218 lines
6.9 KiB
Groff
218 lines
6.9 KiB
Groff
.\" Copyright (c) 2005 David Xu <davidxu@FreeBSD.org>
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer as
|
|
.\" the first lines of this file unmodified other than the possible
|
|
.\" addition of one or more copyright notices.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice(s), this list of conditions and the following disclaimer in
|
|
.\" the documentation and/or other materials provided with the
|
|
.\" distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
|
|
.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
|
|
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE
|
|
.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
|
|
.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
|
|
.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
|
|
.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
|
|
.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
|
|
.\" OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
|
|
.\" EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
.\"
|
|
.\" Portions of this text are reprinted and reproduced in electronic form
|
|
.\" from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
|
|
.\" Portable Operating System Interface (POSIX), The Open Group Base
|
|
.\" Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
|
|
.\" Electrical and Electronics Engineers, Inc and The Open Group. In the
|
|
.\" event of any discrepancy between this version and the original IEEE and
|
|
.\" The Open Group Standard, the original IEEE and The Open Group Standard is
|
|
.\" the referee document. The original Standard can be obtained online at
|
|
.\" http://www.opengroup.org/unix/online.html.
|
|
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd November 29, 2005
|
|
.Dt MQ_RECEIVE 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm mq_receive , mq_timedreceive
|
|
.Nd "receive a message from message queue (REALTIME)"
|
|
.Sh LIBRARY
|
|
.Lb librt
|
|
.Sh SYNOPSIS
|
|
.In mqueue.h
|
|
.Ft ssize_t
|
|
.Fo mq_receive
|
|
.Fa "mqd_t mqdes"
|
|
.Fa "char *msg_ptr"
|
|
.Fa "size_t msg_len"
|
|
.Fa "unsigned *msg_prio"
|
|
.Fc
|
|
.Ft ssize_t
|
|
.Fo mq_timedreceive
|
|
.Fa "mqd_t mqdes"
|
|
.Fa "char *msg_ptr"
|
|
.Fa "size_t msg_len"
|
|
.Fa "unsigned *msg_prio"
|
|
.Fa "const struct timespec *abs_timeout"
|
|
.Fc
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Fn mq_receive
|
|
system call receives oldest of the highest priority message(s) from the
|
|
message queue specified by
|
|
.Fa mqdes .
|
|
If the size of the buffer in bytes, specified by the
|
|
.Fa msg_len
|
|
argument, is less than the
|
|
.Va mq_msgsize
|
|
attribute of the message queue, the system call will fail and return an
|
|
error.
|
|
Otherwise, the selected message will be removed from the queue
|
|
and copied to the buffer pointed to by the
|
|
.Fa msg_ptr
|
|
argument.
|
|
.Pp
|
|
If the argument
|
|
.Fa msg_prio
|
|
is not
|
|
.Dv NULL ,
|
|
the priority of the selected message will be stored in the
|
|
location referenced by
|
|
.Fa msg_prio .
|
|
If the specified message queue is empty and
|
|
.Dv O_NONBLOCK
|
|
is not set in the message queue description associated with
|
|
.Fa mqdes ,
|
|
.Fn mq_receive
|
|
will block until a message is enqueued on the message queue or until
|
|
.Fn mq_receive
|
|
is interrupted by a signal.
|
|
If more than one thread is waiting to receive
|
|
a message when a message arrives at an empty queue and the Priority
|
|
Scheduling option is supported, then the thread of highest priority that
|
|
has been waiting the longest will be selected to receive the message.
|
|
Otherwise, it is unspecified which waiting thread receives the message.
|
|
If the specified message queue is empty and
|
|
.Dv O_NONBLOCK
|
|
is set in the message queue description associated with
|
|
.Fa mqdes ,
|
|
no message
|
|
will be removed from the queue, and
|
|
.Fn mq_receive
|
|
will return an error.
|
|
.Pp
|
|
The
|
|
.Fn mq_timedreceive
|
|
system call will receive the oldest of the highest priority messages from the
|
|
message queue specified by
|
|
.Fa mqdes
|
|
as described for the
|
|
.Fn mq_receive
|
|
system call.
|
|
However, if
|
|
.Dv O_NONBLOCK
|
|
was not specified when the message queue was opened via the
|
|
.Fn mq_open
|
|
system call, and no message exists on the queue to satisfy the receive, the wait
|
|
for such a message will be terminated when the specified timeout expires.
|
|
If
|
|
.Dv O_NONBLOCK
|
|
is set, this system call is equivalent to
|
|
.Fn mq_receive .
|
|
.Pp
|
|
The timeout expires when the absolute time specified by
|
|
.Fa abs_timeout
|
|
passes, as measured by the clock on which timeouts are based (that is, when
|
|
the value of that clock equals or exceeds
|
|
.Fa abs_timeout ) ,
|
|
or if the absolute time specified by
|
|
.Fa abs_timeout
|
|
has already been passed at the time of the call.
|
|
.Pp
|
|
The timeout is based on the
|
|
.Dv CLOCK_REALTIME
|
|
clock.
|
|
.Sh RETURN VALUES
|
|
Upon successful completion, the
|
|
.Fn mq_receive
|
|
and
|
|
.Fn mq_timedreceive
|
|
system calls return the length of the selected message in bytes and the
|
|
message is removed from the queue.
|
|
Otherwise, no message is removed
|
|
from the queue, the system call returns a value of \-1,
|
|
and the global variable
|
|
.Va errno
|
|
is set to indicate the error.
|
|
.Sh ERRORS
|
|
The
|
|
.Fn mq_receive
|
|
and
|
|
.Fn mq_timedreceive
|
|
system calls
|
|
will fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EAGAIN
|
|
.Dv O_NONBLOCK
|
|
flag is set in the message queue description associated with
|
|
.Fa mqdes ,
|
|
and the specified message queue is empty.
|
|
.It Bq Er EBADF
|
|
The
|
|
.Fa mqdes
|
|
argument is not a valid message queue descriptor open for reading.
|
|
.It Bq Er EMSGSIZE
|
|
The specified message buffer size,
|
|
.Fa msg_len ,
|
|
is less than the message size attribute of the message queue.
|
|
.It Bq Er EINTR
|
|
The
|
|
.Fn mq_receive
|
|
or
|
|
.Fn mq_timedreceive
|
|
operation was interrupted by a signal.
|
|
.It Bq Er EINVAL
|
|
The process or thread would have blocked, and the
|
|
.Fa abs_timeout
|
|
parameter specified a nanoseconds field value less than zero or greater
|
|
than or equal to 1000 million.
|
|
.It Bq Er ETIMEDOUT
|
|
The
|
|
.Dv O_NONBLOCK
|
|
flag was not set when the message queue was opened, but no message arrived
|
|
on the queue before the specified timeout expired.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr mq_open 2 ,
|
|
.Xr mq_send 2 ,
|
|
.Xr mq_timedsend 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn mq_receive
|
|
and
|
|
.Fn mq_timedreceive
|
|
system calls conform to
|
|
.St -p1003.1-2004 .
|
|
.Sh HISTORY
|
|
Support for
|
|
.Tn POSIX
|
|
message queues first appeared in
|
|
.Fx 7.0 .
|
|
.Sh COPYRIGHT
|
|
Portions of this text are reprinted and reproduced in electronic form
|
|
from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
|
|
Portable Operating System Interface (POSIX), The Open Group Base
|
|
Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of
|
|
Electrical and Electronics Engineers, Inc and The Open Group. In the
|
|
event of any discrepancy between this version and the original IEEE and
|
|
The Open Group Standard, the original IEEE and The Open Group Standard is
|
|
the referee document. The original Standard can be obtained online at
|
|
http://www.opengroup.org/unix/online.html.
|