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 2, or (at your option) 8 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, write to the Free Software Foundation, 17 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 18 19 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */ 20 21 #ifndef VERIFY_H 22 # define VERIFY_H 1 23 24 /* Each of these macros verifies that its argument R is nonzero. To 25 be portable, R should be an integer constant expression. Unlike 26 assert (R), there is no run-time overhead. 27 28 There are two macros, since no single macro can be used in all 29 contexts in C. verify_true (R) is for scalar contexts, including 30 integer constant expression contexts. verify (R) is for declaration 31 contexts, e.g., the top level. 32 33 Symbols ending in "__" are private to this header. 34 35 The code below uses several ideas. 36 37 * The first step is ((R) ? 1 : -1). Given an expression R, of 38 integral or boolean or floating-point type, this yields an 39 expression of integral type, whose value is later verified to be 40 constant and nonnegative. 41 42 * Next this expression W is wrapped in a type 43 struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }. 44 If W is negative, this yields a compile-time error. No compiler can 45 deal with a bit-field of negative size. 46 47 One might think that an array size check would have the same 48 effect, that is, that the type struct { unsigned int dummy[W]; } 49 would work as well. However, inside a function, some compilers 50 (such as C++ compilers and GNU C) allow local parameters and 51 variables inside array size expressions. With these compilers, 52 an array size check would not properly diagnose this misuse of 53 the verify macro: 54 55 void function (int n) { verify (n < 0); } 56 57 * For the verify macro, the struct verify_type__ will need to 58 somehow be embedded into a declaration. To be portable, this 59 declaration must declare an object, a constant, a function, or a 60 typedef name. If the declared entity uses the type directly, 61 such as in 62 63 struct dummy {...}; 64 typedef struct {...} dummy; 65 extern struct {...} *dummy; 66 extern void dummy (struct {...} *); 67 extern struct {...} *dummy (void); 68 69 two uses of the verify macro would yield colliding declarations 70 if the entity names are not disambiguated. A workaround is to 71 attach the current line number to the entity name: 72 73 #define GL_CONCAT0(x, y) x##y 74 #define GL_CONCAT(x, y) GL_CONCAT0 (x, y) 75 extern struct {...} * GL_CONCAT(dummy,__LINE__); 76 77 But this has the problem that two invocations of verify from 78 within the same macro would collide, since the __LINE__ value 79 would be the same for both invocations. 80 81 A solution is to use the sizeof operator. It yields a number, 82 getting rid of the identity of the type. Declarations like 83 84 extern int dummy [sizeof (struct {...})]; 85 extern void dummy (int [sizeof (struct {...})]); 86 extern int (*dummy (void)) [sizeof (struct {...})]; 87 88 can be repeated. 89 90 * Should the implementation use a named struct or an unnamed struct? 91 Which of the following alternatives can be used? 92 93 extern int dummy [sizeof (struct {...})]; 94 extern int dummy [sizeof (struct verify_type__ {...})]; 95 extern void dummy (int [sizeof (struct {...})]); 96 extern void dummy (int [sizeof (struct verify_type__ {...})]); 97 extern int (*dummy (void)) [sizeof (struct {...})]; 98 extern int (*dummy (void)) [sizeof (struct verify_type__ {...})]; 99 100 In the second and sixth case, the struct type is exported to the 101 outer scope; two such declarations therefore collide. GCC warns 102 about the first, third, and fourth cases. So the only remaining 103 possibility is the fifth case: 104 105 extern int (*dummy (void)) [sizeof (struct {...})]; 106 107 * This implementation exploits the fact that GCC does not warn about 108 the last declaration mentioned above. If a future version of GCC 109 introduces a warning for this, the problem could be worked around 110 by using code specialized to GCC, e.g.,: 111 112 #if 4 <= __GNUC__ 113 # define verify(R) \ 114 extern int (* verify_function__ (void)) \ 115 [__builtin_constant_p (R) && (R) ? 1 : -1] 116 #endif 117 118 * In C++, any struct definition inside sizeof is invalid. 119 Use a template type to work around the problem. */ 120 121 122 /* Verify requirement R at compile-time, as an integer constant expression. 123 Return 1. */ 124 125 # ifdef __cplusplus 126 template <int w> 127 struct verify_type__ { unsigned int verify_error_if_negative_size__: w; }; 128 # define verify_true(R) \ 129 (!!sizeof (verify_type__<(R) ? 1 : -1>)) 130 # else 131 # define verify_true(R) \ 132 (!!sizeof \ 133 (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; })) 134 # endif 135 136 /* Verify requirement R at compile-time, as a declaration without a 137 trailing ';'. */ 138 139 # define verify(R) extern int (* verify_function__ (void)) [verify_true (R)] 140 141 #endif 142