.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "MQ_RECEIVE" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" mq_receive 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
mq_receive, mq_timedreceive \- receive a message from a message queue
(\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <mqueue.h>
.br
.sp
ssize_t mq_receive(mqd_t\fP \fImqdes\fP\fB, char *\fP\fImsg_ptr\fP\fB,
size_t\fP \fImsg_len\fP\fB,
.br
\ \ \ \ \ \  unsigned *\fP\fImsg_prio\fP\fB); \fP
\fB
.br
.sp
\fP 
.LP
\fB
.br
#include <mqueue.h>
.br
#include <time.h>
.br
ssize_t mq_timedreceive(mqd_t\fP \fImqdes\fP\fB, char *restrict\fP
\fImsg_ptr\fP\fB,
.br
\ \ \ \ \ \  size_t\fP \fImsg_len\fP\fB, unsigned *restrict\fP \fImsg_prio\fP\fB,
.br
\ \ \ \ \ \  const struct timespec *restrict\fP \fIabs_timeout\fP\fB);
\fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fImq_receive\fP() function shall receive the oldest of the highest
priority message(s) from the message queue specified by
\fImqdes\fP. If the size of the buffer in bytes, specified by the
\fImsg_len\fP argument, is less than the \fImq_msgsize\fP
attribute of the message queue, the function shall fail and return
an error. Otherwise, the selected message shall be removed from
the queue and copied to the buffer pointed to by the \fImsg_ptr\fP
argument.
.LP
If the value of \fImsg_len\fP is greater than {SSIZE_MAX}, the result
is implementation-defined.
.LP
If the argument \fImsg_prio\fP is not NULL, the priority of the selected
message shall be stored in the location referenced by
\fImsg_prio\fP.
.LP
If the specified message queue is empty and O_NONBLOCK is not set
in the message queue description associated with \fImqdes\fP,
\fImq_receive\fP() shall block until a message is enqueued on the
message queue or until \fImq_receive\fP() 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 shall be selected to receive
the message. Otherwise, it is unspecified which waiting thread receives
the message. If the specified message queue is empty and
O_NONBLOCK is set in the message queue description associated with
\fImqdes\fP, no message shall be removed from the queue, and
\fImq_receive\fP() shall return an error.
.LP
The \fImq_timedreceive\fP() function shall receive the oldest of the
highest priority messages from the message queue specified by
\fImqdes\fP as described for the \fImq_receive\fP() function. However,
if O_NONBLOCK was not specified when the message queue was
opened via the \fImq_open\fP() function, and no message exists on
the queue to satisfy the
receive, the wait for such a message shall be terminated when the
specified timeout expires. If O_NONBLOCK is set, this function is
equivalent to \fImq_receive\fP().
.LP
The timeout expires when the absolute time specified by \fIabs_timeout\fP
passes, as measured by the clock on which timeouts
are based (that is, when the value of that clock equals or exceeds
\fIabs_timeout\fP), or if the absolute time specified by
\fIabs_timeout\fP has already been passed at the time of the call.
.LP
If the Timers option is supported, the timeout shall be based on the
CLOCK_REALTIME clock; if the Timers option is not
supported, the timeout shall be based on the system clock as returned
by the \fItime\fP()
function. 
.LP
The resolution of the timeout shall be the resolution of the clock
on which it is based. The \fItimespec\fP argument is defined in
the \fI<time.h>\fP header.
.LP
Under no circumstance shall the operation fail with a timeout if a
message can be removed from the message queue immediately.
The validity of the \fIabs_timeout\fP parameter need not be checked
if a message can be removed from the message queue
immediately. 
.SH RETURN VALUE
.LP
Upon successful completion, the \fImq_receive\fP()  and \fImq_timedreceive\fP()
functions shall return the length of the selected message in bytes
and the message shall be removed from
the queue. Otherwise, no message shall be removed from the queue,
the functions shall return a value of -1, and set \fIerrno\fP to
indicate the error.
.SH ERRORS
.LP
The \fImq_receive\fP()  and \fImq_timedreceive\fP() 
functions shall fail if:
.TP 7
.B EAGAIN
O_NONBLOCK was set in the message description associated with \fImqdes\fP,
and the specified message queue is empty.
.TP 7
.B EBADF
The \fImqdes\fP argument is not a valid message queue descriptor open
for reading.
.TP 7
.B EMSGSIZE
The specified message buffer size, \fImsg_len\fP, is less than the
message size attribute of the message queue.
.TP 7
.B EINTR
The \fImq_receive\fP()  or \fImq_timedreceive\fP() 
operation was interrupted by a signal.
.TP 7
.B EINVAL
The process or thread would have blocked, and the \fIabs_timeout\fP
parameter specified a nanoseconds field value less than zero
or greater than or equal to 1000 million. 
.TP 7
.B ETIMEDOUT
The O_NONBLOCK flag was not set when the message queue was opened,
but no message arrived on the queue before the specified timeout
expired. 
.sp
.LP
The \fImq_receive\fP()  and \fImq_timedreceive\fP() 
functions may fail if:
.TP 7
.B EBADMSG
The implementation has detected a data corruption problem with the
message.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fImq_open\fP(), \fImq_send\fP(), \fImq_timedsend\fP(), \fImsgctl\fP(),
\fImsgget\fP(), \fImsgrcv\fP(), \fImsgsnd\fP(), \fItime\fP(),
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<mqueue.h>\fP,
\fI<time.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .