173 lines
4.6 KiB
Groff
173 lines
4.6 KiB
Groff
.\" $NetBSD: mq_receive.3,v 1.6 2015/11/19 07:03:13 wiz Exp $
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.\"
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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.\"
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.Dd November 18, 2015
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.Dt MQ_RECEIVE 3
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.Os
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.Sh NAME
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.Nm mq_receive, mq_timedreceive
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.Nd receive a message from a message queue (REALTIME)
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.Sh LIBRARY
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.Lb librt
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.Sh SYNOPSIS
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.In mqueue.h
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.Ft ssize_t
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.Fo mq_receive
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.Fa "mqd_t mqdes"
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.Fa "char *msg_ptr"
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.Fa "size_t msg_len"
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.Fa "unsigned *msg_prio"
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.Fc
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.In mqueue.h
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.In time.h
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.Ft ssize_t
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.Fo mq_timedreceive
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.Fa "mqd_t mqdes"
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.Fa "char *restrict msg_ptr"
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.Fa "size_t msg_len"
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.Fa "unsigned *restrict msg_prio"
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.Fa "const struct timespec *restrict abs_timeout"
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.Fc
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.Sh DESCRIPTION
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The
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.Fn mq_receive
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function receives the oldest of the highest priority message(s)
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from the message queue specified by
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.Fa mqdes .
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If the size of the buffer in bytes, specified by the
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.Fa msg_len
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argument, is less than the
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.Va mq_msgsize
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attribute of the message queue, the function fails and returns an error.
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Otherwise, the selected message will be removed from the queue and copied
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to the buffer pointed to by the
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.Fa msg_ptr
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argument.
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.Pp
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If the argument
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.Fa msg_prio
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is not
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.Dv NULL ,
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the priority of the selected message will be stored in the location
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referenced by
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.Fa msg_prio .
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.Pp
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If the specified message queue is empty and
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.Dv O_NONBLOCK
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is not set in the message queue description associated with
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.Fa mqdes ,
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.Fn mq_receive
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blocks until a message is enqueued on the message queue or until
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.Fn mq_receive
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is interrupted by a signal.
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If more than one thread is waiting to receive a message when a
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message arrives at an empty queue, then the thread of highest
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priority that has been waiting the longest will be selected to
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receive the message.
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If the specified message queue is empty and
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.Dv O_NONBLOCK
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is set in the message queue description associated with
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.Fa mqdes ,
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no message will be removed from the queue, and
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.Fn mq_receive
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returns an error.
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.Pp
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The timeout expires when the absolute time specified by
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.Fa abs_timeout
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passes, as measured by the clock on which timeouts are based (that is,
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when the value of that clock equals or exceeds
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.Fa abs_timeout ) ,
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or if the absolute time specified by
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.Fa abs_timeout
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has already been passed at the time of the call.
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.Pp
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The resolution of the timeout is based on the CLOCK_REALTIME clock.
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The
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.Fa timespec
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argument is defined in the
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.In time.h
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header.
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.Pp
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Under no circumstance will the operation fail with a timeout if a
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message can be removed from the message queue immediately.
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The validity of the
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.Fa abs_timeout
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parameter will not be checked if a message can be removed from the
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message queue immediately.
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.Sh RETURN VALUES
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Upon successful completion, the
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.Fn mq_receive
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and
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.Fn mq_timedreceive
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functions return the length of the selected message in bytes, and the
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message is removed from the queue.
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Otherwise, no message will be removed from the queue,
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the functions return a value of
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\-1, and set the global variable
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.Va errno
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to indicate the error.
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.Sh ERRORS
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The
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.Fn mq_receive
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and
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.Fn mq_timedreceive
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functions fail if:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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.Dv O_NONBLOCK
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was set in the message description associated with
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.Fa mqdes ,
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and the specified message queue is empty.
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.It Bq Er EBADF
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The
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.Fa mqdes
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argument is not a valid message queue descriptor open for reading.
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.It Bq Er EINTR
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The
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.Fn mq_receive
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or
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.Fn mq_timedreceive
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operation was interrupted by a signal.
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.It Bq Er EINVAL
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The process or thread would have blocked, and the
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.Fa abs_timeout
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parameter specified a nanoseconds field value less than zero
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or greater than or equal to 1000 million.
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.It Bq Er EMSGSIZE
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The specified message buffer size,
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.Fa msg_len ,
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is less than the message size attribute of the message queue.
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.It Bq Er ETIMEDOUT
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The
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.Dv O_NONBLOCK
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flag was not set when the message queue was opened,
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but no message arrived on the queue before the specified timeout expired.
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.El
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.Sh SEE ALSO
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.Xr mq 3 ,
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.Xr mq_send 3
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.Sh STANDARDS
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These functions are expected to conform to the
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.St -p1003.1-2001
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standard.
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.Sh HISTORY
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The
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.Fn mq_receive
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and
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.Fn mq_timedreceive
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functions first appeared in
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.Nx 5.0 .
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.Sh COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group.
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In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document.
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The original Standard can be obtained online at
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.Lk http://www.opengroup.org/unix/online.html .
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