.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
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.Dd March 22, 2001
.Dt CRITICAL_ENTER 9
.Os
.Sh NAME
.Nm critical_enter ,
.Nm critical_exit
.Nd enter and exit a critical region
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/systm.h>
.Ft critical_t
.Fn critical_enter "void"
.Ft void
.Fn critical_exit "critical_t savecrit"
.Sh DESCRIPTION
These functions are used to prevent preemption in a critcal region of code.
All that is guaranteed is that the current CPU will not be preempted by an
external interrupt.
The current CPU may still trigger faults and exceptions during a critical
section.
The
.Fn critical_enter
function returns an opaque value of type
.Vt critical_t .
This value must be passed to the
.Fn critical_exit
function when leaving the critical region.
.Pp
Note that these functions are not required to provide any inter-CPU
synchronization, data protection, or memory ordering guarantees and thus
should
.Sy not
be used to protect shared data structures.
.Pp
These functions should be used with care as an infinite loop within a
critical region will deadlock the CPU.
.Sh RETURN VALUES
The
.Fn critical_enter
function returns an opaque value that must be passed to a corresponding call to
.Fn critical_exit .
.Sh EXAMPLES
This example demonstrates the use of
.Fn critical_enter
and
.Fn critical_exit
to guarantee atomic access to the DMA controller.
.Bd -literal -offset indent
int
isa_dmastatus(int chan)
{
        u_long  cnt = 0;
        int     ffport, waport;
        u_long  low1, high1, low2, high2;
        critical_t savecrit;

	...
        savecrit = critical_enter();
        outb(ffport, 0);
        low1 = inb(waport);
        high1 = inb(waport);
        outb(ffport, 0);
        low2 = inb(waport);
        high2 = inb(waport);
        critical_exit(savecrit);
	...
}
.Ed
.Sh HISTORY
These functions were introduced in
.Fx 5.0 .