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freebsd-src/lib/libsys/mq_receive.2
Brooks Davis 8269e7673c libsys: relocate implementations and manpages 
Remove core system call implementations and documentation to lib/libsys
and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>.
Update paths to allow libc to find them in their new home.

Reviewed by:	kib, emaste, imp
Pull Request:	https://github.com/freebsd/freebsd-src/pull/908
2024-02-05 20:34:55 +00:00

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.\" 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.
.\"
.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.
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