1 /* Compile-time assert-like macros.
3 Copyright (C) 2005-2006, 2009-2011 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 <http://www.gnu.org/licenses/>. */
18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
23 /* Define HAVE__STATIC_ASSERT to 1 if _Static_assert works as per the
24 C1X draft N1548 section 6.7.10. This is supported by GCC 4.6.0 and
25 later, and its use here generates easier-to-read diagnostics when
28 For now, use this only with GCC. Eventually whether _Static_assert
29 works should be determined by 'configure'. */
30 # if 4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)
31 # define HAVE__STATIC_ASSERT 1
34 /* Each of these macros verifies that its argument R is nonzero. To
35 be portable, R should be an integer constant expression. Unlike
36 assert (R), there is no run-time overhead.
38 There are two macros, since no single macro can be used in all
39 contexts in C. verify_true (R) is for scalar contexts, including
40 integer constant expression contexts. verify (R) is for declaration
41 contexts, e.g., the top level.
43 Symbols ending in "__" are private to this header.
45 If _Static_assert works, verify (R) uses it directly. Similarly,
46 verify_true (R) works by packaging a _Static_assert inside a struct
47 that is an operand of sizeof.
49 The code below uses several ideas for C++ compilers, and for C
50 compilers that do not support _Static_assert:
52 * The first step is ((R) ? 1 : -1). Given an expression R, of
53 integral or boolean or floating-point type, this yields an
54 expression of integral type, whose value is later verified to be
55 constant and nonnegative.
57 * Next this expression W is wrapped in a type
58 struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
59 If W is negative, this yields a compile-time error. No compiler can
60 deal with a bit-field of negative size.
62 One might think that an array size check would have the same
63 effect, that is, that the type struct { unsigned int dummy[W]; }
64 would work as well. However, inside a function, some compilers
65 (such as C++ compilers and GNU C) allow local parameters and
66 variables inside array size expressions. With these compilers,
67 an array size check would not properly diagnose this misuse of
70 void function (int n) { verify (n < 0); }
72 * For the verify macro, the struct verify_type__ will need to
73 somehow be embedded into a declaration. To be portable, this
74 declaration must declare an object, a constant, a function, or a
75 typedef name. If the declared entity uses the type directly,
79 typedef struct {...} dummy;
80 extern struct {...} *dummy;
81 extern void dummy (struct {...} *);
82 extern struct {...} *dummy (void);
84 two uses of the verify macro would yield colliding declarations
85 if the entity names are not disambiguated. A workaround is to
86 attach the current line number to the entity name:
88 #define _GL_CONCAT0(x, y) x##y
89 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
90 extern struct {...} * _GL_CONCAT (dummy, __LINE__);
92 But this has the problem that two invocations of verify from
93 within the same macro would collide, since the __LINE__ value
94 would be the same for both invocations. (The GCC __COUNTER__
95 macro solves this problem, but is not portable.)
97 A solution is to use the sizeof operator. It yields a number,
98 getting rid of the identity of the type. Declarations like
100 extern int dummy [sizeof (struct {...})];
101 extern void dummy (int [sizeof (struct {...})]);
102 extern int (*dummy (void)) [sizeof (struct {...})];
106 * Should the implementation use a named struct or an unnamed struct?
107 Which of the following alternatives can be used?
109 extern int dummy [sizeof (struct {...})];
110 extern int dummy [sizeof (struct verify_type__ {...})];
111 extern void dummy (int [sizeof (struct {...})]);
112 extern void dummy (int [sizeof (struct verify_type__ {...})]);
113 extern int (*dummy (void)) [sizeof (struct {...})];
114 extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
116 In the second and sixth case, the struct type is exported to the
117 outer scope; two such declarations therefore collide. GCC warns
118 about the first, third, and fourth cases. So the only remaining
119 possibility is the fifth case:
121 extern int (*dummy (void)) [sizeof (struct {...})];
123 * GCC warns about duplicate declarations of the dummy function if
124 -Wredundant_decls is used. GCC 4.3 and later have a builtin
125 __COUNTER__ macro that can let us generate unique identifiers for
126 each dummy function, to suppress this warning.
128 * This implementation exploits the fact that older versions of GCC,
129 which do not support _Static_assert, also do not warn about the
130 last declaration mentioned above.
132 * In C++, any struct definition inside sizeof is invalid.
133 Use a template type to work around the problem. */
135 /* Concatenate two preprocessor tokens. */
136 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
137 # define _GL_CONCAT0(x, y) x##y
139 /* _GL_COUNTER is an integer, preferably one that changes each time we
140 use it. Use __COUNTER__ if it works, falling back on __LINE__
141 otherwise. __LINE__ isn't perfect, but it's better than a
143 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
144 # define _GL_COUNTER __COUNTER__
146 # define _GL_COUNTER __LINE__
149 /* Generate a symbol with the given prefix, making it unique if
151 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
153 /* Verify requirement R at compile-time, as an integer constant expression.
158 struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
159 # define verify_true(R) \
160 (!!sizeof (verify_type__<(R) ? 1 : -1>))
161 # elif HAVE__STATIC_ASSERT
162 # define verify_true(R) \
165 _Static_assert (R, "verify_true (" #R ")"); \
166 int verify_dummy__; \
169 # define verify_true(R) \
171 (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
174 /* Verify requirement R at compile-time, as a declaration without a
177 # if HAVE__STATIC_ASSERT
178 # define verify(R) _Static_assert (R, "verify (" #R ")")
181 extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]