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 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__ || (__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   * GCC warns if -Wnested-externs is enabled and verify() is used
137     within a function body; but inside a function, you can always
138     arrange to use verify_expr() instead.
139
140   * In C++, any struct definition inside sizeof is invalid.
141     Use a template type to work around the problem.  */
142
143/* Concatenate two preprocessor tokens.  */
144# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
145# define _GL_CONCAT0(x, y) x##y
146
147/* _GL_COUNTER is an integer, preferably one that changes each time we
148   use it.  Use __COUNTER__ if it works, falling back on __LINE__
149   otherwise.  __LINE__ isn't perfect, but it's better than a
150   constant.  */
151# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
152#  define _GL_COUNTER __COUNTER__
153# else
154#  define _GL_COUNTER __LINE__
155# endif
156
157/* Generate a symbol with the given prefix, making it unique if
158   possible.  */
159# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
160
161/* Verify requirement R at compile-time, as an integer constant expression
162   that returns 1.  If R is false, fail at compile-time, preferably
163   with a diagnostic that includes the string-literal DIAGNOSTIC.  */
164
165# define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
166    (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
167
168# ifdef __cplusplus
169#  if !GNULIB_defined_struct__gl_verify_type
170template <int w>
171  struct _gl_verify_type {
172    unsigned int _gl_verify_error_if_negative: w;
173  };
174#   define GNULIB_defined_struct__gl_verify_type 1
175#  endif
176#  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
177    _gl_verify_type<(R) ? 1 : -1>
178# elif defined _GL_HAVE__STATIC_ASSERT
179#  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
180     struct {                                   \
181       _Static_assert (R, DIAGNOSTIC);          \
182       int _gl_dummy;                          \
183     }
184# else
185#  define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
186     struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
187# endif
188
189/* Verify requirement R at compile-time, as a declaration without a
190   trailing ';'.  If R is false, fail at compile-time, preferably
191   with a diagnostic that includes the string-literal DIAGNOSTIC.
192
193   Unfortunately, unlike C11, this implementation must appear as an
194   ordinary declaration, and cannot appear inside struct { ... }.  */
195
196# ifdef _GL_HAVE__STATIC_ASSERT
197#  define _GL_VERIFY _Static_assert
198# else
199#  define _GL_VERIFY(R, DIAGNOSTIC)				       \
200     extern int (*_GL_GENSYM (_gl_verify_function) (void))	       \
201       [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
202# endif
203
204/* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h.  */
205# ifdef _GL_STATIC_ASSERT_H
206#  if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
207#   define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
208#  endif
209#  if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
210#   define static_assert _Static_assert /* C11 requires this #define.  */
211#  endif
212# endif
213
214/* @assert.h omit start@  */
215
216/* Each of these macros verifies that its argument R is nonzero.  To
217   be portable, R should be an integer constant expression.  Unlike
218   assert (R), there is no run-time overhead.
219
220   There are two macros, since no single macro can be used in all
221   contexts in C.  verify_true (R) is for scalar contexts, including
222   integer constant expression contexts.  verify (R) is for declaration
223   contexts, e.g., the top level.  */
224
225/* Verify requirement R at compile-time, as an integer constant expression.
226   Return 1.  This is equivalent to verify_expr (R, 1).
227
228   verify_true is obsolescent; please use verify_expr instead.  */
229
230# define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
231
232/* Verify requirement R at compile-time.  Return the value of the
233   expression E.  */
234
235# define verify_expr(R, E) \
236    (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
237
238/* Verify requirement R at compile-time, as a declaration without a
239   trailing ';'.  */
240
241# define verify(R) _GL_VERIFY (R, "verify (" #R ")")
242
243/* @assert.h omit end@  */
244
245#endif
246