1 /* -*- buffer-read-only: t -*- vi: set ro: */
2 /* DO NOT EDIT! GENERATED AUTOMATICALLY! */
3 /* Compile-time assert-like macros.
5 Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.
7 This program is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 3 of the License, or
10 (at your option) any later version.
12 This program is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>. */
20 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
25 /* Each of these macros verifies that its argument R is nonzero. To
26 be portable, R should be an integer constant expression. Unlike
27 assert (R), there is no run-time overhead.
29 There are two macros, since no single macro can be used in all
30 contexts in C. verify_true (R) is for scalar contexts, including
31 integer constant expression contexts. verify (R) is for declaration
32 contexts, e.g., the top level.
34 Symbols ending in "__" are private to this header.
36 The code below uses several ideas.
38 * The first step is ((R) ? 1 : -1). Given an expression R, of
39 integral or boolean or floating-point type, this yields an
40 expression of integral type, whose value is later verified to be
41 constant and nonnegative.
43 * Next this expression W is wrapped in a type
44 struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
45 If W is negative, this yields a compile-time error. No compiler can
46 deal with a bit-field of negative size.
48 One might think that an array size check would have the same
49 effect, that is, that the type struct { unsigned int dummy[W]; }
50 would work as well. However, inside a function, some compilers
51 (such as C++ compilers and GNU C) allow local parameters and
52 variables inside array size expressions. With these compilers,
53 an array size check would not properly diagnose this misuse of
56 void function (int n) { verify (n < 0); }
58 * For the verify macro, the struct verify_type__ will need to
59 somehow be embedded into a declaration. To be portable, this
60 declaration must declare an object, a constant, a function, or a
61 typedef name. If the declared entity uses the type directly,
65 typedef struct {...} dummy;
66 extern struct {...} *dummy;
67 extern void dummy (struct {...} *);
68 extern struct {...} *dummy (void);
70 two uses of the verify macro would yield colliding declarations
71 if the entity names are not disambiguated. A workaround is to
72 attach the current line number to the entity name:
74 #define GL_CONCAT0(x, y) x##y
75 #define GL_CONCAT(x, y) GL_CONCAT0 (x, y)
76 extern struct {...} * GL_CONCAT(dummy,__LINE__);
78 But this has the problem that two invocations of verify from
79 within the same macro would collide, since the __LINE__ value
80 would be the same for both invocations.
82 A solution is to use the sizeof operator. It yields a number,
83 getting rid of the identity of the type. Declarations like
85 extern int dummy [sizeof (struct {...})];
86 extern void dummy (int [sizeof (struct {...})]);
87 extern int (*dummy (void)) [sizeof (struct {...})];
91 * Should the implementation use a named struct or an unnamed struct?
92 Which of the following alternatives can be used?
94 extern int dummy [sizeof (struct {...})];
95 extern int dummy [sizeof (struct verify_type__ {...})];
96 extern void dummy (int [sizeof (struct {...})]);
97 extern void dummy (int [sizeof (struct verify_type__ {...})]);
98 extern int (*dummy (void)) [sizeof (struct {...})];
99 extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
101 In the second and sixth case, the struct type is exported to the
102 outer scope; two such declarations therefore collide. GCC warns
103 about the first, third, and fourth cases. So the only remaining
104 possibility is the fifth case:
106 extern int (*dummy (void)) [sizeof (struct {...})];
108 * This implementation exploits the fact that GCC does not warn about
109 the last declaration mentioned above. If a future version of GCC
110 introduces a warning for this, the problem could be worked around
111 by using code specialized to GCC, e.g.,:
115 extern int (* verify_function__ (void)) \
116 [__builtin_constant_p (R) && (R) ? 1 : -1]
119 * In C++, any struct definition inside sizeof is invalid.
120 Use a template type to work around the problem. */
123 /* Verify requirement R at compile-time, as an integer constant expression.
128 struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
129 # define verify_true(R) \
130 (!!sizeof (verify_type__<(R) ? 1 : -1>))
132 # define verify_true(R) \
134 (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
137 /* Verify requirement R at compile-time, as a declaration without a
140 # define verify(R) extern int (* verify_function__ (void)) [verify_true (R)]