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1 Copyright (C) 2000-2020 Free Software Foundation, Inc.
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2
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3 This file is intended to contain a few notes about writing C code
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4 within GCC so that it compiles without error on the full range of
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5 compilers GCC needs to be able to compile on.
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6
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7 The problem is that many ISO-standard constructs are not accepted by
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8 either old or buggy compilers, and we keep getting bitten by them.
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111
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9 This knowledge until now has been sparsely spread around, so I
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10 thought I'd collect it in one useful place. Please add and correct
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11 any problems as you come across them.
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12
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13 I'm going to start from a base of the ISO C90 standard, since that is
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14 probably what most people code to naturally. Obviously using
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15 constructs introduced after that is not a good idea.
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16
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17 For the complete coding style conventions used in GCC, please read
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18 http://gcc.gnu.org/codingconventions.html
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19
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20
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21 String literals
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22 ---------------
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23
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24 Some compilers like MSVC++ have fairly low limits on the maximum
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25 length of a string literal; 509 is the lowest we've come across. You
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26 may need to break up a long printf statement into many smaller ones.
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27
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28
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29 Empty macro arguments
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30 ---------------------
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31
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32 ISO C (6.8.3 in the 1990 standard) specifies the following:
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33
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34 If (before argument substitution) any argument consists of no
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35 preprocessing tokens, the behavior is undefined.
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36
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37 This was relaxed by ISO C99, but some older compilers emit an error,
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38 so code like
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39
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40 #define foo(x, y) x y
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41 foo (bar, )
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42
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43 needs to be coded in some other way.
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44
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45
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111
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46 Avoid unnecessary test before free
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47 ----------------------------------
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48
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49 Since SunOS 4 stopped being a reasonable portability target,
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50 (which happened around 2007) there has been no need to guard
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51 against "free (NULL)". Thus, any guard like the following
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52 constitutes a redundant test:
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53
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54 if (P)
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55 free (P);
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56
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111
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57 It is better to avoid the test.[*]
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58 Instead, simply free P, regardless of whether it is NULL.
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59
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111
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60 [*] However, if your profiling exposes a test like this in a
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61 performance-critical loop, say where P is nearly always NULL, and
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62 the cost of calling free on a NULL pointer would be prohibitively
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63 high, consider using __builtin_expect, e.g., like this:
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64
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65 if (__builtin_expect (ptr != NULL, 0))
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66 free (ptr);
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67
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68
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69
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70 Trigraphs
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71 ---------
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72
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73 You weren't going to use them anyway, but some otherwise ISO C
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74 compliant compilers do not accept trigraphs.
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75
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76
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77 Suffixes on Integer Constants
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78 -----------------------------
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79
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80 You should never use a 'l' suffix on integer constants ('L' is fine),
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81 since it can easily be confused with the number '1'.
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82
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83
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84 Common Coding Pitfalls
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85 ======================
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86
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87 errno
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88 -----
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89
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90 errno might be declared as a macro.
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91
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92
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93 Implicit int
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94 ------------
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95
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96 In C, the 'int' keyword can often be omitted from type declarations.
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97 For instance, you can write
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98
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99 unsigned variable;
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100
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101 as shorthand for
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102
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103 unsigned int variable;
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104
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105 There are several places where this can cause trouble. First, suppose
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106 'variable' is a long; then you might think
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107
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108 (unsigned) variable
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109
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110 would convert it to unsigned long. It does not. It converts to
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111 unsigned int. This mostly causes problems on 64-bit platforms, where
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112 long and int are not the same size.
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113
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114 Second, if you write a function definition with no return type at
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115 all:
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116
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117 operate (int a, int b)
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118 {
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119 ...
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120 }
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121
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122 that function is expected to return int, *not* void. GCC will warn
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123 about this.
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124
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125 Implicit function declarations always have return type int. So if you
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126 correct the above definition to
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127
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128 void
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129 operate (int a, int b)
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130 ...
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131
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132 but operate() is called above its definition, you will get an error
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133 about a "type mismatch with previous implicit declaration". The cure
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134 is to prototype all functions at the top of the file, or in an
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135 appropriate header.
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136
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137 Char vs unsigned char vs int
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138 ----------------------------
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139
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140 In C, unqualified 'char' may be either signed or unsigned; it is the
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141 implementation's choice. When you are processing 7-bit ASCII, it does
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142 not matter. But when your program must handle arbitrary binary data,
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143 or fully 8-bit character sets, you have a problem. The most obvious
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144 issue is if you have a look-up table indexed by characters.
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145
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146 For instance, the character '\341' in ISO Latin 1 is SMALL LETTER A
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147 WITH ACUTE ACCENT. In the proper locale, isalpha('\341') will be
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148 true. But if you read '\341' from a file and store it in a plain
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149 char, isalpha(c) may look up character 225, or it may look up
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150 character -31. And the ctype table has no entry at offset -31, so
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151 your program will crash. (If you're lucky.)
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152
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153 It is wise to use unsigned char everywhere you possibly can. This
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154 avoids all these problems. Unfortunately, the routines in <string.h>
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155 take plain char arguments, so you have to remember to cast them back
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156 and forth - or avoid the use of strxxx() functions, which is probably
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157 a good idea anyway.
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158
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159 Another common mistake is to use either char or unsigned char to
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160 receive the result of getc() or related stdio functions. They may
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161 return EOF, which is outside the range of values representable by
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162 char. If you use char, some legal character value may be confused
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163 with EOF, such as '\377' (SMALL LETTER Y WITH UMLAUT, in Latin-1).
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164 The correct choice is int.
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165
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166 A more subtle version of the same mistake might look like this:
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167
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168 unsigned char pushback[NPUSHBACK];
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169 int pbidx;
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170 #define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c))
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171 #define get(c) (pbidx ? pushback[--pbidx] : getchar())
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172 ...
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173 unget(EOF);
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174
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175 which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y
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176 WITH UMLAUT.
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177
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178
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179 Other common pitfalls
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180 ---------------------
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181
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182 o Expecting 'plain' char to be either sign or unsigned extending.
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183
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184 o Shifting an item by a negative amount or by greater than or equal to
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185 the number of bits in a type (expecting shifts by 32 to be sensible
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186 has caused quite a number of bugs at least in the early days).
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187
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188 o Expecting ints shifted right to be sign extended.
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189
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190 o Modifying the same value twice within one sequence point.
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191
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192 o Host vs. target floating point representation, including emitting NaNs
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193 and Infinities in a form that the assembler handles.
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194
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195 o qsort being an unstable sort function (unstable in the sense that
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196 multiple items that sort the same may be sorted in different orders
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197 by different qsort functions).
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198
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199 o Passing incorrect types to fprintf and friends.
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200
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201 o Adding a function declaration for a module declared in another file to
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202 a .c file instead of to a .h file.
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