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
       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.

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 attributes(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 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

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