1 /* Compile-time assert-like macros. 2 3 Copyright (C) 2005, 2006 Free Software Foundation, Inc. 4 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. 9 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. 14 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/>. */ 17 18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */ 19 20 #ifndef VERIFY_H 21 # define VERIFY_H 1 22 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. 26 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. 31 32 Symbols ending in "__" are private to this header. 33 34 The code below uses several ideas. 35 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. 40 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. 45 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 52 the verify macro: 53 54 void function (int n) { verify (n < 0); } 55 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, 60 such as in 61 62 struct dummy {...}; 63 typedef struct {...} dummy; 64 extern struct {...} *dummy; 65 extern void dummy (struct {...} *); 66 extern struct {...} *dummy (void); 67 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: 71 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__); 75 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. 79 80 A solution is to use the sizeof operator. It yields a number, 81 getting rid of the identity of the type. Declarations like 82 83 extern int dummy [sizeof (struct {...})]; 84 extern void dummy (int [sizeof (struct {...})]); 85 extern int (*dummy (void)) [sizeof (struct {...})]; 86 87 can be repeated. 88 89 * Should the implementation use a named struct or an unnamed struct? 90 Which of the following alternatives can be used? 91 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__ {...})]; 98 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: 103 104 extern int (*dummy (void)) [sizeof (struct {...})]; 105 106 * 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.,: 110 111 #if 4 <= __GNUC__ 112 # define verify(R) \ 113 extern int (* verify_function__ (void)) \ 114 [__builtin_constant_p (R) && (R) ? 1 : -1] 115 #endif 116 117 * In C++, any struct definition inside sizeof is invalid. 118 Use a template type to work around the problem. */ 119 120 121 /* Verify requirement R at compile-time, as an integer constant expression. 122 Return 1. */ 123 124 # ifdef __cplusplus 125 template <int w> 126 struct verify_type__ { unsigned int verify_error_if_negative_size__: w; }; 127 # define verify_true(R) \ 128 (!!sizeof (verify_type__<(R) ? 1 : -1>)) 129 # else 130 # define verify_true(R) \ 131 (!!sizeof \ 132 (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; })) 133 # endif 134 135 /* Verify requirement R at compile-time, as a declaration without a 136 trailing ';'. */ 137 138 # define verify(R) extern int (* verify_function__ (void)) [verify_true (R)] 139 140 #endif 141
