mq_receive

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():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION
       mq_receive()  removes the oldest message with the highest priority from
       the message queue referred to by the message  queue  descriptor  mqdes,
       and  places  it in the buffer pointed to by msg_ptr.  The msg_len argu-
       ment 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 mes-
       sage.

       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 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.

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 or not opened  for
              reading.

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

       +--------------------------------+---------------+---------+
       |Interface                       | Attribute     | Value   |
       +--------------------------------+---------------+---------+
       |mq_receive(), mq_timedreceive() | Thread safety | MT-Safe |
       +--------------------------------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

NOTES
       On Linux, mq_timedreceive() is a system call, and mq_receive() is a li-
       brary 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 5.05 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                     MQ_RECEIVE(3)