1/* Compile-time assert-like macros.
2
3 Copyright (C) 2005-2006, 2009-2012 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 Lesser General Public License as published by
7 the Free Software Foundation; either version 2.1 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 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser 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 * In C++, any struct definition inside sizeof is invalid.
137 Use a template type to work around the problem. */
138
139/* Concatenate two preprocessor tokens. */
140# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
141# define _GL_CONCAT0(x, y) x##y
142
143/* _GL_COUNTER is an integer, preferably one that changes each time we
144 use it. Use __COUNTER__ if it works, falling back on __LINE__
145 otherwise. __LINE__ isn't perfect, but it's better than a
146 constant. */
147# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
148# define _GL_COUNTER __COUNTER__
149# else
150# define _GL_COUNTER __LINE__
151# endif
152
153/* Generate a symbol with the given prefix, making it unique if
154 possible. */
155# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
156
157/* Verify requirement R at compile-time, as an integer constant expression
158 that returns 1. If R is false, fail at compile-time, preferably
159 with a diagnostic that includes the string-literal DIAGNOSTIC. */
160
161# define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
162 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
163
164# ifdef __cplusplus
165# if !GNULIB_defined_struct__gl_verify_type
166template <int w>
167 struct _gl_verify_type {
168 unsigned int _gl_verify_error_if_negative: w;
169 };
170# define GNULIB_defined_struct__gl_verify_type 1
171# endif
172# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
173 _gl_verify_type<(R) ? 1 : -1>
174# elif defined _GL_HAVE__STATIC_ASSERT
175# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
176 struct { \
177 _Static_assert (R, DIAGNOSTIC); \
178 int _gl_dummy; \
179 }
180# else
181# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
182 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
183# endif
184
185/* Verify requirement R at compile-time, as a declaration without a
186 trailing ';'. If R is false, fail at compile-time, preferably
187 with a diagnostic that includes the string-literal DIAGNOSTIC.
188
189 Unfortunately, unlike C11, this implementation must appear as an
190 ordinary declaration, and cannot appear inside struct { ... }. */
191
192# ifdef _GL_HAVE__STATIC_ASSERT
193# define _GL_VERIFY _Static_assert
194# else
195# define _GL_VERIFY(R, DIAGNOSTIC) \
196 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
197 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
198# endif
199
200/* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
201# ifdef _GL_STATIC_ASSERT_H
202# if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
203# define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
204# endif
205# if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
206# define static_assert _Static_assert /* C11 requires this #define. */
207# endif
208# endif
209
210/* @assert.h omit start@ */
211
212/* Each of these macros verifies that its argument R is nonzero. To
213 be portable, R should be an integer constant expression. Unlike
214 assert (R), there is no run-time overhead.
215
216 There are two macros, since no single macro can be used in all
217 contexts in C. verify_true (R) is for scalar contexts, including
218 integer constant expression contexts. verify (R) is for declaration
219 contexts, e.g., the top level. */
220
221/* Verify requirement R at compile-time, as an integer constant expression.
222 Return 1. This is equivalent to verify_expr (R, 1).
223
224 verify_true is obsolescent; please use verify_expr instead. */
225
226# define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
227
228/* Verify requirement R at compile-time. Return the value of the
229 expression E. */
230
231# define verify_expr(R, E) \
232 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
233
234/* Verify requirement R at compile-time, as a declaration without a
235 trailing ';'. */
236
237# define verify(R) _GL_VERIFY (R, "verify (" #R ")")
238
239/* @assert.h omit end@ */
240
241#endif
242