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/* Compile-time assert-like macros.
Copyright (C) 2005, 2006 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 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 General Public License for more details.
You should have received a copy of the GNU 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.
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 {...})];
* 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, e.g.,:
#if 4 <= __GNUC__
# define verify(R) \
extern int (* verify_function__ (void)) \
[__builtin_constant_p (R) && (R) ? 1 : -1]
#endif
* In C++, any struct definition inside sizeof is invalid.
Use a template type to work around the problem. */
/* 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 (* verify_function__ (void)) [verify_true (R)]
#endif
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