119 lines
4.9 KiB
Plaintext
119 lines
4.9 KiB
Plaintext
@c GNU verify module documentation
|
|
|
|
@c Copyright (C) 2006, 2009--2024 Free Software Foundation, Inc.
|
|
|
|
@c Permission is granted to copy, distribute and/or modify this document
|
|
@c under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
@c any later version published by the Free Software Foundation; with no
|
|
@c Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
|
@c copy of the license is at <https://www.gnu.org/licenses/fdl-1.3.en.html>.
|
|
|
|
@node Compile-time Assertions
|
|
@section Compile-time Assertions
|
|
|
|
@cindex assertion
|
|
@findex verify
|
|
@findex verify_expr
|
|
|
|
This module provides a header file @file{verify.h} that defines
|
|
macros related to compile-time verification.
|
|
|
|
Two of these macros are @code{verify (@var{V})} and @code{verify_expr
|
|
(@var{V}, @var{EXPR})}. Both accept an integer constant expression
|
|
argument @var{V} and verify that it is nonzero. If not, a compile-time error
|
|
results.
|
|
|
|
These two macros implement compile-time tests, as opposed to
|
|
the standard @code{assert} macro which supports only runtime tests.
|
|
Since the tests occur at compile-time, they are more reliable, and
|
|
they require no runtime overhead.
|
|
|
|
@code{verify (@var{V});} is a declaration; it can occur outside of
|
|
functions. In contrast, @code{verify_expr (@var{V}, @var{EXPR})} is
|
|
an expression that returns the value of @var{EXPR}; it can be used in
|
|
macros that expand to expressions. If @var{EXPR} is an integer
|
|
constant expression, then @code{verify_expr (@var{V}, @var{EXPR})} is
|
|
also an integer constant expression. Although @var{EXPR} and
|
|
@code{verify_expr (@var{V}, @var{EXPR})}@ are guaranteed to have the
|
|
same side effects and value and type (after integer promotion), they
|
|
need not have the same type if @var{EXPR}'s type is an integer that is
|
|
narrower than @code{int} or @code{unsigned int}.
|
|
|
|
@var{V} should be an integer constant expression in the sense
|
|
of the C standard. Its leaf operands should be integer, enumeration,
|
|
or character constants; or @code{sizeof} expressions that return
|
|
constants; or floating constants that are the immediate operands of
|
|
casts. Outside a @code{sizeof} subexpression, @var{V} should
|
|
not contain any assignments, function calls, comma operators, casts to
|
|
non-integer types, or subexpressions whose values are outside the
|
|
representable ranges for their types. If @var{V} is not an
|
|
integer constant expression, then a compiler might reject a usage like
|
|
@samp{verify (@var{V});} even when @var{V} is
|
|
nonzero.
|
|
|
|
Although the standard @code{assert} macro is a runtime test, C23 and C++17
|
|
specify a builtin @code{static_assert (@var{V})}, which differs
|
|
from @code{verify} in two major ways. First, it can also be used
|
|
within a @code{struct} or @code{union} specifier, in place of an
|
|
ordinary member declaration. Second, it allows the programmer to
|
|
specify, as an optional second argument, a compile-time diagnostic as
|
|
a string literal. If your program is not intended to be portable to
|
|
compilers that lack C23 or C++17 @code{static_assert}, the only
|
|
advantage of @code{verify} is that its name is a bit shorter.
|
|
|
|
The @file{verify.h} header defines one more macro, @code{assume
|
|
(@var{E})}, which expands to an expression of type @code{void}
|
|
that causes the compiler to assume that @var{E} yields a nonzero
|
|
value. @var{E} should be a scalar expression, and should not
|
|
have side effects; it may or may not be evaluated. The behavior is
|
|
undefined if @var{E} would yield zero. The main use of @code{assume}
|
|
is optimization, as the compiler may be able to generate better code
|
|
if it assumes @var{E}. For best results, @var{E} should be simple
|
|
enough that a compiler can determine that it has no side effects: if
|
|
@var{E} calls an external function or accesses volatile storage the
|
|
compiler may not be able to optimize @var{E} away and @code{assume
|
|
(@var{E})} may therefore slow down the program.
|
|
|
|
Here are some example uses of these macros.
|
|
|
|
@example
|
|
#include <verify.h>
|
|
|
|
#include <limits.h>
|
|
#include <time.h>
|
|
|
|
/* Verify that time_t is an integer type. */
|
|
verify ((time_t) 1.5 == 1);
|
|
|
|
/* Verify that time_t is no smaller than int. */
|
|
verify (sizeof (int) <= sizeof (time_t));
|
|
|
|
/* Verify that time_t is signed. */
|
|
verify ((time_t) -1 < 0);
|
|
|
|
/* Verify that time_t uses two's complement representation. */
|
|
verify (~ (time_t) -1 == 0);
|
|
|
|
/* Return the maximum value of the integer type T,
|
|
verifying that T is an unsigned integer type.
|
|
The cast to (T) is outside the call to verify_expr
|
|
so that the result is of type T
|
|
even when T is narrower than unsigned int. */
|
|
#define MAX_UNSIGNED_VAL(t) \
|
|
((T) verify_expr (0 < (T) -1, -1))
|
|
|
|
/* Return T divided by CHAR_MAX + 1, where behavior is
|
|
undefined if T < 0. In the common case where CHAR_MAX
|
|
is 127 the compiler can therefore implement the division
|
|
by shifting T right 7 bits, an optimization that would
|
|
not be valid if T were negative. */
|
|
time_t
|
|
time_index (time_t t)
|
|
@{
|
|
assume (0 <= t);
|
|
return t / (CHAR_MAX + 1);
|
|
@}
|
|
|
|
|
|
@end example
|