1 /* Compile-time assert-like macros. 2 3 Copyright (C) 2005-2006, 2009-2013 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 C11. 25 This is supported by GCC 4.6.0 and later, in C mode, and its use 26 here generates easier-to-read diagnostics when verify (R) fails. 27 28 Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11. 29 This will likely be supported by future GCC versions, in C++ mode. 30 31 Use this only with GCC. If we were willing to slow 'configure' 32 down we could also use it with other compilers, but since this 33 affects only the quality of diagnostics, why bother? */ 34 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus 35 # define _GL_HAVE__STATIC_ASSERT 1 36 # endif 37 /* The condition (99 < __GNUC__) is temporary, until we know about the 38 first G++ release that supports static_assert. */ 39 # if (99 < __GNUC__) && defined __cplusplus 40 # define _GL_HAVE_STATIC_ASSERT 1 41 # endif 42 43 /* Each of these macros verifies that its argument R is nonzero. To 44 be portable, R should be an integer constant expression. Unlike 45 assert (R), there is no run-time overhead. 46 47 If _Static_assert works, verify (R) uses it directly. Similarly, 48 _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct 49 that is an operand of sizeof. 50 51 The code below uses several ideas for C++ compilers, and for C 52 compilers that do not support _Static_assert: 53 54 * The first step is ((R) ? 1 : -1). Given an expression R, of 55 integral or boolean or floating-point type, this yields an 56 expression of integral type, whose value is later verified to be 57 constant and nonnegative. 58 59 * Next this expression W is wrapped in a type 60 struct _gl_verify_type { 61 unsigned int _gl_verify_error_if_negative: W; 62 }. 63 If W is negative, this yields a compile-time error. No compiler can 64 deal with a bit-field of negative size. 65 66 One might think that an array size check would have the same 67 effect, that is, that the type struct { unsigned int dummy[W]; } 68 would work as well. However, inside a function, some compilers 69 (such as C++ compilers and GNU C) allow local parameters and 70 variables inside array size expressions. With these compilers, 71 an array size check would not properly diagnose this misuse of 72 the verify macro: 73 74 void function (int n) { verify (n < 0); } 75 76 * For the verify macro, the struct _gl_verify_type will need to 77 somehow be embedded into a declaration. To be portable, this 78 declaration must declare an object, a constant, a function, or a 79 typedef name. If the declared entity uses the type directly, 80 such as in 81 82 struct dummy {...}; 83 typedef struct {...} dummy; 84 extern struct {...} *dummy; 85 extern void dummy (struct {...} *); 86 extern struct {...} *dummy (void); 87 88 two uses of the verify macro would yield colliding declarations 89 if the entity names are not disambiguated. A workaround is to 90 attach the current line number to the entity name: 91 92 #define _GL_CONCAT0(x, y) x##y 93 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 94 extern struct {...} * _GL_CONCAT (dummy, __LINE__); 95 96 But this has the problem that two invocations of verify from 97 within the same macro would collide, since the __LINE__ value 98 would be the same for both invocations. (The GCC __COUNTER__ 99 macro solves this problem, but is not portable.) 100 101 A solution is to use the sizeof operator. It yields a number, 102 getting rid of the identity of the type. Declarations like 103 104 extern int dummy [sizeof (struct {...})]; 105 extern void dummy (int [sizeof (struct {...})]); 106 extern int (*dummy (void)) [sizeof (struct {...})]; 107 108 can be repeated. 109 110 * Should the implementation use a named struct or an unnamed struct? 111 Which of the following alternatives can be used? 112 113 extern int dummy [sizeof (struct {...})]; 114 extern int dummy [sizeof (struct _gl_verify_type {...})]; 115 extern void dummy (int [sizeof (struct {...})]); 116 extern void dummy (int [sizeof (struct _gl_verify_type {...})]); 117 extern int (*dummy (void)) [sizeof (struct {...})]; 118 extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})]; 119 120 In the second and sixth case, the struct type is exported to the 121 outer scope; two such declarations therefore collide. GCC warns 122 about the first, third, and fourth cases. So the only remaining 123 possibility is the fifth case: 124 125 extern int (*dummy (void)) [sizeof (struct {...})]; 126 127 * GCC warns about duplicate declarations of the dummy function if 128 -Wredundant-decls is used. GCC 4.3 and later have a builtin 129 __COUNTER__ macro that can let us generate unique identifiers for 130 each dummy function, to suppress this warning. 131 132 * This implementation exploits the fact that older versions of GCC, 133 which do not support _Static_assert, also do not warn about the 134 last declaration mentioned above. 135 136 * GCC warns if -Wnested-externs is enabled and verify() is used 137 within a function body; but inside a function, you can always 138 arrange to use verify_expr() instead. 139 140 * In C++, any struct definition inside sizeof is invalid. 141 Use a template type to work around the problem. */ 142 143 /* Concatenate two preprocessor tokens. */ 144 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 145 # define _GL_CONCAT0(x, y) x##y 146 147 /* _GL_COUNTER is an integer, preferably one that changes each time we 148 use it. Use __COUNTER__ if it works, falling back on __LINE__ 149 otherwise. __LINE__ isn't perfect, but it's better than a 150 constant. */ 151 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__ 152 # define _GL_COUNTER __COUNTER__ 153 # else 154 # define _GL_COUNTER __LINE__ 155 # endif 156 157 /* Generate a symbol with the given prefix, making it unique if 158 possible. */ 159 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER) 160 161 /* Verify requirement R at compile-time, as an integer constant expression 162 that returns 1. If R is false, fail at compile-time, preferably 163 with a diagnostic that includes the string-literal DIAGNOSTIC. */ 164 165 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \ 166 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC))) 167 168 # ifdef __cplusplus 169 # if !GNULIB_defined_struct__gl_verify_type 170 template <int w> 171 struct _gl_verify_type { 172 unsigned int _gl_verify_error_if_negative: w; 173 }; 174 # define GNULIB_defined_struct__gl_verify_type 1 175 # endif 176 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 177 _gl_verify_type<(R) ? 1 : -1> 178 # elif defined _GL_HAVE__STATIC_ASSERT 179 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 180 struct { \ 181 _Static_assert (R, DIAGNOSTIC); \ 182 int _gl_dummy; \ 183 } 184 # else 185 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 186 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; } 187 # endif 188 189 /* Verify requirement R at compile-time, as a declaration without a 190 trailing ';'. If R is false, fail at compile-time, preferably 191 with a diagnostic that includes the string-literal DIAGNOSTIC. 192 193 Unfortunately, unlike C11, this implementation must appear as an 194 ordinary declaration, and cannot appear inside struct { ... }. */ 195 196 # ifdef _GL_HAVE__STATIC_ASSERT 197 # define _GL_VERIFY _Static_assert 198 # else 199 # define _GL_VERIFY(R, DIAGNOSTIC) \ 200 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \ 201 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)] 202 # endif 203 204 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */ 205 # ifdef _GL_STATIC_ASSERT_H 206 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert 207 # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC) 208 # endif 209 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert 210 # define static_assert _Static_assert /* C11 requires this #define. */ 211 # endif 212 # endif 213 214 /* @assert.h omit start@ */ 215 216 /* Each of these macros verifies that its argument R is nonzero. To 217 be portable, R should be an integer constant expression. Unlike 218 assert (R), there is no run-time overhead. 219 220 There are two macros, since no single macro can be used in all 221 contexts in C. verify_true (R) is for scalar contexts, including 222 integer constant expression contexts. verify (R) is for declaration 223 contexts, e.g., the top level. */ 224 225 /* Verify requirement R at compile-time, as an integer constant expression. 226 Return 1. This is equivalent to verify_expr (R, 1). 227 228 verify_true is obsolescent; please use verify_expr instead. */ 229 230 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")") 231 232 /* Verify requirement R at compile-time. Return the value of the 233 expression E. */ 234 235 # define verify_expr(R, E) \ 236 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E)) 237 238 /* Verify requirement R at compile-time, as a declaration without a 239 trailing ';'. */ 240 241 # define verify(R) _GL_VERIFY (R, "verify (" #R ")") 242 243 /* @assert.h omit end@ */ 244 245 #endif 246