'\" te
.\"  Copyright (c) 2006, Sun Microsystems, Inc.  All Rights Reserved
.\" The contents of this file are subject to the terms of the Common Development and Distribution License (the "License").  You may not use this file except in compliance with the License.
.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE or http://www.opensolaris.org/os/licensing.  See the License for the specific language governing permissions and limitations under the License.
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.TH ddi_enter_critical 9F "16 Jan 2006" "SunOS 5.11" "Kernel Functions for Drivers"
.SH NAME
ddi_enter_critical, ddi_exit_critical \- enter and exit a critical region of
control
.SH SYNOPSIS
.LP
.nf
#include <sys/conf.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>



\fBunsigned int\fR \fBddi_enter_critical\fR(\fBvoid\fR);
.fi

.LP
.nf
\fBvoid\fR \fBddi_exit_critical\fR(\fBunsignedint\fR \fIddic\fR);
.fi

.SH INTERFACE LEVEL
.sp
.LP
Solaris DDI specific (Solaris DDI).
.SH PARAMETERS
.sp
.ne 2
.mk
.na
\fB\fIddic\fR\fR
.ad
.RS 8n
.rt  
The returned value from the call to \fBddi_enter_critical()\fR must be passed
to \fBddi_exit_critical()\fR.
.RE

.SH DESCRIPTION
.sp
.LP
Nearly all driver operations can be done without any special synchronization
and protection mechanisms beyond those provided by, for example, mutexes (see
\fBmutex\fR(9F)). However, for certain devices there can exist a very short
critical region of code which \fBmust\fR be allowed to run uninterrupted. The
function \fBddi_enter_critical()\fR provides a mechanism by which a driver can
ask the system to guarantee to the best of its ability that the current thread
of execution will neither be preempted nor interrupted. This stays in effect
until a bracketing call to \fBddi_exit_critical()\fR is made (with an argument
which was the returned value from \fBddi_enter_critical()\fR).
.sp
.LP
The driver may not call any functions external to itself in between the time it
calls \fBddi_enter_critical()\fR and the time it calls
\fBddi_exit_critical()\fR.
.SH RETURN VALUES
.sp
.LP
The \fBddi_enter_critical()\fR function returns an opaque unsigned integer
which must be used in the subsequent call to \fBddi_exit_critical()\fR.
.SH CONTEXT
.sp
.LP
This function can be called from user, interrupt, or kernel context.
.SH WARNINGS
.sp
.LP
Driver writers should note that in a multiple processor system  this function
does not temporarily suspend other processors from executing. This function
also cannot guarantee to actually block the hardware from doing such things as
interrupt acknowledge cycles. What it \fBcan\fR do is guarantee that the
currently executing thread will not be preempted.
.sp
.LP
Do not write code  bracketed by \fBddi_enter_critical()\fR and
\fBddi_exit_critical()\fR that can get caught in an infinite loop, as the
machine may crash if you do.
.SH SEE ALSO
.sp
.LP
\fBmutex\fR(9F)
.sp
.LP
\fIWriting Device Drivers\fR