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83 lines
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Plaintext
83 lines
3.0 KiB
Plaintext
@c GNU verify module documentation
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@c Copyright (C) 2006 Free Software Foundation, Inc.
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@c Permission is granted to copy, distribute and/or modify this document
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@c under the terms of the GNU Free Documentation License, Version 1.2
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@c or any later version published by the Free Software Foundation;
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@c with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
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@c Texts. A copy of the license is included in the ``GNU Free
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@c Documentation License'' file as part of this distribution.
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@node Compile-time Assertions
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@section Compile-time Assertions
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@cindex assertion
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@findex verify
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@findex verify_true
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The @samp{verify} module supports compile-time tests, as opposed to
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the standard @file{assert.h} header which supports only runtime tests.
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Since the tests occur at compile-time, they are more reliable, and
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they require no runtime overhead.
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This module provides a header file @file{verify.h} that defines two
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macros: @code{verify (@var{EXPRESSION})} and @code{verify_true
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(@var{EXPRESSION})}. Both accept an integer constant expression
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argument and verify that it is nonzero. If not, a compile-time error
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results.
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@code{verify (@var{EXPRESSION});} is a declaration; it can occur
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outside of functions. In contrast, @code{verify_true
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(@var{EXPRESSION})} is an integer constant expression that always
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evaluates to 1; it can be used in macros that expand to
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expressions.
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@var{EXPRESSION} should be an integer constant expression in the sense
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of the C standard. Its leaf operands should be integer, enumeration,
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or character constants; or @code{sizeof} expressions that return
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constants; or floating constants that are the immediate operands of
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casts. Outside a @code{sizeof} subexpression, @var{EXPRESSION} should
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not contain any assignments, function calls, comma operators, casts to
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non-integer types, or subexpressions whose values are outside the
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representable ranges for their types. If @var{EXPRESSION} is not an
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integer constant expression, then a compiler might reject a usage like
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@samp{verify (@var{EXPRESSION});} even when @var{EXPRESSION} is
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nonzero.
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Here are some example uses.
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@example
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#include <verify.h>
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#include <limits.h>
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#include <time.h>
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/* Verify that time_t is an integer type. */
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verify ((time_t) 1.5 == 1);
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/* Verify that time_t is at least as wide as int. */
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verify (INT_MIN == (time_t) INT_MIN);
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verify (INT_MAX == (time_t) INT_MAX);
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/* Verify that time_t is signed. */
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verify ((time_t) -1 < 0);
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/* Verify that time_t uses two's complement representation. */
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verify (~ (time_t) -1 == 0);
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/* Return the maximum value of the integer type T,
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verifying that T is an unsigned integer type. */
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#define MAX_UNSIGNED_VAL_WITH_COMMA(t) \
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(verify_true (0 < (T) -1), (T) -1)
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/* Same as MAX_UNSIGNED_VAL_WITH_COMMA,
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but expand to an integer constant expression,
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which cannot contain a comma operator.
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The cast to (T) is outside the conditional expression
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so that the result is of type T
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even when T is narrower than unsigned int. */
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#define MAX_UNSIGNED_VAL(t) ((T) \
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((T) (verify_true (0 < (T) -1) ? -1 : 0))
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@end example
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