1 /* Compile-time assert-like macros. 2 3 Copyright (C) 2005-2006, 2009-2011 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 _GL_VERIFY_H 21 # define _GL_VERIFY_H 22 23 24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per the 25 C1X draft N1548 section 6.7.10. This is supported by GCC 4.6.0 and 26 later, in C mode, and its use here generates easier-to-read diagnostics 27 when verify (R) fails. 28 29 Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per the 30 C++0X draft N3242 section 7.(4). 31 This will likely be supported by future GCC versions, in C++ mode. 32 33 Use this only with GCC. If we were willing to slow 'configure' 34 down we could also use it with other compilers, but since this 35 affects only the quality of diagnostics, why bother? */ 36 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus 37 # define _GL_HAVE__STATIC_ASSERT 1 38 # endif 39 /* The condition (99 < __GNUC__) is temporary, until we know about the 40 first G++ release that supports static_assert. */ 41 # if (99 < __GNUC__) && defined __cplusplus 42 # define _GL_HAVE_STATIC_ASSERT 1 43 # endif 44 45 /* Each of these macros verifies that its argument R is nonzero. To 46 be portable, R should be an integer constant expression. Unlike 47 assert (R), there is no run-time overhead. 48 49 If _Static_assert works, verify (R) uses it directly. Similarly, 50 _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct 51 that is an operand of sizeof. 52 53 The code below uses several ideas for C++ compilers, and for C 54 compilers that do not support _Static_assert: 55 56 * The first step is ((R) ? 1 : -1). Given an expression R, of 57 integral or boolean or floating-point type, this yields an 58 expression of integral type, whose value is later verified to be 59 constant and nonnegative. 60 61 * Next this expression W is wrapped in a type 62 struct _gl_verify_type { 63 unsigned int _gl_verify_error_if_negative: W; 64 }. 65 If W is negative, this yields a compile-time error. No compiler can 66 deal with a bit-field of negative size. 67 68 One might think that an array size check would have the same 69 effect, that is, that the type struct { unsigned int dummy[W]; } 70 would work as well. However, inside a function, some compilers 71 (such as C++ compilers and GNU C) allow local parameters and 72 variables inside array size expressions. With these compilers, 73 an array size check would not properly diagnose this misuse of 74 the verify macro: 75 76 void function (int n) { verify (n < 0); } 77 78 * For the verify macro, the struct _gl_verify_type will need to 79 somehow be embedded into a declaration. To be portable, this 80 declaration must declare an object, a constant, a function, or a 81 typedef name. If the declared entity uses the type directly, 82 such as in 83 84 struct dummy {...}; 85 typedef struct {...} dummy; 86 extern struct {...} *dummy; 87 extern void dummy (struct {...} *); 88 extern struct {...} *dummy (void); 89 90 two uses of the verify macro would yield colliding declarations 91 if the entity names are not disambiguated. A workaround is to 92 attach the current line number to the entity name: 93 94 #define _GL_CONCAT0(x, y) x##y 95 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 96 extern struct {...} * _GL_CONCAT (dummy, __LINE__); 97 98 But this has the problem that two invocations of verify from 99 within the same macro would collide, since the __LINE__ value 100 would be the same for both invocations. (The GCC __COUNTER__ 101 macro solves this problem, but is not portable.) 102 103 A solution is to use the sizeof operator. It yields a number, 104 getting rid of the identity of the type. Declarations like 105 106 extern int dummy [sizeof (struct {...})]; 107 extern void dummy (int [sizeof (struct {...})]); 108 extern int (*dummy (void)) [sizeof (struct {...})]; 109 110 can be repeated. 111 112 * Should the implementation use a named struct or an unnamed struct? 113 Which of the following alternatives can be used? 114 115 extern int dummy [sizeof (struct {...})]; 116 extern int dummy [sizeof (struct _gl_verify_type {...})]; 117 extern void dummy (int [sizeof (struct {...})]); 118 extern void dummy (int [sizeof (struct _gl_verify_type {...})]); 119 extern int (*dummy (void)) [sizeof (struct {...})]; 120 extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})]; 121 122 In the second and sixth case, the struct type is exported to the 123 outer scope; two such declarations therefore collide. GCC warns 124 about the first, third, and fourth cases. So the only remaining 125 possibility is the fifth case: 126 127 extern int (*dummy (void)) [sizeof (struct {...})]; 128 129 * GCC warns about duplicate declarations of the dummy function if 130 -Wredundant_decls is used. GCC 4.3 and later have a builtin 131 __COUNTER__ macro that can let us generate unique identifiers for 132 each dummy function, to suppress this warning. 133 134 * This implementation exploits the fact that older versions of GCC, 135 which do not support _Static_assert, also do not warn about the 136 last declaration mentioned above. 137 138 * In C++, any struct definition inside sizeof is invalid. 139 Use a template type to work around the problem. */ 140 141 /* Concatenate two preprocessor tokens. */ 142 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 143 # define _GL_CONCAT0(x, y) x##y 144 145 /* _GL_COUNTER is an integer, preferably one that changes each time we 146 use it. Use __COUNTER__ if it works, falling back on __LINE__ 147 otherwise. __LINE__ isn't perfect, but it's better than a 148 constant. */ 149 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__ 150 # define _GL_COUNTER __COUNTER__ 151 # else 152 # define _GL_COUNTER __LINE__ 153 # endif 154 155 /* Generate a symbol with the given prefix, making it unique if 156 possible. */ 157 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER) 158 159 /* Verify requirement R at compile-time, as an integer constant expression 160 that returns 1. If R is false, fail at compile-time, preferably 161 with a diagnostic that includes the string-literal DIAGNOSTIC. */ 162 163 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \ 164 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC))) 165 166 # ifdef __cplusplus 167 # if !GNULIB_defined_struct__gl_verify_type 168 template <int w> 169 struct _gl_verify_type { 170 unsigned int _gl_verify_error_if_negative: w; 171 }; 172 # define GNULIB_defined_struct__gl_verify_type 1 173 # endif 174 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 175 _gl_verify_type<(R) ? 1 : -1> 176 # elif defined _GL_HAVE__STATIC_ASSERT 177 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 178 struct { \ 179 _Static_assert (R, DIAGNOSTIC); \ 180 int _gl_dummy; \ 181 } 182 # else 183 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 184 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; } 185 # endif 186 187 /* Verify requirement R at compile-time, as a declaration without a 188 trailing ';'. If R is false, fail at compile-time, preferably 189 with a diagnostic that includes the string-literal DIAGNOSTIC. 190 191 Unfortunately, unlike C1X, this implementation must appear as an 192 ordinary declaration, and cannot appear inside struct { ... }. */ 193 194 # ifdef _GL_HAVE__STATIC_ASSERT 195 # define _GL_VERIFY _Static_assert 196 # else 197 # define _GL_VERIFY(R, DIAGNOSTIC) \ 198 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \ 199 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)] 200 # endif 201 202 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */ 203 # ifdef _GL_STATIC_ASSERT_H 204 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert 205 # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC) 206 # endif 207 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert 208 # define static_assert _Static_assert /* Draft C1X requires this #define. */ 209 # endif 210 # endif 211 212 /* @assert.h omit start@ */ 213 214 /* Each of these macros verifies that its argument R is nonzero. To 215 be portable, R should be an integer constant expression. Unlike 216 assert (R), there is no run-time overhead. 217 218 There are two macros, since no single macro can be used in all 219 contexts in C. verify_true (R) is for scalar contexts, including 220 integer constant expression contexts. verify (R) is for declaration 221 contexts, e.g., the top level. */ 222 223 /* Verify requirement R at compile-time, as an integer constant expression. 224 Return 1. This is equivalent to verify_expr (R, 1). 225 226 verify_true is obsolescent; please use verify_expr instead. */ 227 228 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")") 229 230 /* Verify requirement R at compile-time. Return the value of the 231 expression E. */ 232 233 # define verify_expr(R, E) \ 234 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E)) 235 236 /* Verify requirement R at compile-time, as a declaration without a 237 trailing ';'. */ 238 239 # define verify(R) _GL_VERIFY (R, "verify (" #R ")") 240 241 /* @assert.h omit end@ */ 242 243 #endif 244