.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" Copyright (c) 2000 Jonathan M. Bresler
.\" All rights reserved.
.\" Copyright (c) 2023 The FreeBSD Foundation
.\"
.\" Portions of this documentation were written by Mitchell Horne
.\" under sponsorship from the FreeBSD Foundation.
.\"
.\" This program is free software.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.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 .