'\" te
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.TH DDI_ENTER_CRITICAL 9F "Jan 16, 2006"
.SH NAME
ddi_enter_critical, ddi_exit_critical \- enter and exit a critical region of
control
.SH SYNOPSIS
.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
illumos DDI specific (illumos DDI).
.SH PARAMETERS
.ne 2
.na
\fB\fIddic\fR\fR
.ad
.RS 8n
The returned value from the call to \fBddi_enter_critical()\fR must be passed
to \fBddi_exit_critical()\fR.
.RE

.SH DESCRIPTION
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
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
This function can be called from user, interrupt, or kernel context.
.SH WARNINGS
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
.BR mutex (9F)
.sp
.LP
\fIWriting Device Drivers\fR