1
/* Compile-time assert-like macros.
3
Copyright (C) 2005, 2006 Free Software Foundation, Inc.
5
This program is free software: you can redistribute it and/or modify
6
it under the terms of the GNU General Public License as published by
7
the Free Software Foundation; either version 3 of the License, or
8
(at your option) any later version.
10
This program is distributed in the hope that it will be useful,
11
but WITHOUT ANY WARRANTY; without even the implied warranty of
12
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13
GNU General Public License for more details.
15
You should have received a copy of the GNU General Public License
16
along with this program. If not, see <http://www.gnu.org/licenses/>. */
18
/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
23
/* Each of these macros verifies that its argument R is nonzero. To
24
be portable, R should be an integer constant expression. Unlike
25
assert (R), there is no run-time overhead.
27
There are two macros, since no single macro can be used in all
28
contexts in C. verify_true (R) is for scalar contexts, including
29
integer constant expression contexts. verify (R) is for declaration
30
contexts, e.g., the top level.
32
Symbols ending in "__" are private to this header.
34
The code below uses several ideas.
36
* The first step is ((R) ? 1 : -1). Given an expression R, of
37
integral or boolean or floating-point type, this yields an
38
expression of integral type, whose value is later verified to be
39
constant and nonnegative.
41
* Next this expression W is wrapped in a type
42
struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
43
If W is negative, this yields a compile-time error. No compiler can
44
deal with a bit-field of negative size.
46
One might think that an array size check would have the same
47
effect, that is, that the type struct { unsigned int dummy[W]; }
48
would work as well. However, inside a function, some compilers
49
(such as C++ compilers and GNU C) allow local parameters and
50
variables inside array size expressions. With these compilers,
51
an array size check would not properly diagnose this misuse of
54
void function (int n) { verify (n < 0); }
56
* For the verify macro, the struct verify_type__ will need to
57
somehow be embedded into a declaration. To be portable, this
58
declaration must declare an object, a constant, a function, or a
59
typedef name. If the declared entity uses the type directly,
63
typedef struct {...} dummy;
64
extern struct {...} *dummy;
65
extern void dummy (struct {...} *);
66
extern struct {...} *dummy (void);
68
two uses of the verify macro would yield colliding declarations
69
if the entity names are not disambiguated. A workaround is to
70
attach the current line number to the entity name:
72
#define GL_CONCAT0(x, y) x##y
73
#define GL_CONCAT(x, y) GL_CONCAT0 (x, y)
74
extern struct {...} * GL_CONCAT(dummy,__LINE__);
76
But this has the problem that two invocations of verify from
77
within the same macro would collide, since the __LINE__ value
78
would be the same for both invocations.
80
A solution is to use the sizeof operator. It yields a number,
81
getting rid of the identity of the type. Declarations like
83
extern int dummy [sizeof (struct {...})];
84
extern void dummy (int [sizeof (struct {...})]);
85
extern int (*dummy (void)) [sizeof (struct {...})];
89
* Should the implementation use a named struct or an unnamed struct?
90
Which of the following alternatives can be used?
92
extern int dummy [sizeof (struct {...})];
93
extern int dummy [sizeof (struct verify_type__ {...})];
94
extern void dummy (int [sizeof (struct {...})]);
95
extern void dummy (int [sizeof (struct verify_type__ {...})]);
96
extern int (*dummy (void)) [sizeof (struct {...})];
97
extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
99
In the second and sixth case, the struct type is exported to the
100
outer scope; two such declarations therefore collide. GCC warns
101
about the first, third, and fourth cases. So the only remaining
102
possibility is the fifth case:
104
extern int (*dummy (void)) [sizeof (struct {...})];
106
* This implementation exploits the fact that GCC does not warn about
107
the last declaration mentioned above. If a future version of GCC
108
introduces a warning for this, the problem could be worked around
109
by using code specialized to GCC, e.g.,:
113
extern int (* verify_function__ (void)) \
114
[__builtin_constant_p (R) && (R) ? 1 : -1]
117
* In C++, any struct definition inside sizeof is invalid.
118
Use a template type to work around the problem. */
121
/* Verify requirement R at compile-time, as an integer constant expression.
126
struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
127
# define verify_true(R) \
128
(!!sizeof (verify_type__<(R) ? 1 : -1>))
130
# define verify_true(R) \
132
(struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
135
/* Verify requirement R at compile-time, as a declaration without a
138
# define verify(R) extern int (* verify_function__ (void)) [verify_true (R)]
added
removed
lib/verify.h
