1 /* Compile-time assert-like macros. 2 3 Copyright (C) 2005-2006, 2009-2010 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. (The GCC __COUNTER__ 79 macro solves this problem, but is not portable.) 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 * GCC warns about duplicate declarations of the dummy function if 108 -Wredundant_decls is used. GCC 4.3 and later have a builtin 109 __COUNTER__ macro that can let us generate unique identifiers for 110 each dummy function, to suppress this warning. 111 112 * This implementation exploits the fact that GCC does not warn about 113 the last declaration mentioned above. If a future version of GCC 114 introduces a warning for this, the problem could be worked around 115 by using code specialized to GCC, just as __COUNTER__ is already 116 being used if available. 117 118 #if 4 <= __GNUC__ 119 # define verify(R) [another version to keep GCC happy] 120 #endif 121 122 * In C++, any struct definition inside sizeof is invalid. 123 Use a template type to work around the problem. */ 124 125 /* Concatenate two preprocessor tokens. */ 126 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 127 # define _GL_CONCAT0(x, y) x##y 128 129 /* _GL_COUNTER is an integer, preferably one that changes each time we 130 use it. Use __COUNTER__ if it works, falling back on __LINE__ 131 otherwise. __LINE__ isn't perfect, but it's better than a 132 constant. */ 133 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__ 134 # define _GL_COUNTER __COUNTER__ 135 # else 136 # define _GL_COUNTER __LINE__ 137 # endif 138 139 /* Generate a symbol with the given prefix, making it unique if 140 possible. */ 141 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER) 142 143 /* Verify requirement R at compile-time, as an integer constant expression. 144 Return 1. */ 145 146 # ifdef __cplusplus 147 template <int w> 148 struct verify_type__ { unsigned int verify_error_if_negative_size__: w; }; 149 # define verify_true(R) \ 150 (!!sizeof (verify_type__<(R) ? 1 : -1>)) 151 # else 152 # define verify_true(R) \ 153 (!!sizeof \ 154 (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; })) 155 # endif 156 157 /* Verify requirement R at compile-time, as a declaration without a 158 trailing ';'. */ 159 160 # define verify(R) \ 161 extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)] 162 163 #endif 164