diff --git a/share/man/man9/KASSERT.9 b/share/man/man9/KASSERT.9 index 8d9dd98014a8..bab8efe5cfac 100644 --- a/share/man/man9/KASSERT.9 +++ b/share/man/man9/KASSERT.9 @@ -1,192 +1,201 @@ .\" SPDX-License-Identifier: BSD-2-Clause .\" .\" Copyright (c) 2000 Jonathan M. Bresler .\" All rights reserved. .\" Copyright (c) 2023-2024 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. .\" .Dd March 19, 2024 .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 simple assertion message including file and line information. .Ss Assertion Guidelines When adding new assertions, keep in mind their primary purpose: to aid in identifying and debugging of complex error conditions. .Pp The panic messages resulting from assertion failures should be useful without the resulting kernel dump; the message may be included in a bug report, and should contain the relevant information needed to discern how the assertion was violated. This is especially important when the error condition is difficult or impossible for the developer to reproduce locally. .Pp Therefore, assertions should adhere to the following guidelines: .Bl -enum .It Whenever possible, the value of a runtime variable checked by an assertion condition should appear in its message. .It Unrelated conditions must appear in separate assertions. .It Multiple related conditions should be distinguishable (e.g. by value), or split into separate assertions. .It When in doubt, print more information, not less. .El .Pp Combined, this gives greater clarity into the exact cause of an assertion panic; see .Sx EXAMPLES below. .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, flags=%x", __func__, fp, fp->foo_flags)); ... } .Ed .Pp This assertion provides the full flag set for the object, as well as the memory pointer, which may be used by a debugger to examine the object in detail .Po for example with a 'show foo' command in .Xr ddb 4 .Pc . .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 .Pp This is a simple condition, and the message provides enough information to investigate the failure. .Pp The assertion .Bd -literal -offset indent MPASS(td == curthread && (sz >= SIZE_MIN && sz <= SIZE_MAX)); .Ed .Pp is .Em NOT useful enough. The message doesn't indicate which part of the assertion was violated, nor does it report the value of .Dv sz , which may be critical to understanding .Em why the assertion failed. .Pp According to the guidelines above, this would be correctly expressed as: .Bd -literal -offset indent MPASS(td == curthread); KASSERT(sz >= SIZE_MIN && sz <= SIZE_MAX, ("invalid size argument: %u", sz)); .Ed +.Sh HISTORY +The +.Nm MPASS +macro first appeared in +.Bsx +and was imported into +.Fx 5.0 . +The name originates as an acronym of "multi-processor assert", but has evolved +to mean "must pass", or "must-pass assert". .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 .