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-/* Compile-time assert-like macros.
-
- Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.
-
- This program is free software: you can redistribute it and/or modify
- it under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
-
- This program is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- GNU Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public License
- along with this program. If not, see <http://www.gnu.org/licenses/>. */
-
-/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
-
-#ifndef VERIFY_H
-# define VERIFY_H 1
-
-/* Each of these macros verifies that its argument R is nonzero. To
- be portable, R should be an integer constant expression. Unlike
- assert (R), there is no run-time overhead.
-
- There are two macros, since no single macro can be used in all
- contexts in C. verify_true (R) is for scalar contexts, including
- integer constant expression contexts. verify (R) is for declaration
- contexts, e.g., the top level.
-
- Symbols ending in "__" are private to this header.
-
- The code below uses several ideas.
-
- * The first step is ((R) ? 1 : -1). Given an expression R, of
- integral or boolean or floating-point type, this yields an
- expression of integral type, whose value is later verified to be
- constant and nonnegative.
-
- * Next this expression W is wrapped in a type
- struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
- If W is negative, this yields a compile-time error. No compiler can
- deal with a bit-field of negative size.
-
- One might think that an array size check would have the same
- effect, that is, that the type struct { unsigned int dummy[W]; }
- would work as well. However, inside a function, some compilers
- (such as C++ compilers and GNU C) allow local parameters and
- variables inside array size expressions. With these compilers,
- an array size check would not properly diagnose this misuse of
- the verify macro:
-
- void function (int n) { verify (n < 0); }
-
- * For the verify macro, the struct verify_type__ will need to
- somehow be embedded into a declaration. To be portable, this
- declaration must declare an object, a constant, a function, or a
- typedef name. If the declared entity uses the type directly,
- such as in
-
- struct dummy {...};
- typedef struct {...} dummy;
- extern struct {...} *dummy;
- extern void dummy (struct {...} *);
- extern struct {...} *dummy (void);
-
- two uses of the verify macro would yield colliding declarations
- if the entity names are not disambiguated. A workaround is to
- attach the current line number to the entity name:
-
- #define _GL_CONCAT0(x, y) x##y
- #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
- extern struct {...} * _GL_CONCAT (dummy, __LINE__);
-
- But this has the problem that two invocations of verify from
- within the same macro would collide, since the __LINE__ value
- would be the same for both invocations. (The GCC __COUNTER__
- macro solves this problem, but is not portable.)
-
- A solution is to use the sizeof operator. It yields a number,
- getting rid of the identity of the type. Declarations like
-
- extern int dummy [sizeof (struct {...})];
- extern void dummy (int [sizeof (struct {...})]);
- extern int (*dummy (void)) [sizeof (struct {...})];
-
- can be repeated.
-
- * Should the implementation use a named struct or an unnamed struct?
- Which of the following alternatives can be used?
-
- extern int dummy [sizeof (struct {...})];
- extern int dummy [sizeof (struct verify_type__ {...})];
- extern void dummy (int [sizeof (struct {...})]);
- extern void dummy (int [sizeof (struct verify_type__ {...})]);
- extern int (*dummy (void)) [sizeof (struct {...})];
- extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
-
- In the second and sixth case, the struct type is exported to the
- outer scope; two such declarations therefore collide. GCC warns
- about the first, third, and fourth cases. So the only remaining
- possibility is the fifth case:
-
- extern int (*dummy (void)) [sizeof (struct {...})];
-
- * GCC warns about duplicate declarations of the dummy function if
- -Wredundant_decls is used. GCC 4.3 and later have a builtin
- __COUNTER__ macro that can let us generate unique identifiers for
- each dummy function, to suppress this warning.
-
- * This implementation exploits the fact that GCC does not warn about
- the last declaration mentioned above. If a future version of GCC
- introduces a warning for this, the problem could be worked around
- by using code specialized to GCC, just as __COUNTER__ is already
- being used if available.
-
- #if 4 <= __GNUC__
- # define verify(R) [another version to keep GCC happy]
- #endif
-
- * In C++, any struct definition inside sizeof is invalid.
- Use a template type to work around the problem. */
-
-/* Concatenate two preprocessor tokens. */
-# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
-# define _GL_CONCAT0(x, y) x##y
-
-/* _GL_COUNTER is an integer, preferably one that changes each time we
- use it. Use __COUNTER__ if it works, falling back on __LINE__
- otherwise. __LINE__ isn't perfect, but it's better than a
- constant. */
-# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
-# define _GL_COUNTER __COUNTER__
-# else
-# define _GL_COUNTER __LINE__
-# endif
-
-/* Generate a symbol with the given prefix, making it unique if
- possible. */
-# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
-
-/* Verify requirement R at compile-time, as an integer constant expression.
- Return 1. */
-
-# ifdef __cplusplus
-template <int w>
- struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
-# define verify_true(R) \
- (!!sizeof (verify_type__<(R) ? 1 : -1>))
-# else
-# define verify_true(R) \
- (!!sizeof \
- (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
-# endif
-
-/* Verify requirement R at compile-time, as a declaration without a
- trailing ';'. */
-
-# define verify(R) \
- extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]
-
-#endif