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@c GNU verify module documentation
@c Copyright (C) 2006, 2009-2017 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
@c or any later version published by the Free Software Foundation;
@c with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
@c Texts. A copy of the license is included in the ``GNU Free
@c Documentation License'' file as part of this distribution.
@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, C11
specifies a builtin @code{_Static_assert (@var{V},
@var{STRING-LITERAL})}, its @file{assert.h} header has a similar macro
named @code{static_assert}, and C++11 has a similar
@code{static_assert} builtin. These builtins and macros differ
from @code{verify} in two major ways. First, they can also be used
within a @code{struct} or @code{union} specifier, in place of an
ordinary member declaration. Second, they require the programmer to
specify a compile-time diagnostic as a string literal.
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
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