33f8d79d76
Reviewed by: imp, 0mp MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D29833
144 lines
4.2 KiB
Groff
144 lines
4.2 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)assert.3 8.1 (Berkeley) 6/9/93
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.\" $FreeBSD$
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.\"
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.Dd April 20, 2021
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.Dt ASSERT 3
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.Os
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.Sh NAME
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.Nm assert ,
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.Nm static_assert
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.Nd expression verification macro
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.Sh SYNOPSIS
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.In assert.h
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.Fn assert expression
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.Fn static_assert expression
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.Fn static_assert expression message
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.Sh DESCRIPTION
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The
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.Fn assert
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macro tests the given
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.Ar expression
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and if it is false,
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the calling process is terminated.
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A diagnostic message is written to
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.Dv stderr
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and the function
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.Xr abort 3
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is called, effectively terminating the program.
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.Pp
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If
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.Ar expression
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is true,
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the
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.Fn assert
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macro does nothing.
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.Pp
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The
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.Fn assert
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macro
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may be removed at compile time by defining
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.Dv NDEBUG
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as a macro
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(e.g., by using the
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.Xr cc 1
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option
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.Fl D Ns Dv NDEBUG ) .
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Unlike most other include files,
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.In assert.h
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may be included multiple times.
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Each time whether or not
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.Dv NDEBUG
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is defined determines the behavior of assert from that point forward
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until the end of the unit or another include of
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.In assert.h .
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.Pp
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The
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.Fn assert
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macro should only be used for ensuring the developer's expectations
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hold true.
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It is not appropriate for regular run-time error detection.
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.Pp
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The
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.Fn static_assert
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macro expands to
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.Fn _Static_assert ,
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and, contrarily to
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.Fn assert ,
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makes assertions at compile-time.
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Once the constraint is violated, the compiler produces a diagnostic
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message including the string literal message, if provided.
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The initial form of the
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.Fn _Static_assert
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containing a string literal message was introduced in C11 standard, and
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the other form with no string literal is to be implemented by C2x and
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some compilers may lack its adoption at present.
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.Sh EXAMPLES
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The assertion:
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.Dl "assert(1 == 0);"
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generates a diagnostic message similar to the following:
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.Dl "Assertion failed: (1 == 0), function main, file main.c, line 100."
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.Pp
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The following assert tries to assert there was no partial read:
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.Dl "assert(read(fd, buf, nbytes) == nbytes);"
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However, there are two problems.
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First, it checks for normal conditions, rather than conditions that
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indicate a bug.
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Second, the code will disappear if
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.Dv NDEBUG
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is defined, changing the semantics of the program.
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.Pp
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The following asserts that the size of the S structure is 16.
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Otherwise, it produces a diagnostic message which points at the
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constraint and includes the provided string literal:
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.Dl "static_assert(sizeof(struct S) == 16, ""size mismatch"");"
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If none is provided, it only points at the constraint.
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.Sh SEE ALSO
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.Xr abort2 2 ,
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.Xr abort 3
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.Sh STANDARDS
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.Rs
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The
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.Fn assert
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macro conforms to
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.St -isoC-99 .
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.Re
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.Pp
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.Rs
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The
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.Fn static_assert
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macro conforms to
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.St -isoC-2011 .
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.Re
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.Sh HISTORY
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An
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.Nm
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macro appeared in
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.At v7 .
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