1 /* Compile-time assert-like macros.
3 Copyright (C) 2005-2006, 2009-2020 Free Software Foundation, Inc.
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.
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.
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/>. */
18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
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
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.
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? */
37 # if (201112L <= __STDC_VERSION__ \
38 || (!defined __STRICT_ANSI__ && 4 < __GNUC__ + (6 <= __GNUC_MINOR__)))
39 # define _GL_HAVE__STATIC_ASSERT 1
41 # if (202000L <= __STDC_VERSION__ \
42 || (!defined __STRICT_ANSI__ && 9 <= __GNUC__))
43 # define _GL_HAVE__STATIC_ASSERT1 1
46 # if 201703L <= __cplusplus || 9 <= __GNUC__
47 # define _GL_HAVE_STATIC_ASSERT1 1
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
56 # undef _Static_assert
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.
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.
67 The code below uses several ideas for C++ compilers, and for C
68 compilers that do not support _Static_assert:
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.
75 * Next this expression W is wrapped in a type
76 struct _gl_verify_type {
77 unsigned int _gl_verify_error_if_negative: W;
79 If W is negative, this yields a compile-time error. No compiler can
80 deal with a bit-field of negative size.
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
90 void function (int n) { verify (n < 0); }
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,
99 typedef struct {...} dummy;
100 extern struct {...} *dummy;
101 extern void dummy (struct {...} *);
102 extern struct {...} *dummy (void);
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:
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__);
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.)
117 A solution is to use the sizeof operator. It yields a number,
118 getting rid of the identity of the type. Declarations like
120 extern int dummy [sizeof (struct {...})];
121 extern void dummy (int [sizeof (struct {...})]);
122 extern int (*dummy (void)) [sizeof (struct {...})];
126 * Should the implementation use a named struct or an unnamed struct?
127 Which of the following alternatives can be used?
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 {...})];
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:
141 extern int (*dummy (void)) [sizeof (struct {...})];
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.
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.
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.
156 * In C++, any struct definition inside sizeof is invalid.
157 Use a template type to work around the problem. */
159 /* Concatenate two preprocessor tokens. */
160 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
161 #define _GL_CONCAT0(x, y) x##y
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
167 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
168 # define _GL_COUNTER __COUNTER__
170 # define _GL_COUNTER __LINE__
173 /* Generate a symbol with the given prefix, making it unique if
175 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
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. */
181 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
182 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
185 # if !GNULIB_defined_struct__gl_verify_type
187 struct _gl_verify_type
{
188 unsigned int _gl_verify_error_if_negative
: w
;
190 # define GNULIB_defined_struct__gl_verify_type 1
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) \
197 _Static_assert (R, DIAGNOSTIC); \
201 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
202 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
205 /* Verify requirement R at compile-time, as a declaration without a
206 trailing ';'. If R is false, fail at compile-time.
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.
212 Unfortunately, unlike C11, this implementation must appear as an
213 ordinary declaration, and cannot appear inside struct { ... }. */
215 #if defined _GL_HAVE__STATIC_ASSERT
216 # define _GL_VERIFY(R, DIAGNOSTIC, ...) _Static_assert (R, DIAGNOSTIC)
218 # define _GL_VERIFY(R, DIAGNOSTIC, ...) \
219 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
220 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
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", -)
229 # if !defined _GL_HAVE_STATIC_ASSERT1 && !defined static_assert
230 # define static_assert _Static_assert /* C11 requires this #define. */
234 /* @assert.h omit start@ */
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)
241 # define _GL_HAS_BUILTIN_TRAP 0
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)
249 # define _GL_HAS_BUILTIN_UNREACHABLE 0
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.
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. */
261 /* Verify requirement R at compile-time. Return the value of the
264 #define verify_expr(R, E) \
265 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
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. */
273 /* PGI barfs if R is long. */
274 # define verify(R) _GL_VERIFY (R, "verify (...)", -)
276 # define verify(R) _GL_VERIFY (R, "verify (" #R ")", -)
279 /* Assume that R always holds. Behavior is undefined if R is false,
280 fails to evaluate, or has side effects.
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.
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)'.
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. */
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 ())
307 /* Some tools grok NOTREACHED, e.g., Oracle Studio 12.6. */
308 # define assume(R) ((R) ? (void) 0 : /*NOTREACHED*/ (void) 0)
311 /* @assert.h omit end@ */