1 /* Compile-time assert-like macros. 2 3 Copyright (C) 2005-2006, 2009-2020 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 <https://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 (R, DIAGNOSTIC) 25 works as per C11. This is supported by GCC 4.6.0 and later, in C 26 mode. 27 28 Define _GL_HAVE__STATIC_ASSERT1 to 1 if _Static_assert (R) works as 29 per C2X, and define _GL_HAVE_STATIC_ASSERT1 if static_assert (R) 30 works as per C++17. This is supported by GCC 9.1 and later. 31 32 Support compilers claiming conformance to the relevant standard, 33 and also support GCC when not pedantic. If we were willing to slow 34 'configure' down we could also use it with other compilers, but 35 since this affects only the quality of diagnostics, why bother? */ 36 #ifndef __cplusplus 37 # if (201112L <= __STDC_VERSION__ \ 38 || (!defined __STRICT_ANSI__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__))) 39 # define _GL_HAVE__STATIC_ASSERT 1 40 # endif 41 # if (202000L <= __STDC_VERSION__ \ 42 || (!defined __STRICT_ANSI__ && 9 <= __GNUC__)) 43 # define _GL_HAVE__STATIC_ASSERT1 1 44 # endif 45 #else 46 # if 201703L <= __cplusplus || 9 <= __GNUC__ 47 # define _GL_HAVE_STATIC_ASSERT1 1 48 # endif 49 #endif 50 51 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other 52 system headers, defines a conflicting _Static_assert that is no 53 better than ours; override it. */ 54 #ifndef _GL_HAVE__STATIC_ASSERT 55 # include <stddef.h> 56 # undef _Static_assert 57 #endif 58 59 /* Each of these macros verifies that its argument R is nonzero. To 60 be portable, R should be an integer constant expression. Unlike 61 assert (R), there is no run-time overhead. 62 63 If _Static_assert works, verify (R) uses it directly. Similarly, 64 _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct 65 that is an operand of sizeof. 66 67 The code below uses several ideas for C++ compilers, and for C 68 compilers that do not support _Static_assert: 69 70 * The first step is ((R) ? 1 : -1). Given an expression R, of 71 integral or boolean or floating-point type, this yields an 72 expression of integral type, whose value is later verified to be 73 constant and nonnegative. 74 75 * Next this expression W is wrapped in a type 76 struct _gl_verify_type { 77 unsigned int _gl_verify_error_if_negative: W; 78 }. 79 If W is negative, this yields a compile-time error. No compiler can 80 deal with a bit-field of negative size. 81 82 One might think that an array size check would have the same 83 effect, that is, that the type struct { unsigned int dummy[W]; } 84 would work as well. However, inside a function, some compilers 85 (such as C++ compilers and GNU C) allow local parameters and 86 variables inside array size expressions. With these compilers, 87 an array size check would not properly diagnose this misuse of 88 the verify macro: 89 90 void function (int n) { verify (n < 0); } 91 92 * For the verify macro, the struct _gl_verify_type will need to 93 somehow be embedded into a declaration. To be portable, this 94 declaration must declare an object, a constant, a function, or a 95 typedef name. If the declared entity uses the type directly, 96 such as in 97 98 struct dummy {...}; 99 typedef struct {...} dummy; 100 extern struct {...} *dummy; 101 extern void dummy (struct {...} *); 102 extern struct {...} *dummy (void); 103 104 two uses of the verify macro would yield colliding declarations 105 if the entity names are not disambiguated. A workaround is to 106 attach the current line number to the entity name: 107 108 #define _GL_CONCAT0(x, y) x##y 109 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 110 extern struct {...} * _GL_CONCAT (dummy, __LINE__); 111 112 But this has the problem that two invocations of verify from 113 within the same macro would collide, since the __LINE__ value 114 would be the same for both invocations. (The GCC __COUNTER__ 115 macro solves this problem, but is not portable.) 116 117 A solution is to use the sizeof operator. It yields a number, 118 getting rid of the identity of the type. Declarations like 119 120 extern int dummy [sizeof (struct {...})]; 121 extern void dummy (int [sizeof (struct {...})]); 122 extern int (*dummy (void)) [sizeof (struct {...})]; 123 124 can be repeated. 125 126 * Should the implementation use a named struct or an unnamed struct? 127 Which of the following alternatives can be used? 128 129 extern int dummy [sizeof (struct {...})]; 130 extern int dummy [sizeof (struct _gl_verify_type {...})]; 131 extern void dummy (int [sizeof (struct {...})]); 132 extern void dummy (int [sizeof (struct _gl_verify_type {...})]); 133 extern int (*dummy (void)) [sizeof (struct {...})]; 134 extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})]; 135 136 In the second and sixth case, the struct type is exported to the 137 outer scope; two such declarations therefore collide. GCC warns 138 about the first, third, and fourth cases. So the only remaining 139 possibility is the fifth case: 140 141 extern int (*dummy (void)) [sizeof (struct {...})]; 142 143 * GCC warns about duplicate declarations of the dummy function if 144 -Wredundant-decls is used. GCC 4.3 and later have a builtin 145 __COUNTER__ macro that can let us generate unique identifiers for 146 each dummy function, to suppress this warning. 147 148 * This implementation exploits the fact that older versions of GCC, 149 which do not support _Static_assert, also do not warn about the 150 last declaration mentioned above. 151 152 * GCC warns if -Wnested-externs is enabled and 'verify' is used 153 within a function body; but inside a function, you can always 154 arrange to use verify_expr instead. 155 156 * In C++, any struct definition inside sizeof is invalid. 157 Use a template type to work around the problem. */ 158 159 /* Concatenate two preprocessor tokens. */ 160 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) 161 #define _GL_CONCAT0(x, y) x##y 162 163 /* _GL_COUNTER is an integer, preferably one that changes each time we 164 use it. Use __COUNTER__ if it works, falling back on __LINE__ 165 otherwise. __LINE__ isn't perfect, but it's better than a 166 constant. */ 167 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__ 168 # define _GL_COUNTER __COUNTER__ 169 #else 170 # define _GL_COUNTER __LINE__ 171 #endif 172 173 /* Generate a symbol with the given prefix, making it unique if 174 possible. */ 175 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER) 176 177 /* Verify requirement R at compile-time, as an integer constant expression 178 that returns 1. If R is false, fail at compile-time, preferably 179 with a diagnostic that includes the string-literal DIAGNOSTIC. */ 180 181 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \ 182 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC))) 183 184 #ifdef __cplusplus 185 # if !GNULIB_defined_struct__gl_verify_type 186 template <int w> 187 struct _gl_verify_type { 188 unsigned int _gl_verify_error_if_negative: w; 189 }; 190 # define GNULIB_defined_struct__gl_verify_type 1 191 # endif 192 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 193 _gl_verify_type<(R) ? 1 : -1> 194 #elif defined _GL_HAVE__STATIC_ASSERT 195 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 196 struct { \ 197 _Static_assert (R, DIAGNOSTIC); \ 198 int _gl_dummy; \ 199 } 200 #else 201 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ 202 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; } 203 #endif 204 205 /* Verify requirement R at compile-time, as a declaration without a 206 trailing ';'. If R is false, fail at compile-time. 207 208 This macro requires three or more arguments but uses at most the first 209 two, so that the _Static_assert macro optionally defined below supports 210 both the C11 two-argument syntax and the C2X one-argument syntax. 211 212 Unfortunately, unlike C11, this implementation must appear as an 213 ordinary declaration, and cannot appear inside struct { ... }. */ 214 215 #if defined _GL_HAVE__STATIC_ASSERT 216 # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC) 217 #else 218 # define _GL_VERIFY(R, DIAGNOSTIC, ...) \ 219 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \ 220 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)] 221 #endif 222 223 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */ 224 #ifdef _GL_STATIC_ASSERT_H 225 # if !defined _GL_HAVE__STATIC_ASSERT1 && !defined _Static_assert 226 # define _Static_assert(...) \ 227 _GL_VERIFY (__VA_ARGS__, "static assertion failed", -) 228 # endif 229 # if !defined _GL_HAVE_STATIC_ASSERT1 && !defined static_assert 230 # define static_assert _Static_assert /* C11 requires this #define. */ 231 # endif 232 #endif 233 234 /* @assert.h omit start@ */ 235 236 #if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__)) 237 # define _GL_HAS_BUILTIN_TRAP 1 238 #elif defined __has_builtin 239 # define _GL_HAS_BUILTIN_TRAP __has_builtin (__builtin_trap) 240 #else 241 # define _GL_HAS_BUILTIN_TRAP 0 242 #endif 243 244 #if 4 < __GNUC__ + (5 <= __GNUC_MINOR__) 245 # define _GL_HAS_BUILTIN_UNREACHABLE 1 246 #elif defined __has_builtin 247 # define _GL_HAS_BUILTIN_UNREACHABLE __has_builtin (__builtin_unreachable) 248 #else 249 # define _GL_HAS_BUILTIN_UNREACHABLE 0 250 #endif 251 252 /* Each of these macros verifies that its argument R is nonzero. To 253 be portable, R should be an integer constant expression. Unlike 254 assert (R), there is no run-time overhead. 255 256 There are two macros, since no single macro can be used in all 257 contexts in C. verify_expr (R, E) is for scalar contexts, including 258 integer constant expression contexts. verify (R) is for declaration 259 contexts, e.g., the top level. */ 260 261 /* Verify requirement R at compile-time. Return the value of the 262 expression E. */ 263 264 #define verify_expr(R, E) \ 265 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E)) 266 267 /* Verify requirement R at compile-time, as a declaration without a 268 trailing ';'. verify (R) acts like static_assert (R) except that 269 it is portable to C11/C++14 and earlier, it can issue better 270 diagnostics, and its name is shorter and may be more convenient. */ 271 272 #ifdef __PGI 273 /* PGI barfs if R is long. */ 274 # define verify(R) _GL_VERIFY (R, "verify (...)", -) 275 #else 276 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -) 277 #endif 278 279 /* Assume that R always holds. Behavior is undefined if R is false, 280 fails to evaluate, or has side effects. 281 282 'assume (R)' is a directive from the programmer telling the 283 compiler that R is true so the compiler needn't generate code to 284 test R. This is why 'assume' is in verify.h: it's related to 285 static checking (in this case, static checking done by the 286 programmer), not dynamic checking. 287 288 'assume (R)' can affect compilation of all the code, not just code 289 that happens to be executed after the assume (R) is "executed". 290 For example, if the code mistakenly does 'assert (R); assume (R);' 291 the compiler is entitled to optimize away the 'assert (R)'. 292 293 Although assuming R can help a compiler generate better code or 294 diagnostics, performance can suffer if R uses hard-to-optimize 295 features such as function calls not inlined by the compiler. */ 296 297 #if _GL_HAS_BUILTIN_UNREACHABLE 298 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ()) 299 #elif 1200 <= _MSC_VER 300 # define assume(R) __assume (R) 301 #elif (defined GCC_LINT || defined lint) && _GL_HAS_BUILTIN_TRAP 302 /* Doing it this way helps various packages when configured with 303 --enable-gcc-warnings, which compiles with -Dlint. It's nicer 304 when 'assume' silences warnings even with older GCCs. */ 305 # define assume(R) ((R) ? (void) 0 : __builtin_trap ()) 306 #else 307 /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6. */ 308 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0) 309 #endif 310 311 /* @assert.h omit end@ */ 312 313 #endif 314