1 | #include <stdio.h> |
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2 | #include <assert.h> |
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3 | #include <stdlib.h> |
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4 | |
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5 | #define SHOW(x, fmt) printf( #x ": " fmt "\n", x ) |
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6 | |
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7 | #ifdef ERRS |
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8 | #define ERR(...) __VA_ARGS__ |
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9 | #else |
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10 | #define ERR(...) |
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11 | #endif |
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12 | |
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13 | // int main( int argc, const char *argv[] ) { |
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14 | // assert(argc == 2); |
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15 | // const int n = atoi(argv[1]); |
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16 | // assert(0 < n && n < 1000); |
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17 | |
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18 | // float a1[42]; |
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19 | // float a2[n]; |
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20 | // SHOW(sizeof(a1), "%zd"); |
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21 | // SHOW(sizeof(a2), "%zd"); |
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22 | |
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23 | // } |
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24 | |
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25 | |
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26 | // SHOW(sizeof( a ), "%zd"); |
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27 | // SHOW(sizeof(&a ), "%zd"); |
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28 | // SHOW(sizeof( a[0]), "%zd"); |
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29 | // SHOW(sizeof(&a[0]), "%zd"); |
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30 | |
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31 | |
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32 | |
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33 | int main() { |
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34 | |
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35 | /* |
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36 | When a programmer works with an array, C semantics provide access to a type that is different in every way from ``pointer to its first element.'' |
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37 | |
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38 | Its qualities become apparent by inspecting the declaration |
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39 | */ |
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40 | float a[10]; |
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41 | /* |
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42 | |
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43 | The inspection begins by using @sizeof@ to provide definite program semantics for the intuition of an expression's type. |
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44 | |
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45 | Assuming a target platform keeps things concrete: |
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46 | */ |
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47 | static_assert(sizeof(float)==4); // floats (array elements) are 4 bytes |
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48 | static_assert(sizeof(void*)==8); // pointers are 8 bytes |
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49 | /* |
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50 | |
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51 | Consider the sizes of expressions derived from @a@, modified by adding ``pointer to'' and ``first element'' (and including unnecessary parentheses to avoid confusion about precedence). |
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52 | */ |
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53 | static_assert(sizeof( a ) == 40); // array |
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54 | static_assert(sizeof(& a ) == 8 ); // pointer to array |
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55 | static_assert(sizeof( a[0] ) == 4 ); // first element |
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56 | static_assert(sizeof(&(a[0])) == 8 ); // pointer to first element |
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57 | /* |
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58 | That @a@ takes up 40 bytes is common reasoning for C programmers. |
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59 | Set aside for a moment the claim that this first assertion is giving information about a type. |
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60 | For now, note that an array and a pointer to its first element are, sometimes, different things. |
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61 | |
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62 | The idea that there is such a thing as a pointer to an array may be surprising. |
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63 | It is not the same thing as a pointer to the first element: |
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64 | */ |
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65 | typeof(& a ) x; // x is pointer to array |
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66 | typeof(&(a[0])) y; // y is pointer to first element |
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67 | ERR( |
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68 | x = y; // ill-typed |
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69 | y = x; // ill-typed |
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70 | ) |
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71 | /* |
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72 | The first gets |
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73 | warning: warning: assignment to `float (*)[10]' from incompatible pointer type `float *' |
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74 | and the second gets the opposite. |
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75 | */ |
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76 | |
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77 | /* |
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78 | We now refute a concern that @sizeof(a)@ is reporting on special knowledge from @a@ being an local variable, |
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79 | say that it is informing about an allocation, rather than simply a type. |
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80 | |
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81 | First, recognizing that @sizeof@ has two forms, one operating on an expression, the other on a type, we observe that the original answers are unaffected by using the type-parameterized form: |
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82 | */ |
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83 | static_assert(sizeof(typeof( a )) == 40); |
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84 | static_assert(sizeof(typeof(& a )) == 8 ); |
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85 | static_assert(sizeof(typeof( a[0] )) == 4 ); |
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86 | static_assert(sizeof(typeof(&(a[0]))) == 8 ); |
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87 | |
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88 | /* |
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89 | Finally, the same sizing is reported when there is no allocation at all, and we launch the analysis instead from the pointer-to-array type. |
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90 | */ |
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91 | void f( float (*pa)[10] ) { |
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92 | static_assert(sizeof( *pa ) == 40); // array |
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93 | static_assert(sizeof( pa ) == 8 ); // pointer to array |
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94 | static_assert(sizeof( (*pa)[0] ) == 4 ); // first element |
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95 | static_assert(sizeof(&((*pa)[0])) == 8 ); // pointer to first element |
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96 | } |
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97 | f( & a ); |
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98 | |
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99 | /* |
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100 | So, in spite of considerable programmer success enabled by an understanding that |
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101 | an array just a pointer to its first element (revisited TODO pointer decay), |
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102 | this understanding is simplistic. |
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103 | */ |
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104 | |
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105 | /* |
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106 | A shortened form for declaring local variables exists, provided that length information is given in the initializer: |
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107 | */ |
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108 | float fs[] = {3.14, 1.707}; |
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109 | char cs[] = "hello"; |
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110 | |
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111 | static_assert( sizeof(fs) == 2 * sizeof(float) ); |
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112 | static_assert( sizeof(cs) == 6 * sizeof(char) ); // 5 letters + 1 null terminator |
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113 | |
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114 | /* |
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115 | In these declarations, the resulting types are both arrays, but their lengths are inferred. |
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116 | */ |
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117 | |
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118 | } |
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119 | |
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120 | |
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121 | void syntaxReferenceCheck(void) { |
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122 | // $\rightarrow$ & (base element) |
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123 | // & @float@ |
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124 | // & @float x;@ |
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125 | // & @[ float ]@ |
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126 | // & @[ float ]@ |
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127 | float x0; |
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128 | |
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129 | // $\rightarrow$ & pointer |
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130 | // & @float *@ |
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131 | // & @float * x;@ |
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132 | // & @[ * float ]@ |
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133 | // & @[ * float ]@ |
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134 | float * x1; |
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135 | |
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136 | // $\rightarrow$ & array |
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137 | // & @float[10]@ |
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138 | // & @float x[10];@ |
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139 | // & @[ [10] float ]@ |
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140 | // & @[ array(float, 10) ]@ |
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141 | float x2[10]; |
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142 | |
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143 | typeof(float[10]) x2b; |
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144 | |
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145 | // & array of pointers |
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146 | // & @(float*)[10]@ |
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147 | // & @float *x[10];@ |
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148 | // & @[ [10] * float ]@ |
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149 | // & @[ array(*float, 10) ]@ |
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150 | float *x3[10]; |
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151 | // (float *)x3a[10]; NO |
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152 | |
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153 | // $\rightarrow$ & pointer to array |
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154 | // & @float(*)[10]@ |
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155 | // & @float (*x)[10];@ |
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156 | // & @[ * [10] float ]@ |
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157 | // & @[ * array(float, 10) ]@ |
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158 | float (*x4)[10]; |
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159 | |
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160 | // & pointer to array |
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161 | // & @(float*)(*)[10]@ |
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162 | // & @float *(*x)[10];@ |
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163 | // & @[ * [10] * float ]@ |
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164 | // & @[ * array(*float, 10) ]@ |
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165 | float *(*x5)[10]; |
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166 | x5 = (float*(*)[10]) x4; |
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167 | // x5 = (float(*)[10]) x4; // wrong target type; meta test suggesting above cast uses correct type |
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168 | |
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169 | // [here] |
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170 | // const |
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171 | |
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172 | // [later] |
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173 | // static |
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174 | // star as dimension |
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175 | // under pointer decay: int p1[const 3] being int const *p1 |
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176 | |
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177 | const float * y1; |
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178 | float const * y2; |
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179 | float * const y3; |
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180 | |
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181 | y1 = 0; |
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182 | y2 = 0; |
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183 | // y3 = 0; // bad |
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184 | |
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185 | // *y1 = 3.14; // bad |
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186 | // *y2 = 3.14; // bad |
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187 | *y3 = 3.14; |
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188 | |
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189 | const float z1 = 1.414; |
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190 | float const z2 = 1.414; |
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191 | |
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192 | // z1 = 3.14; // bad |
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193 | // z2 = 3.14; // bad |
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194 | |
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195 | |
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196 | } |
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197 | |
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198 | #define T float |
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199 | void stx2() { const T x[10]; |
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200 | // x[5] = 3.14; // bad |
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201 | } |
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202 | void stx3() { T const x[10]; |
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203 | // x[5] = 3.14; // bad |
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204 | } |
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