245 lines
9.6 KiB
C++
245 lines
9.6 KiB
C++
/* Compile-time assert-like macros.
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Copyright (C) 2005-2006, 2009-2013 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
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#ifndef _GL_VERIFY_H
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# define _GL_VERIFY_H
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/* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11.
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This is supported by GCC 4.6.0 and later, in C mode, and its use
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here generates easier-to-read diagnostics when verify (R) fails.
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Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11.
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This will likely be supported by future GCC versions, in C++ mode.
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Use this only with GCC. If we were willing to slow 'configure'
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down we could also use it with other compilers, but since this
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affects only the quality of diagnostics, why bother? */
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# if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus
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# define _GL_HAVE__STATIC_ASSERT 1
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# endif
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/* The condition (99 < __GNUC__) is temporary, until we know about the
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first G++ release that supports static_assert. */
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# if (99 < __GNUC__) && defined __cplusplus
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# define _GL_HAVE_STATIC_ASSERT 1
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# endif
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/* Each of these macros verifies that its argument R is nonzero. To
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be portable, R should be an integer constant expression. Unlike
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assert (R), there is no run-time overhead.
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If _Static_assert works, verify (R) uses it directly. Similarly,
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_GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
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that is an operand of sizeof.
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The code below uses several ideas for C++ compilers, and for C
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compilers that do not support _Static_assert:
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* The first step is ((R) ? 1 : -1). Given an expression R, of
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integral or boolean or floating-point type, this yields an
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expression of integral type, whose value is later verified to be
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constant and nonnegative.
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* Next this expression W is wrapped in a type
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struct _gl_verify_type {
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unsigned int _gl_verify_error_if_negative: W;
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}.
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If W is negative, this yields a compile-time error. No compiler can
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deal with a bit-field of negative size.
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One might think that an array size check would have the same
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effect, that is, that the type struct { unsigned int dummy[W]; }
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would work as well. However, inside a function, some compilers
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(such as C++ compilers and GNU C) allow local parameters and
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variables inside array size expressions. With these compilers,
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an array size check would not properly diagnose this misuse of
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the verify macro:
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void function (int n) { verify (n < 0); }
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* For the verify macro, the struct _gl_verify_type will need to
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somehow be embedded into a declaration. To be portable, this
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declaration must declare an object, a constant, a function, or a
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typedef name. If the declared entity uses the type directly,
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such as in
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struct dummy {...};
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typedef struct {...} dummy;
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extern struct {...} *dummy;
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extern void dummy (struct {...} *);
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extern struct {...} *dummy (void);
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two uses of the verify macro would yield colliding declarations
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if the entity names are not disambiguated. A workaround is to
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attach the current line number to the entity name:
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#define _GL_CONCAT0(x, y) x##y
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#define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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extern struct {...} * _GL_CONCAT (dummy, __LINE__);
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But this has the problem that two invocations of verify from
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within the same macro would collide, since the __LINE__ value
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would be the same for both invocations. (The GCC __COUNTER__
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macro solves this problem, but is not portable.)
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A solution is to use the sizeof operator. It yields a number,
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getting rid of the identity of the type. Declarations like
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extern int dummy [sizeof (struct {...})];
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extern void dummy (int [sizeof (struct {...})]);
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extern int (*dummy (void)) [sizeof (struct {...})];
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can be repeated.
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* Should the implementation use a named struct or an unnamed struct?
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Which of the following alternatives can be used?
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extern int dummy [sizeof (struct {...})];
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extern int dummy [sizeof (struct _gl_verify_type {...})];
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extern void dummy (int [sizeof (struct {...})]);
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extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
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extern int (*dummy (void)) [sizeof (struct {...})];
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extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
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In the second and sixth case, the struct type is exported to the
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outer scope; two such declarations therefore collide. GCC warns
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about the first, third, and fourth cases. So the only remaining
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possibility is the fifth case:
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extern int (*dummy (void)) [sizeof (struct {...})];
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* GCC warns about duplicate declarations of the dummy function if
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-Wredundant-decls is used. GCC 4.3 and later have a builtin
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__COUNTER__ macro that can let us generate unique identifiers for
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each dummy function, to suppress this warning.
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* This implementation exploits the fact that older versions of GCC,
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which do not support _Static_assert, also do not warn about the
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last declaration mentioned above.
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* GCC warns if -Wnested-externs is enabled and verify() is used
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within a function body; but inside a function, you can always
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arrange to use verify_expr() instead.
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* In C++, any struct definition inside sizeof is invalid.
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Use a template type to work around the problem. */
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/* Concatenate two preprocessor tokens. */
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# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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# define _GL_CONCAT0(x, y) x##y
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/* _GL_COUNTER is an integer, preferably one that changes each time we
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use it. Use __COUNTER__ if it works, falling back on __LINE__
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otherwise. __LINE__ isn't perfect, but it's better than a
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constant. */
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# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
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# define _GL_COUNTER __COUNTER__
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# else
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# define _GL_COUNTER __LINE__
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# endif
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/* Generate a symbol with the given prefix, making it unique if
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possible. */
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# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
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/* Verify requirement R at compile-time, as an integer constant expression
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that returns 1. If R is false, fail at compile-time, preferably
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with a diagnostic that includes the string-literal DIAGNOSTIC. */
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# define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
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(!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
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# ifdef __cplusplus
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# if !GNULIB_defined_struct__gl_verify_type
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template <int w>
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struct _gl_verify_type {
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unsigned int _gl_verify_error_if_negative: w;
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};
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# define GNULIB_defined_struct__gl_verify_type 1
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# endif
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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_gl_verify_type<(R) ? 1 : -1>
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# elif defined _GL_HAVE__STATIC_ASSERT
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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struct { \
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_Static_assert (R, DIAGNOSTIC); \
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int _gl_dummy; \
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}
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# else
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# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
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# endif
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/* Verify requirement R at compile-time, as a declaration without a
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trailing ';'. If R is false, fail at compile-time, preferably
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with a diagnostic that includes the string-literal DIAGNOSTIC.
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Unfortunately, unlike C11, this implementation must appear as an
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ordinary declaration, and cannot appear inside struct { ... }. */
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# ifdef _GL_HAVE__STATIC_ASSERT
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# define _GL_VERIFY _Static_assert
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# else
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# define _GL_VERIFY(R, DIAGNOSTIC) \
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extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
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[_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
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# endif
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/* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
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# ifdef _GL_STATIC_ASSERT_H
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# if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
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# define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
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# endif
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# if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
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# define static_assert _Static_assert /* C11 requires this #define. */
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# endif
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# endif
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/* @assert.h omit start@ */
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/* Each of these macros verifies that its argument R is nonzero. To
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be portable, R should be an integer constant expression. Unlike
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assert (R), there is no run-time overhead.
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There are two macros, since no single macro can be used in all
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contexts in C. verify_true (R) is for scalar contexts, including
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integer constant expression contexts. verify (R) is for declaration
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contexts, e.g., the top level. */
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/* Verify requirement R at compile-time, as an integer constant expression.
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Return 1. This is equivalent to verify_expr (R, 1).
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verify_true is obsolescent; please use verify_expr instead. */
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# define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
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/* Verify requirement R at compile-time. Return the value of the
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expression E. */
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# define verify_expr(R, E) \
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(_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
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/* Verify requirement R at compile-time, as a declaration without a
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trailing ';'. */
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# define verify(R) _GL_VERIFY (R, "verify (" #R ")")
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/* @assert.h omit end@ */
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#endif
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