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1 office 1 /* Compile-time assert-like macros.
2  
3 Copyright (C) 2005-2006, 2009-2015 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__ + (6 <= __GNUC_MINOR__) \
35 && (201112L <= __STDC_VERSION__ || !defined __STRICT_ANSI__) \
36 && !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 /* FreeBSD 9.1 <sys/cdefs.h>, included by <stddef.h> and lots of other
46 system headers, defines a conflicting _Static_assert that is no
47 better than ours; override it. */
48 #ifndef _GL_HAVE_STATIC_ASSERT
49 # include <stddef.h>
50 # undef _Static_assert
51 #endif
52  
53 /* Each of these macros verifies that its argument R is nonzero. To
54 be portable, R should be an integer constant expression. Unlike
55 assert (R), there is no run-time overhead.
56  
57 If _Static_assert works, verify (R) uses it directly. Similarly,
58 _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
59 that is an operand of sizeof.
60  
61 The code below uses several ideas for C++ compilers, and for C
62 compilers that do not support _Static_assert:
63  
64 * The first step is ((R) ? 1 : -1). Given an expression R, of
65 integral or boolean or floating-point type, this yields an
66 expression of integral type, whose value is later verified to be
67 constant and nonnegative.
68  
69 * Next this expression W is wrapped in a type
70 struct _gl_verify_type {
71 unsigned int _gl_verify_error_if_negative: W;
72 }.
73 If W is negative, this yields a compile-time error. No compiler can
74 deal with a bit-field of negative size.
75  
76 One might think that an array size check would have the same
77 effect, that is, that the type struct { unsigned int dummy[W]; }
78 would work as well. However, inside a function, some compilers
79 (such as C++ compilers and GNU C) allow local parameters and
80 variables inside array size expressions. With these compilers,
81 an array size check would not properly diagnose this misuse of
82 the verify macro:
83  
84 void function (int n) { verify (n < 0); }
85  
86 * For the verify macro, the struct _gl_verify_type will need to
87 somehow be embedded into a declaration. To be portable, this
88 declaration must declare an object, a constant, a function, or a
89 typedef name. If the declared entity uses the type directly,
90 such as in
91  
92 struct dummy {...};
93 typedef struct {...} dummy;
94 extern struct {...} *dummy;
95 extern void dummy (struct {...} *);
96 extern struct {...} *dummy (void);
97  
98 two uses of the verify macro would yield colliding declarations
99 if the entity names are not disambiguated. A workaround is to
100 attach the current line number to the entity name:
101  
102 #define _GL_CONCAT0(x, y) x##y
103 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
104 extern struct {...} * _GL_CONCAT (dummy, __LINE__);
105  
106 But this has the problem that two invocations of verify from
107 within the same macro would collide, since the __LINE__ value
108 would be the same for both invocations. (The GCC __COUNTER__
109 macro solves this problem, but is not portable.)
110  
111 A solution is to use the sizeof operator. It yields a number,
112 getting rid of the identity of the type. Declarations like
113  
114 extern int dummy [sizeof (struct {...})];
115 extern void dummy (int [sizeof (struct {...})]);
116 extern int (*dummy (void)) [sizeof (struct {...})];
117  
118 can be repeated.
119  
120 * Should the implementation use a named struct or an unnamed struct?
121 Which of the following alternatives can be used?
122  
123 extern int dummy [sizeof (struct {...})];
124 extern int dummy [sizeof (struct _gl_verify_type {...})];
125 extern void dummy (int [sizeof (struct {...})]);
126 extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
127 extern int (*dummy (void)) [sizeof (struct {...})];
128 extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
129  
130 In the second and sixth case, the struct type is exported to the
131 outer scope; two such declarations therefore collide. GCC warns
132 about the first, third, and fourth cases. So the only remaining
133 possibility is the fifth case:
134  
135 extern int (*dummy (void)) [sizeof (struct {...})];
136  
137 * GCC warns about duplicate declarations of the dummy function if
138 -Wredundant-decls is used. GCC 4.3 and later have a builtin
139 __COUNTER__ macro that can let us generate unique identifiers for
140 each dummy function, to suppress this warning.
141  
142 * This implementation exploits the fact that older versions of GCC,
143 which do not support _Static_assert, also do not warn about the
144 last declaration mentioned above.
145  
146 * GCC warns if -Wnested-externs is enabled and verify() is used
147 within a function body; but inside a function, you can always
148 arrange to use verify_expr() instead.
149  
150 * In C++, any struct definition inside sizeof is invalid.
151 Use a template type to work around the problem. */
152  
153 /* Concatenate two preprocessor tokens. */
154 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
155 #define _GL_CONCAT0(x, y) x##y
156  
157 /* _GL_COUNTER is an integer, preferably one that changes each time we
158 use it. Use __COUNTER__ if it works, falling back on __LINE__
159 otherwise. __LINE__ isn't perfect, but it's better than a
160 constant. */
161 #if defined __COUNTER__ && __COUNTER__ != __COUNTER__
162 # define _GL_COUNTER __COUNTER__
163 #else
164 # define _GL_COUNTER __LINE__
165 #endif
166  
167 /* Generate a symbol with the given prefix, making it unique if
168 possible. */
169 #define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
170  
171 /* Verify requirement R at compile-time, as an integer constant expression
172 that returns 1. If R is false, fail at compile-time, preferably
173 with a diagnostic that includes the string-literal DIAGNOSTIC. */
174  
175 #define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
176 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
177  
178 #ifdef __cplusplus
179 # if !GNULIB_defined_struct__gl_verify_type
180 template <int w>
181 struct _gl_verify_type {
182 unsigned int _gl_verify_error_if_negative: w;
183 };
184 # define GNULIB_defined_struct__gl_verify_type 1
185 # endif
186 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
187 _gl_verify_type<(R) ? 1 : -1>
188 #elif defined _GL_HAVE__STATIC_ASSERT
189 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
190 struct { \
191 _Static_assert (R, DIAGNOSTIC); \
192 int _gl_dummy; \
193 }
194 #else
195 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
196 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
197 #endif
198  
199 /* Verify requirement R at compile-time, as a declaration without a
200 trailing ';'. If R is false, fail at compile-time, preferably
201 with a diagnostic that includes the string-literal DIAGNOSTIC.
202  
203 Unfortunately, unlike C11, this implementation must appear as an
204 ordinary declaration, and cannot appear inside struct { ... }. */
205  
206 #ifdef _GL_HAVE__STATIC_ASSERT
207 # define _GL_VERIFY _Static_assert
208 #else
209 # define _GL_VERIFY(R, DIAGNOSTIC) \
210 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
211 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
212 #endif
213  
214 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
215 #ifdef _GL_STATIC_ASSERT_H
216 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
217 # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
218 # endif
219 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
220 # define static_assert _Static_assert /* C11 requires this #define. */
221 # endif
222 #endif
223  
224 /* @assert.h omit start@ */
225  
226 /* Each of these macros verifies that its argument R is nonzero. To
227 be portable, R should be an integer constant expression. Unlike
228 assert (R), there is no run-time overhead.
229  
230 There are two macros, since no single macro can be used in all
231 contexts in C. verify_true (R) is for scalar contexts, including
232 integer constant expression contexts. verify (R) is for declaration
233 contexts, e.g., the top level. */
234  
235 /* Verify requirement R at compile-time, as an integer constant expression.
236 Return 1. This is equivalent to verify_expr (R, 1).
237  
238 verify_true is obsolescent; please use verify_expr instead. */
239  
240 #define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
241  
242 /* Verify requirement R at compile-time. Return the value of the
243 expression E. */
244  
245 #define verify_expr(R, E) \
246 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
247  
248 /* Verify requirement R at compile-time, as a declaration without a
249 trailing ';'. */
250  
251 #define verify(R) _GL_VERIFY (R, "verify (" #R ")")
252  
253 #ifndef __has_builtin
254 # define __has_builtin(x) 0
255 #endif
256  
257 /* Assume that R always holds. This lets the compiler optimize
258 accordingly. R should not have side-effects; it may or may not be
259 evaluated. Behavior is undefined if R is false. */
260  
261 #if (__has_builtin (__builtin_unreachable) \
262 || 4 < __GNUC__ + (5 <= __GNUC_MINOR__))
263 # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
264 #elif 1200 <= _MSC_VER
265 # define assume(R) __assume (R)
266 #elif (defined lint \
267 && (__has_builtin (__builtin_trap) \
268 || 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))))
269 /* Doing it this way helps various packages when configured with
270 --enable-gcc-warnings, which compiles with -Dlint. It's nicer
271 when 'assume' silences warnings even with older GCCs. */
272 # define assume(R) ((R) ? (void) 0 : __builtin_trap ())
273 #else
274 # define assume(R) ((void) (0 && (R)))
275 #endif
276  
277 /* @assert.h omit end@ */
278  
279 #endif