summaryrefslogtreecommitdiffstats
path: root/gl/verify.h
blob: 4ad780c8fa00839fe66d5202edf8afc9165af3a4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
/* Compile-time assert-like macros.

   Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

/* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */

#ifndef VERIFY_H
# define VERIFY_H 1

/* Each of these macros verifies that its argument R is nonzero.  To
   be portable, R should be an integer constant expression.  Unlike
   assert (R), there is no run-time overhead.

   There are two macros, since no single macro can be used in all
   contexts in C.  verify_true (R) is for scalar contexts, including
   integer constant expression contexts.  verify (R) is for declaration
   contexts, e.g., the top level.

   Symbols ending in "__" are private to this header.

   The code below uses several ideas.

   * The first step is ((R) ? 1 : -1).  Given an expression R, of
     integral or boolean or floating-point type, this yields an
     expression of integral type, whose value is later verified to be
     constant and nonnegative.

   * Next this expression W is wrapped in a type
     struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
     If W is negative, this yields a compile-time error.  No compiler can
     deal with a bit-field of negative size.

     One might think that an array size check would have the same
     effect, that is, that the type struct { unsigned int dummy[W]; }
     would work as well.  However, inside a function, some compilers
     (such as C++ compilers and GNU C) allow local parameters and
     variables inside array size expressions.  With these compilers,
     an array size check would not properly diagnose this misuse of
     the verify macro:

       void function (int n) { verify (n < 0); }

   * For the verify macro, the struct verify_type__ will need to
     somehow be embedded into a declaration.  To be portable, this
     declaration must declare an object, a constant, a function, or a
     typedef name.  If the declared entity uses the type directly,
     such as in

       struct dummy {...};
       typedef struct {...} dummy;
       extern struct {...} *dummy;
       extern void dummy (struct {...} *);
       extern struct {...} *dummy (void);

     two uses of the verify macro would yield colliding declarations
     if the entity names are not disambiguated.  A workaround is to
     attach the current line number to the entity name:

       #define _GL_CONCAT0(x, y) x##y
       #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
       extern struct {...} * _GL_CONCAT (dummy, __LINE__);

     But this has the problem that two invocations of verify from
     within the same macro would collide, since the __LINE__ value
     would be the same for both invocations.  (The GCC __COUNTER__
     macro solves this problem, but is not portable.)

     A solution is to use the sizeof operator.  It yields a number,
     getting rid of the identity of the type.  Declarations like

       extern int dummy [sizeof (struct {...})];
       extern void dummy (int [sizeof (struct {...})]);
       extern int (*dummy (void)) [sizeof (struct {...})];

     can be repeated.

   * Should the implementation use a named struct or an unnamed struct?
     Which of the following alternatives can be used?

       extern int dummy [sizeof (struct {...})];
       extern int dummy [sizeof (struct verify_type__ {...})];
       extern void dummy (int [sizeof (struct {...})]);
       extern void dummy (int [sizeof (struct verify_type__ {...})]);
       extern int (*dummy (void)) [sizeof (struct {...})];
       extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];

     In the second and sixth case, the struct type is exported to the
     outer scope; two such declarations therefore collide.  GCC warns
     about the first, third, and fourth cases.  So the only remaining
     possibility is the fifth case:

       extern int (*dummy (void)) [sizeof (struct {...})];

   * GCC warns about duplicate declarations of the dummy function if
     -Wredundant_decls is used.  GCC 4.3 and later have a builtin
     __COUNTER__ macro that can let us generate unique identifiers for
     each dummy function, to suppress this warning.

   * This implementation exploits the fact that GCC does not warn about
     the last declaration mentioned above.  If a future version of GCC
     introduces a warning for this, the problem could be worked around
     by using code specialized to GCC, just as __COUNTER__ is already
     being used if available.

       #if 4 <= __GNUC__
       # define verify(R) [another version to keep GCC happy]
       #endif

   * In C++, any struct definition inside sizeof is invalid.
     Use a template type to work around the problem.  */

/* Concatenate two preprocessor tokens.  */
# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
# define _GL_CONCAT0(x, y) x##y

/* _GL_COUNTER is an integer, preferably one that changes each time we
   use it.  Use __COUNTER__ if it works, falling back on __LINE__
   otherwise.  __LINE__ isn't perfect, but it's better than a
   constant.  */
# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
#  define _GL_COUNTER __COUNTER__
# else
#  define _GL_COUNTER __LINE__
# endif

/* Generate a symbol with the given prefix, making it unique if
   possible.  */
# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)

/* Verify requirement R at compile-time, as an integer constant expression.
   Return 1.  */

# ifdef __cplusplus
template <int w>
  struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
#  define verify_true(R) \
     (!!sizeof (verify_type__<(R) ? 1 : -1>))
# else
#  define verify_true(R) \
     (!!sizeof \
      (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
# endif

/* Verify requirement R at compile-time, as a declaration without a
   trailing ';'.  */

# define verify(R) \
    extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]

#endif