06-03                     MQ_RECEIVE(3)

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