| 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 | template <int w> |
| 168 | struct _gl_verify_type { |
| 169 | unsigned int _gl_verify_error_if_negative: w; |
| 170 | }; |
| 171 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ |
| 172 | _gl_verify_type<(R) ? 1 : -1> |
| 173 | # elif defined _GL_HAVE__STATIC_ASSERT |
| 174 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ |
| 175 | struct { \ |
| 176 | _Static_assert (R, DIAGNOSTIC); \ |
| 177 | int _gl_dummy; \ |
| 178 | } |
| 179 | # else |
| 180 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ |
| 181 | struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; } |
| 182 | # endif |
| 183 | |
| 184 | /* Verify requirement R at compile-time, as a declaration without a |
| 185 | trailing ';'. If R is false, fail at compile-time, preferably |
| 186 | with a diagnostic that includes the string-literal DIAGNOSTIC. |
| 187 | |
| 188 | Unfortunately, unlike C1X, this implementation must appear as an |
| 189 | ordinary declaration, and cannot appear inside struct { ... }. */ |
| 190 | |
| 191 | # ifdef _GL_HAVE__STATIC_ASSERT |
| 192 | # define _GL_VERIFY _Static_assert |
| 193 | # else |
| 194 | # define _GL_VERIFY(R, DIAGNOSTIC) \ |
| 195 | extern int (*_GL_GENSYM (_gl_verify_function) (void)) \ |
| 196 | [_GL_VERIFY_TRUE (R, DIAGNOSTIC)] |
| 197 | # endif |
| 198 | |
| 199 | /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */ |
| 200 | # ifdef _GL_STATIC_ASSERT_H |
| 201 | # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert |
| 202 | # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC) |
| 203 | # endif |
| 204 | # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert |
| 205 | # define static_assert _Static_assert /* Draft C1X requires this #define. */ |
| 206 | # endif |
| 207 | # endif |
| 208 | |
| 209 | # ifdef _GL_VERIFY_H |
| 210 | |
| 211 | /* Each of these macros verifies that its argument R is nonzero. To |
| 212 | be portable, R should be an integer constant expression. Unlike |
| 213 | assert (R), there is no run-time overhead. |
| 214 | |
| 215 | There are two macros, since no single macro can be used in all |
| 216 | contexts in C. verify_true (R) is for scalar contexts, including |
| 217 | integer constant expression contexts. verify (R) is for declaration |
| 218 | contexts, e.g., the top level. */ |
| 219 | |
| 220 | /* Verify requirement R at compile-time, as an integer constant expression. |
| 221 | Return 1. */ |
| 222 | |
| 223 | # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")") |
| 224 | |
| 225 | /* Verify requirement R at compile-time, as a declaration without a |
| 226 | trailing ';'. */ |
| 227 | |
| 228 | # define verify(R) _GL_VERIFY (R, "verify (" #R ")") |
| 229 | |
| 230 | # endif |
| 231 | |
| 232 | #endif |