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.\" Copyright (c) 2000 Jonathan M. Bresler
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.Dd March 16, 2023
.Dt KASSERT 9
.Os
.Sh NAME
.Nm KASSERT
.Nd kernel expression verification macros
.Sh SYNOPSIS
.Cd "options INVARIANTS"
.Pp
.In sys/param.h
.In sys/systm.h
.Fn KASSERT expression msg
.Fn MPASS expression
.Sh DESCRIPTION
Assertions are widely used within the
.Fx
kernel to verify programmatic assumptions.
For violations of run-time assumptions and invariants, it is desirable to fail
as soon and as loudly as possible.
Assertions are optional code; for non-recoverable error conditions an explicit
call to
.Xr panic 9
is usually preferred.
.Pp
The
.Fn KASSERT
macro tests the given boolean
.Fa expression .
If
.Fa expression
evaluates to
.Dv false ,
and the kernel is compiled with
.Cd "options INVARIANTS" ,
the
.Xr panic 9
function is called.
This terminates the running system at the point of the error, possibly dropping
into the kernel debugger or initiating a kernel core dump.
The second argument,
.Fa msg ,
is a
.Xr printf 9
format string and its arguments,
enclosed in parentheses.
The formatted string will become the panic string.
.Pp
In a kernel that is built without
.Cd "options INVARIANTS" ,
the assertion macros are defined to be no-ops.
This eliminates the runtime overhead of widespread assertions from release
builds of the kernel.
Therefore, checks which can be performed in a constant amount of time can be
added as assertions without concern about their performance impact.
More expensive checks, such as those that output to console, or verify the
integrity of a chain of objects are generally best hidden behind the
.Cd DIAGNOSTIC
kernel option.
.Pp
The
.Fn MPASS
macro (read as: "must-pass")
is a convenience wrapper around
.Fn KASSERT
that automatically generates a sensible assertion message including file and
line information.
.Sh EXAMPLES
A hypothetical
.Vt struct foo
object must not have its 'active' flag set when calling
.Fn foo_dealloc :
.Bd -literal -offset indent
void
foo_dealloc(struct foo *fp)
{

	KASSERT((fp->foo_flags & FOO_ACTIVE) == 0,
	    ("%s: fp %p is still active", __func__, fp));
	...
}
.Ed
.Pp
The assertion
.Bd -literal -offset indent
MPASS(td == curthread);
.Ed
.Pp
located on line 87 of a file named foo.c would generate the following panic
message:
.Bd -literal -offset indent
panic: Assertion td == curthread failed at foo.c:87
.Ed
.Sh SEE ALSO
.Xr panic 9
.Sh AUTHORS
This manual page was written by
.An Jonathan M. Bresler Aq Mt jmb@FreeBSD.org
and
.An Mitchell Horne Aq Mt mhorne@FreeBSD.org .