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annotate lib/verify.h @ 17255:d81be792518a

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update from texinfo
author Karl Berry <karl@freefriends.org>
date Tue, 01 Jan 2013 15:51:49 -0800
parents e542fd46ad6f
children 6105f1dfb98e
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 /* Compile-time assert-like macros.
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2
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3 Copyright (C) 2005-2006, 2009-2013 Free Software Foundation, Inc.
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4
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5 This program is free software: you can redistribute it and/or modify
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6 it under the terms of the GNU General Public License as published by
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7 the Free Software Foundation; either version 3 of the License, or
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8 (at your option) any later version.
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9
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10 This program is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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13 GNU General Public License for more details.
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14
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15 You should have received a copy of the GNU General Public License
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16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
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17
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18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
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20 #ifndef _GL_VERIFY_H
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21 # define _GL_VERIFY_H
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23
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24 /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11.
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25 This is supported by GCC 4.6.0 and later, in C mode, and its use
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26 here generates easier-to-read diagnostics when verify (R) fails.
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27
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28 Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11.
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29 This will likely be supported by future GCC versions, in C++ mode.
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30
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31 Use this only with GCC. If we were willing to slow 'configure'
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32 down we could also use it with other compilers, but since this
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33 affects only the quality of diagnostics, why bother? */
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34 # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus
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35 # define _GL_HAVE__STATIC_ASSERT 1
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36 # endif
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37 /* The condition (99 < __GNUC__) is temporary, until we know about the
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38 first G++ release that supports static_assert. */
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39 # if (99 < __GNUC__) && defined __cplusplus
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40 # define _GL_HAVE_STATIC_ASSERT 1
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41 # endif
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42
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43 /* Each of these macros verifies that its argument R is nonzero. To
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44 be portable, R should be an integer constant expression. Unlike
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45 assert (R), there is no run-time overhead.
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46
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47 If _Static_assert works, verify (R) uses it directly. Similarly,
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48 _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct
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49 that is an operand of sizeof.
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50
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51 The code below uses several ideas for C++ compilers, and for C
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52 compilers that do not support _Static_assert:
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54 * The first step is ((R) ? 1 : -1). Given an expression R, of
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55 integral or boolean or floating-point type, this yields an
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56 expression of integral type, whose value is later verified to be
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57 constant and nonnegative.
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58
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59 * Next this expression W is wrapped in a type
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60 struct _gl_verify_type {
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61 unsigned int _gl_verify_error_if_negative: W;
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62 }.
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63 If W is negative, this yields a compile-time error. No compiler can
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64 deal with a bit-field of negative size.
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65
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66 One might think that an array size check would have the same
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67 effect, that is, that the type struct { unsigned int dummy[W]; }
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68 would work as well. However, inside a function, some compilers
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69 (such as C++ compilers and GNU C) allow local parameters and
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70 variables inside array size expressions. With these compilers,
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71 an array size check would not properly diagnose this misuse of
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72 the verify macro:
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73
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74 void function (int n) { verify (n < 0); }
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75
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76 * For the verify macro, the struct _gl_verify_type will need to
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77 somehow be embedded into a declaration. To be portable, this
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78 declaration must declare an object, a constant, a function, or a
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79 typedef name. If the declared entity uses the type directly,
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80 such as in
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81
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82 struct dummy {...};
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83 typedef struct {...} dummy;
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84 extern struct {...} *dummy;
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85 extern void dummy (struct {...} *);
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86 extern struct {...} *dummy (void);
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87
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88 two uses of the verify macro would yield colliding declarations
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89 if the entity names are not disambiguated. A workaround is to
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90 attach the current line number to the entity name:
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91
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92 #define _GL_CONCAT0(x, y) x##y
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93 #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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94 extern struct {...} * _GL_CONCAT (dummy, __LINE__);
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95
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96 But this has the problem that two invocations of verify from
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97 within the same macro would collide, since the __LINE__ value
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98 would be the same for both invocations. (The GCC __COUNTER__
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99 macro solves this problem, but is not portable.)
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100
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101 A solution is to use the sizeof operator. It yields a number,
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102 getting rid of the identity of the type. Declarations like
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103
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104 extern int dummy [sizeof (struct {...})];
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105 extern void dummy (int [sizeof (struct {...})]);
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106 extern int (*dummy (void)) [sizeof (struct {...})];
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107
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108 can be repeated.
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109
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110 * Should the implementation use a named struct or an unnamed struct?
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111 Which of the following alternatives can be used?
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112
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113 extern int dummy [sizeof (struct {...})];
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114 extern int dummy [sizeof (struct _gl_verify_type {...})];
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115 extern void dummy (int [sizeof (struct {...})]);
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116 extern void dummy (int [sizeof (struct _gl_verify_type {...})]);
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117 extern int (*dummy (void)) [sizeof (struct {...})];
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118 extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})];
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119
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120 In the second and sixth case, the struct type is exported to the
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121 outer scope; two such declarations therefore collide. GCC warns
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122 about the first, third, and fourth cases. So the only remaining
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123 possibility is the fifth case:
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124
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125 extern int (*dummy (void)) [sizeof (struct {...})];
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126
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127 * GCC warns about duplicate declarations of the dummy function if
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128 -Wredundant-decls is used. GCC 4.3 and later have a builtin
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129 __COUNTER__ macro that can let us generate unique identifiers for
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130 each dummy function, to suppress this warning.
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131
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132 * This implementation exploits the fact that older versions of GCC,
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133 which do not support _Static_assert, also do not warn about the
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134 last declaration mentioned above.
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135
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136 * GCC warns if -Wnested-externs is enabled and verify() is used
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137 within a function body; but inside a function, you can always
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138 arrange to use verify_expr() instead.
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139
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140 * In C++, any struct definition inside sizeof is invalid.
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141 Use a template type to work around the problem. */
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142
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143 /* Concatenate two preprocessor tokens. */
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144 # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
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145 # define _GL_CONCAT0(x, y) x##y
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146
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147 /* _GL_COUNTER is an integer, preferably one that changes each time we
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148 use it. Use __COUNTER__ if it works, falling back on __LINE__
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149 otherwise. __LINE__ isn't perfect, but it's better than a
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150 constant. */
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151 # if defined __COUNTER__ && __COUNTER__ != __COUNTER__
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152 # define _GL_COUNTER __COUNTER__
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153 # else
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154 # define _GL_COUNTER __LINE__
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155 # endif
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156
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157 /* Generate a symbol with the given prefix, making it unique if
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158 possible. */
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159 # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
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160
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161 /* Verify requirement R at compile-time, as an integer constant expression
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162 that returns 1. If R is false, fail at compile-time, preferably
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163 with a diagnostic that includes the string-literal DIAGNOSTIC. */
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164
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165 # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
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166 (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
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167
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168 # ifdef __cplusplus
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169 # if !GNULIB_defined_struct__gl_verify_type
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170 template <int w>
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171 struct _gl_verify_type {
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172 unsigned int _gl_verify_error_if_negative: w;
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173 };
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174 # define GNULIB_defined_struct__gl_verify_type 1
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175 # endif
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176 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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177 _gl_verify_type<(R) ? 1 : -1>
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178 # elif defined _GL_HAVE__STATIC_ASSERT
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179 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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180 struct { \
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181 _Static_assert (R, DIAGNOSTIC); \
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182 int _gl_dummy; \
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183 }
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184 # else
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185 # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
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186 struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
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187 # endif
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188
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189 /* Verify requirement R at compile-time, as a declaration without a
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190 trailing ';'. If R is false, fail at compile-time, preferably
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191 with a diagnostic that includes the string-literal DIAGNOSTIC.
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192
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193 Unfortunately, unlike C11, this implementation must appear as an
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194 ordinary declaration, and cannot appear inside struct { ... }. */
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195
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196 # ifdef _GL_HAVE__STATIC_ASSERT
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197 # define _GL_VERIFY _Static_assert
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198 # else
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199 # define _GL_VERIFY(R, DIAGNOSTIC) \
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200 extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
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201 [_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
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202 # endif
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203
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204 /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */
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205 # ifdef _GL_STATIC_ASSERT_H
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206 # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert
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207 # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC)
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208 # endif
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209 # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert
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210 # define static_assert _Static_assert /* C11 requires this #define. */
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211 # endif
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212 # endif
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213
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214 /* @assert.h omit start@ */
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215
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216 /* Each of these macros verifies that its argument R is nonzero. To
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217 be portable, R should be an integer constant expression. Unlike
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218 assert (R), there is no run-time overhead.
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219
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220 There are two macros, since no single macro can be used in all
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221 contexts in C. verify_true (R) is for scalar contexts, including
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222 integer constant expression contexts. verify (R) is for declaration
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223 contexts, e.g., the top level. */
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224
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225 /* Verify requirement R at compile-time, as an integer constant expression.
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226 Return 1. This is equivalent to verify_expr (R, 1).
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227
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228 verify_true is obsolescent; please use verify_expr instead. */
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229
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230 # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")")
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231
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232 /* Verify requirement R at compile-time. Return the value of the
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233 expression E. */
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234
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235 # define verify_expr(R, E) \
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236 (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
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237
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238 /* Verify requirement R at compile-time, as a declaration without a
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239 trailing ';'. */
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240
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241 # define verify(R) _GL_VERIFY (R, "verify (" #R ")")
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242
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243 /* @assert.h omit end@ */
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244
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245 #endif