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1 @c Copyright (C) 2001-2020 Free Software Foundation, Inc.
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2 @c This is part of the GCC manual.
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3 @c For copying conditions, see the file gcc.texi.
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4
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5 @node C Implementation
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111
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6 @chapter C Implementation-Defined Behavior
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7 @cindex implementation-defined behavior, C language
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8
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9 A conforming implementation of ISO C is required to document its
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10 choice of behavior in each of the areas that are designated
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11 ``implementation defined''. The following lists all such areas,
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12 along with the section numbers from the ISO/IEC 9899:1990, ISO/IEC
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13 9899:1999 and ISO/IEC 9899:2011 standards. Some areas are only
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14 implementation-defined in one version of the standard.
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15
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16 Some choices depend on the externally determined ABI for the platform
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17 (including standard character encodings) which GCC follows; these are
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18 listed as ``determined by ABI'' below. @xref{Compatibility, , Binary
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19 Compatibility}, and @uref{http://gcc.gnu.org/readings.html}. Some
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20 choices are documented in the preprocessor manual.
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21 @xref{Implementation-defined behavior, , Implementation-defined
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22 behavior, cpp, The C Preprocessor}. Some choices are made by the
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23 library and operating system (or other environment when compiling for
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24 a freestanding environment); refer to their documentation for details.
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25
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26 @menu
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27 * Translation implementation::
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28 * Environment implementation::
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29 * Identifiers implementation::
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30 * Characters implementation::
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31 * Integers implementation::
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32 * Floating point implementation::
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33 * Arrays and pointers implementation::
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34 * Hints implementation::
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35 * Structures unions enumerations and bit-fields implementation::
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36 * Qualifiers implementation::
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37 * Declarators implementation::
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38 * Statements implementation::
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39 * Preprocessing directives implementation::
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40 * Library functions implementation::
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41 * Architecture implementation::
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42 * Locale-specific behavior implementation::
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43 @end menu
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44
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45 @node Translation implementation
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46 @section Translation
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47
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48 @itemize @bullet
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49 @item
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50 @cite{How a diagnostic is identified (C90 3.7, C99 and C11 3.10, C90,
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51 C99 and C11 5.1.1.3).}
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52
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53 Diagnostics consist of all the output sent to stderr by GCC@.
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54
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55 @item
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56 @cite{Whether each nonempty sequence of white-space characters other than
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57 new-line is retained or replaced by one space character in translation
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58 phase 3 (C90, C99 and C11 5.1.1.2).}
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59
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60 @xref{Implementation-defined behavior, , Implementation-defined
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61 behavior, cpp, The C Preprocessor}.
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62
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63 @end itemize
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64
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65 @node Environment implementation
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66 @section Environment
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67
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68 The behavior of most of these points are dependent on the implementation
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69 of the C library, and are not defined by GCC itself.
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70
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71 @itemize @bullet
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72 @item
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73 @cite{The mapping between physical source file multibyte characters
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74 and the source character set in translation phase 1 (C90, C99 and C11
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75 5.1.1.2).}
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76
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77 @xref{Implementation-defined behavior, , Implementation-defined
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78 behavior, cpp, The C Preprocessor}.
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79
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80 @end itemize
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81
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82 @node Identifiers implementation
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83 @section Identifiers
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84
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85 @itemize @bullet
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86 @item
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87 @cite{Which additional multibyte characters may appear in identifiers
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88 and their correspondence to universal character names (C99 and C11 6.4.2).}
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89
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90 @xref{Implementation-defined behavior, , Implementation-defined
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91 behavior, cpp, The C Preprocessor}.
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92
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93 @item
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94 @cite{The number of significant initial characters in an identifier
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95 (C90 6.1.2, C90, C99 and C11 5.2.4.1, C99 and C11 6.4.2).}
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96
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97 For internal names, all characters are significant. For external names,
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98 the number of significant characters are defined by the linker; for
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99 almost all targets, all characters are significant.
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100
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101 @item
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102 @cite{Whether case distinctions are significant in an identifier with
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103 external linkage (C90 6.1.2).}
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104
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105 This is a property of the linker. C99 and C11 require that case distinctions
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106 are always significant in identifiers with external linkage and
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107 systems without this property are not supported by GCC@.
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108
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109 @end itemize
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110
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111 @node Characters implementation
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112 @section Characters
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113
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114 @itemize @bullet
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115 @item
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116 @cite{The number of bits in a byte (C90 3.4, C99 and C11 3.6).}
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117
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118 Determined by ABI@.
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119
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120 @item
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121 @cite{The values of the members of the execution character set (C90,
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122 C99 and C11 5.2.1).}
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123
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124 Determined by ABI@.
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125
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126 @item
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127 @cite{The unique value of the member of the execution character set produced
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128 for each of the standard alphabetic escape sequences (C90, C99 and C11
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129 5.2.2).}
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130
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131 Determined by ABI@.
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132
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133 @item
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134 @cite{The value of a @code{char} object into which has been stored any
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135 character other than a member of the basic execution character set
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136 (C90 6.1.2.5, C99 and C11 6.2.5).}
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137
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138 Determined by ABI@.
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139
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140 @item
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141 @cite{Which of @code{signed char} or @code{unsigned char} has the same
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142 range, representation, and behavior as ``plain'' @code{char} (C90
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143 6.1.2.5, C90 6.2.1.1, C99 and C11 6.2.5, C99 and C11 6.3.1.1).}
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144
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145 @opindex fsigned-char
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146 @opindex funsigned-char
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147 Determined by ABI@. The options @option{-funsigned-char} and
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148 @option{-fsigned-char} change the default. @xref{C Dialect Options, ,
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149 Options Controlling C Dialect}.
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150
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151 @item
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152 @cite{The mapping of members of the source character set (in character
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153 constants and string literals) to members of the execution character
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154 set (C90 6.1.3.4, C99 and C11 6.4.4.4, C90, C99 and C11 5.1.1.2).}
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155
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156 Determined by ABI@.
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157
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158 @item
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159 @cite{The value of an integer character constant containing more than one
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160 character or containing a character or escape sequence that does not map
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161 to a single-byte execution character (C90 6.1.3.4, C99 and C11 6.4.4.4).}
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162
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163 @xref{Implementation-defined behavior, , Implementation-defined
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164 behavior, cpp, The C Preprocessor}.
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165
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166 @item
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167 @cite{The value of a wide character constant containing more than one
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168 multibyte character or a single multibyte character that maps to
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169 multiple members of the extended execution character set, or
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170 containing a multibyte character or escape sequence not represented in
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171 the extended execution character set (C90 6.1.3.4, C99 and C11
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172 6.4.4.4).}
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173
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174 @xref{Implementation-defined behavior, , Implementation-defined
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175 behavior, cpp, The C Preprocessor}.
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176
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177 @item
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178 @cite{The current locale used to convert a wide character constant consisting
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179 of a single multibyte character that maps to a member of the extended
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180 execution character set into a corresponding wide character code (C90
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181 6.1.3.4, C99 and C11 6.4.4.4).}
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182
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183 @xref{Implementation-defined behavior, , Implementation-defined
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184 behavior, cpp, The C Preprocessor}.
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185
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186 @item
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187 @cite{Whether differently-prefixed wide string literal tokens can be
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188 concatenated and, if so, the treatment of the resulting multibyte
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189 character sequence (C11 6.4.5).}
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190
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191 Such tokens may not be concatenated.
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192
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193 @item
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194 @cite{The current locale used to convert a wide string literal into
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195 corresponding wide character codes (C90 6.1.4, C99 and C11 6.4.5).}
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196
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197 @xref{Implementation-defined behavior, , Implementation-defined
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198 behavior, cpp, The C Preprocessor}.
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199
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200 @item
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201 @cite{The value of a string literal containing a multibyte character or escape
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202 sequence not represented in the execution character set (C90 6.1.4,
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203 C99 and C11 6.4.5).}
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204
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205 @xref{Implementation-defined behavior, , Implementation-defined
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206 behavior, cpp, The C Preprocessor}.
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207
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208 @item
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209 @cite{The encoding of any of @code{wchar_t}, @code{char16_t}, and
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210 @code{char32_t} where the corresponding standard encoding macro
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211 (@code{__STDC_ISO_10646__}, @code{__STDC_UTF_16__}, or
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212 @code{__STDC_UTF_32__}) is not defined (C11 6.10.8.2).}
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213
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214 @xref{Implementation-defined behavior, , Implementation-defined
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215 behavior, cpp, The C Preprocessor}. @code{char16_t} and
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216 @code{char32_t} literals are always encoded in UTF-16 and UTF-32
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217 respectively.
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218
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219 @end itemize
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220
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221 @node Integers implementation
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222 @section Integers
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223
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224 @itemize @bullet
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225 @item
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226 @cite{Any extended integer types that exist in the implementation (C99
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227 and C11 6.2.5).}
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228
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229 GCC does not support any extended integer types.
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230 @c The __mode__ attribute might create types of precisions not
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231 @c otherwise supported, but the syntax isn't right for use everywhere
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232 @c the standard type names might be used. Predefined typedefs should
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233 @c be used if any extended integer types are to be defined. The
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234 @c __int128_t and __uint128_t typedefs are not extended integer types
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235 @c as they are generally longer than the ABI-specified intmax_t.
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236
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237 @item
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238 @cite{Whether signed integer types are represented using sign and magnitude,
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239 two's complement, or one's complement, and whether the extraordinary value
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240 is a trap representation or an ordinary value (C99 and C11 6.2.6.2).}
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241
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242 GCC supports only two's complement integer types, and all bit patterns
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243 are ordinary values.
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244
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245 @item
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246 @cite{The rank of any extended integer type relative to another extended
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247 integer type with the same precision (C99 and C11 6.3.1.1).}
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248
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249 GCC does not support any extended integer types.
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250 @c If it did, there would only be one of each precision and signedness.
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251
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252 @item
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253 @cite{The result of, or the signal raised by, converting an integer to a
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254 signed integer type when the value cannot be represented in an object of
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255 that type (C90 6.2.1.2, C99 and C11 6.3.1.3).}
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256
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257 For conversion to a type of width @math{N}, the value is reduced
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258 modulo @math{2^N} to be within range of the type; no signal is raised.
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259
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260 @item
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261 @cite{The results of some bitwise operations on signed integers (C90
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262 6.3, C99 and C11 6.5).}
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263
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264 Bitwise operators act on the representation of the value including
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265 both the sign and value bits, where the sign bit is considered
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266 immediately above the highest-value value bit. Signed @samp{>>} acts
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267 on negative numbers by sign extension.
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268
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111
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269 As an extension to the C language, GCC does not use the latitude given in
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270 C99 and C11 only to treat certain aspects of signed @samp{<<} as undefined.
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271 However, @option{-fsanitize=shift} (and @option{-fsanitize=undefined}) will
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272 diagnose such cases. They are also diagnosed where constant
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273 expressions are required.
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274
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275 @item
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276 @cite{The sign of the remainder on integer division (C90 6.3.5).}
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277
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278 GCC always follows the C99 and C11 requirement that the result of division is
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279 truncated towards zero.
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280
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281 @end itemize
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282
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283 @node Floating point implementation
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111
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284 @section Floating Point
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285
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286 @itemize @bullet
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287 @item
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288 @cite{The accuracy of the floating-point operations and of the library
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289 functions in @code{<math.h>} and @code{<complex.h>} that return floating-point
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290 results (C90, C99 and C11 5.2.4.2.2).}
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291
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292 The accuracy is unknown.
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293
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294 @item
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295 @cite{The rounding behaviors characterized by non-standard values
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296 of @code{FLT_ROUNDS} @gol
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297 (C90, C99 and C11 5.2.4.2.2).}
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298
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299 GCC does not use such values.
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300
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301 @item
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302 @cite{The evaluation methods characterized by non-standard negative
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303 values of @code{FLT_EVAL_METHOD} (C99 and C11 5.2.4.2.2).}
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304
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305 GCC does not use such values.
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306
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307 @item
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308 @cite{The direction of rounding when an integer is converted to a
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309 floating-point number that cannot exactly represent the original
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310 value (C90 6.2.1.3, C99 and C11 6.3.1.4).}
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311
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312 C99 Annex F is followed.
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313
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314 @item
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315 @cite{The direction of rounding when a floating-point number is
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111
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316 converted to a narrower floating-point number (C90 6.2.1.4, C99 and C11
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317 6.3.1.5).}
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318
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319 C99 Annex F is followed.
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320
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321 @item
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322 @cite{How the nearest representable value or the larger or smaller
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323 representable value immediately adjacent to the nearest representable
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324 value is chosen for certain floating constants (C90 6.1.3.1, C99 and C11
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325 6.4.4.2).}
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326
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327 C99 Annex F is followed.
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328
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329 @item
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330 @cite{Whether and how floating expressions are contracted when not
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331 disallowed by the @code{FP_CONTRACT} pragma (C99 and C11 6.5).}
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332
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111
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333 Expressions are currently only contracted if @option{-ffp-contract=fast},
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334 @option{-funsafe-math-optimizations} or @option{-ffast-math} are used.
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335 This is subject to change.
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336
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337 @item
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111
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338 @cite{The default state for the @code{FENV_ACCESS} pragma (C99 and C11
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339 7.6.1).}
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340
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341 This pragma is not implemented, but the default is to ``off'' unless
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342 @option{-frounding-math} is used in which case it is ``on''.
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343
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344 @item
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345 @cite{Additional floating-point exceptions, rounding modes, environments,
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111
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346 and classifications, and their macro names (C99 and C11 7.6, C99 and
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347 C11 7.12).}
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348
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349 This is dependent on the implementation of the C library, and is not
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350 defined by GCC itself.
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351
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352 @item
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111
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353 @cite{The default state for the @code{FP_CONTRACT} pragma (C99 and C11
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354 7.12.2).}
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355
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356 This pragma is not implemented. Expressions are currently only
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357 contracted if @option{-ffp-contract=fast},
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358 @option{-funsafe-math-optimizations} or @option{-ffast-math} are used.
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359 This is subject to change.
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360
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361 @item
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362 @cite{Whether the ``inexact'' floating-point exception can be raised
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363 when the rounded result actually does equal the mathematical result
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364 in an IEC 60559 conformant implementation (C99 F.9).}
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365
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366 This is dependent on the implementation of the C library, and is not
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367 defined by GCC itself.
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368
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369 @item
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370 @cite{Whether the ``underflow'' (and ``inexact'') floating-point
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371 exception can be raised when a result is tiny but not inexact in an
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372 IEC 60559 conformant implementation (C99 F.9).}
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373
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374 This is dependent on the implementation of the C library, and is not
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375 defined by GCC itself.
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376
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377 @end itemize
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378
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379 @node Arrays and pointers implementation
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111
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380 @section Arrays and Pointers
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0
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381
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382 @itemize @bullet
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383 @item
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384 @cite{The result of converting a pointer to an integer or
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|
111
|
385 vice versa (C90 6.3.4, C99 and C11 6.3.2.3).}
|
|
0
|
386
|
|
|
387 A cast from pointer to integer discards most-significant bits if the
|
|
|
388 pointer representation is larger than the integer type,
|
|
|
389 sign-extends@footnote{Future versions of GCC may zero-extend, or use
|
|
|
390 a target-defined @code{ptr_extend} pattern. Do not rely on sign extension.}
|
|
|
391 if the pointer representation is smaller than the integer type, otherwise
|
|
|
392 the bits are unchanged.
|
|
|
393 @c ??? We've always claimed that pointers were unsigned entities.
|
|
|
394 @c Shouldn't we therefore be doing zero-extension? If so, the bug
|
|
|
395 @c is in convert_to_integer, where we call type_for_size and request
|
|
|
396 @c a signed integral type. On the other hand, it might be most useful
|
|
|
397 @c for the target if we extend according to POINTERS_EXTEND_UNSIGNED.
|
|
|
398
|
|
|
399 A cast from integer to pointer discards most-significant bits if the
|
|
|
400 pointer representation is smaller than the integer type, extends according
|
|
|
401 to the signedness of the integer type if the pointer representation
|
|
|
402 is larger than the integer type, otherwise the bits are unchanged.
|
|
|
403
|
|
|
404 When casting from pointer to integer and back again, the resulting
|
|
|
405 pointer must reference the same object as the original pointer, otherwise
|
|
|
406 the behavior is undefined. That is, one may not use integer arithmetic to
|
|
|
407 avoid the undefined behavior of pointer arithmetic as proscribed in
|
|
111
|
408 C99 and C11 6.5.6/8.
|
|
0
|
409
|
|
|
410 @item
|
|
|
411 @cite{The size of the result of subtracting two pointers to elements
|
|
111
|
412 of the same array (C90 6.3.6, C99 and C11 6.5.6).}
|
|
0
|
413
|
|
|
414 The value is as specified in the standard and the type is determined
|
|
|
415 by the ABI@.
|
|
|
416
|
|
|
417 @end itemize
|
|
|
418
|
|
|
419 @node Hints implementation
|
|
|
420 @section Hints
|
|
|
421
|
|
|
422 @itemize @bullet
|
|
|
423 @item
|
|
|
424 @cite{The extent to which suggestions made by using the @code{register}
|
|
111
|
425 storage-class specifier are effective (C90 6.5.1, C99 and C11 6.7.1).}
|
|
0
|
426
|
|
|
427 The @code{register} specifier affects code generation only in these ways:
|
|
|
428
|
|
|
429 @itemize @bullet
|
|
|
430 @item
|
|
|
431 When used as part of the register variable extension, see
|
|
111
|
432 @ref{Explicit Register Variables}.
|
|
0
|
433
|
|
|
434 @item
|
|
|
435 When @option{-O0} is in use, the compiler allocates distinct stack
|
|
|
436 memory for all variables that do not have the @code{register}
|
|
|
437 storage-class specifier; if @code{register} is specified, the variable
|
|
|
438 may have a shorter lifespan than the code would indicate and may never
|
|
|
439 be placed in memory.
|
|
|
440
|
|
|
441 @item
|
|
|
442 On some rare x86 targets, @code{setjmp} doesn't save the registers in
|
|
|
443 all circumstances. In those cases, GCC doesn't allocate any variables
|
|
|
444 in registers unless they are marked @code{register}.
|
|
|
445
|
|
|
446 @end itemize
|
|
|
447
|
|
|
448 @item
|
|
|
449 @cite{The extent to which suggestions made by using the inline function
|
|
111
|
450 specifier are effective (C99 and C11 6.7.4).}
|
|
0
|
451
|
|
|
452 GCC will not inline any functions if the @option{-fno-inline} option is
|
|
|
453 used or if @option{-O0} is used. Otherwise, GCC may still be unable to
|
|
|
454 inline a function for many reasons; the @option{-Winline} option may be
|
|
|
455 used to determine if a function has not been inlined and why not.
|
|
|
456
|
|
|
457 @end itemize
|
|
|
458
|
|
|
459 @node Structures unions enumerations and bit-fields implementation
|
|
111
|
460 @section Structures, Unions, Enumerations, and Bit-Fields
|
|
0
|
461
|
|
|
462 @itemize @bullet
|
|
|
463 @item
|
|
|
464 @cite{A member of a union object is accessed using a member of a
|
|
|
465 different type (C90 6.3.2.3).}
|
|
|
466
|
|
|
467 The relevant bytes of the representation of the object are treated as
|
|
|
468 an object of the type used for the access. @xref{Type-punning}. This
|
|
|
469 may be a trap representation.
|
|
|
470
|
|
|
471 @item
|
|
|
472 @cite{Whether a ``plain'' @code{int} bit-field is treated as a
|
|
|
473 @code{signed int} bit-field or as an @code{unsigned int} bit-field
|
|
111
|
474 (C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).}
|
|
0
|
475
|
|
|
476 @opindex funsigned-bitfields
|
|
|
477 By default it is treated as @code{signed int} but this may be changed
|
|
|
478 by the @option{-funsigned-bitfields} option.
|
|
|
479
|
|
|
480 @item
|
|
|
481 @cite{Allowable bit-field types other than @code{_Bool}, @code{signed int},
|
|
111
|
482 and @code{unsigned int} (C99 and C11 6.7.2.1).}
|
|
|
483
|
|
|
484 Other integer types, such as @code{long int}, and enumerated types are
|
|
|
485 permitted even in strictly conforming mode.
|
|
0
|
486
|
|
111
|
487 @item
|
|
|
488 @cite{Whether atomic types are permitted for bit-fields (C11 6.7.2.1).}
|
|
|
489
|
|
|
490 Atomic types are not permitted for bit-fields.
|
|
0
|
491
|
|
|
492 @item
|
|
|
493 @cite{Whether a bit-field can straddle a storage-unit boundary (C90
|
|
111
|
494 6.5.2.1, C99 and C11 6.7.2.1).}
|
|
0
|
495
|
|
|
496 Determined by ABI@.
|
|
|
497
|
|
|
498 @item
|
|
|
499 @cite{The order of allocation of bit-fields within a unit (C90
|
|
111
|
500 6.5.2.1, C99 and C11 6.7.2.1).}
|
|
0
|
501
|
|
|
502 Determined by ABI@.
|
|
|
503
|
|
|
504 @item
|
|
|
505 @cite{The alignment of non-bit-field members of structures (C90
|
|
111
|
506 6.5.2.1, C99 and C11 6.7.2.1).}
|
|
0
|
507
|
|
|
508 Determined by ABI@.
|
|
|
509
|
|
|
510 @item
|
|
|
511 @cite{The integer type compatible with each enumerated type (C90
|
|
111
|
512 6.5.2.2, C99 and C11 6.7.2.2).}
|
|
0
|
513
|
|
|
514 @opindex fshort-enums
|
|
|
515 Normally, the type is @code{unsigned int} if there are no negative
|
|
|
516 values in the enumeration, otherwise @code{int}. If
|
|
|
517 @option{-fshort-enums} is specified, then if there are negative values
|
|
|
518 it is the first of @code{signed char}, @code{short} and @code{int}
|
|
|
519 that can represent all the values, otherwise it is the first of
|
|
|
520 @code{unsigned char}, @code{unsigned short} and @code{unsigned int}
|
|
|
521 that can represent all the values.
|
|
|
522 @c On a few unusual targets with 64-bit int, this doesn't agree with
|
|
|
523 @c the code and one of the types accessed via mode attributes (which
|
|
|
524 @c are not currently considered extended integer types) may be used.
|
|
|
525 @c If these types are made extended integer types, it would still be
|
|
|
526 @c the case that -fshort-enums stops the implementation from
|
|
|
527 @c conforming to C90 on those targets.
|
|
|
528
|
|
|
529 On some targets, @option{-fshort-enums} is the default; this is
|
|
|
530 determined by the ABI@.
|
|
|
531
|
|
|
532 @end itemize
|
|
|
533
|
|
|
534 @node Qualifiers implementation
|
|
|
535 @section Qualifiers
|
|
|
536
|
|
|
537 @itemize @bullet
|
|
|
538 @item
|
|
|
539 @cite{What constitutes an access to an object that has volatile-qualified
|
|
111
|
540 type (C90 6.5.3, C99 and C11 6.7.3).}
|
|
0
|
541
|
|
|
542 Such an object is normally accessed by pointers and used for accessing
|
|
|
543 hardware. In most expressions, it is intuitively obvious what is a read
|
|
|
544 and what is a write. For example
|
|
|
545
|
|
|
546 @smallexample
|
|
|
547 volatile int *dst = @var{somevalue};
|
|
|
548 volatile int *src = @var{someothervalue};
|
|
|
549 *dst = *src;
|
|
|
550 @end smallexample
|
|
|
551
|
|
|
552 @noindent
|
|
|
553 will cause a read of the volatile object pointed to by @var{src} and store the
|
|
|
554 value into the volatile object pointed to by @var{dst}. There is no
|
|
|
555 guarantee that these reads and writes are atomic, especially for objects
|
|
|
556 larger than @code{int}.
|
|
|
557
|
|
|
558 However, if the volatile storage is not being modified, and the value of
|
|
|
559 the volatile storage is not used, then the situation is less obvious.
|
|
|
560 For example
|
|
|
561
|
|
|
562 @smallexample
|
|
|
563 volatile int *src = @var{somevalue};
|
|
|
564 *src;
|
|
|
565 @end smallexample
|
|
|
566
|
|
|
567 According to the C standard, such an expression is an rvalue whose type
|
|
145
|
568 is the unqualified version of its original type, i.e.@: @code{int}. Whether
|
|
0
|
569 GCC interprets this as a read of the volatile object being pointed to or
|
|
131
|
570 only as a request to evaluate the expression for its side effects depends
|
|
0
|
571 on this type.
|
|
|
572
|
|
|
573 If it is a scalar type, or on most targets an aggregate type whose only
|
|
|
574 member object is of a scalar type, or a union type whose member objects
|
|
|
575 are of scalar types, the expression is interpreted by GCC as a read of
|
|
|
576 the volatile object; in the other cases, the expression is only evaluated
|
|
131
|
577 for its side effects.
|
|
0
|
578
|
|
|
579 @end itemize
|
|
|
580
|
|
|
581 @node Declarators implementation
|
|
|
582 @section Declarators
|
|
|
583
|
|
|
584 @itemize @bullet
|
|
|
585 @item
|
|
|
586 @cite{The maximum number of declarators that may modify an arithmetic,
|
|
|
587 structure or union type (C90 6.5.4).}
|
|
|
588
|
|
|
589 GCC is only limited by available memory.
|
|
|
590
|
|
|
591 @end itemize
|
|
|
592
|
|
|
593 @node Statements implementation
|
|
|
594 @section Statements
|
|
|
595
|
|
|
596 @itemize @bullet
|
|
|
597 @item
|
|
|
598 @cite{The maximum number of @code{case} values in a @code{switch}
|
|
|
599 statement (C90 6.6.4.2).}
|
|
|
600
|
|
|
601 GCC is only limited by available memory.
|
|
|
602
|
|
|
603 @end itemize
|
|
|
604
|
|
|
605 @node Preprocessing directives implementation
|
|
111
|
606 @section Preprocessing Directives
|
|
0
|
607
|
|
|
608 @xref{Implementation-defined behavior, , Implementation-defined
|
|
|
609 behavior, cpp, The C Preprocessor}, for details of these aspects of
|
|
|
610 implementation-defined behavior.
|
|
|
611
|
|
|
612 @itemize @bullet
|
|
|
613 @item
|
|
111
|
614 @cite{The locations within @code{#pragma} directives where header name
|
|
|
615 preprocessing tokens are recognized (C11 6.4, C11 6.4.7).}
|
|
|
616
|
|
|
617 @item
|
|
0
|
618 @cite{How sequences in both forms of header names are mapped to headers
|
|
111
|
619 or external source file names (C90 6.1.7, C99 and C11 6.4.7).}
|
|
0
|
620
|
|
|
621 @item
|
|
|
622 @cite{Whether the value of a character constant in a constant expression
|
|
|
623 that controls conditional inclusion matches the value of the same character
|
|
111
|
624 constant in the execution character set (C90 6.8.1, C99 and C11 6.10.1).}
|
|
0
|
625
|
|
|
626 @item
|
|
|
627 @cite{Whether the value of a single-character character constant in a
|
|
|
628 constant expression that controls conditional inclusion may have a
|
|
111
|
629 negative value (C90 6.8.1, C99 and C11 6.10.1).}
|
|
0
|
630
|
|
|
631 @item
|
|
|
632 @cite{The places that are searched for an included @samp{<>} delimited
|
|
|
633 header, and how the places are specified or the header is
|
|
111
|
634 identified (C90 6.8.2, C99 and C11 6.10.2).}
|
|
0
|
635
|
|
|
636 @item
|
|
|
637 @cite{How the named source file is searched for in an included @samp{""}
|
|
111
|
638 delimited header (C90 6.8.2, C99 and C11 6.10.2).}
|
|
0
|
639
|
|
|
640 @item
|
|
|
641 @cite{The method by which preprocessing tokens (possibly resulting from
|
|
|
642 macro expansion) in a @code{#include} directive are combined into a header
|
|
111
|
643 name (C90 6.8.2, C99 and C11 6.10.2).}
|
|
0
|
644
|
|
|
645 @item
|
|
|
646 @cite{The nesting limit for @code{#include} processing (C90 6.8.2, C99
|
|
111
|
647 and C11 6.10.2).}
|
|
0
|
648
|
|
|
649 @item
|
|
|
650 @cite{Whether the @samp{#} operator inserts a @samp{\} character before
|
|
|
651 the @samp{\} character that begins a universal character name in a
|
|
111
|
652 character constant or string literal (C99 and C11 6.10.3.2).}
|
|
0
|
653
|
|
|
654 @item
|
|
|
655 @cite{The behavior on each recognized non-@code{STDC #pragma}
|
|
111
|
656 directive (C90 6.8.6, C99 and C11 6.10.6).}
|
|
0
|
657
|
|
|
658 @xref{Pragmas, , Pragmas, cpp, The C Preprocessor}, for details of
|
|
|
659 pragmas accepted by GCC on all targets. @xref{Pragmas, , Pragmas
|
|
|
660 Accepted by GCC}, for details of target-specific pragmas.
|
|
|
661
|
|
|
662 @item
|
|
|
663 @cite{The definitions for @code{__DATE__} and @code{__TIME__} when
|
|
|
664 respectively, the date and time of translation are not available (C90
|
|
111
|
665 6.8.8, C99 6.10.8, C11 6.10.8.1).}
|
|
0
|
666
|
|
|
667 @end itemize
|
|
|
668
|
|
|
669 @node Library functions implementation
|
|
111
|
670 @section Library Functions
|
|
0
|
671
|
|
|
672 The behavior of most of these points are dependent on the implementation
|
|
|
673 of the C library, and are not defined by GCC itself.
|
|
|
674
|
|
|
675 @itemize @bullet
|
|
|
676 @item
|
|
|
677 @cite{The null pointer constant to which the macro @code{NULL} expands
|
|
111
|
678 (C90 7.1.6, C99 7.17, C11 7.19).}
|
|
0
|
679
|
|
|
680 In @code{<stddef.h>}, @code{NULL} expands to @code{((void *)0)}. GCC
|
|
|
681 does not provide the other headers which define @code{NULL} and some
|
|
|
682 library implementations may use other definitions in those headers.
|
|
|
683
|
|
|
684 @end itemize
|
|
|
685
|
|
|
686 @node Architecture implementation
|
|
|
687 @section Architecture
|
|
|
688
|
|
|
689 @itemize @bullet
|
|
|
690 @item
|
|
|
691 @cite{The values or expressions assigned to the macros specified in the
|
|
|
692 headers @code{<float.h>}, @code{<limits.h>}, and @code{<stdint.h>}
|
|
111
|
693 (C90, C99 and C11 5.2.4.2, C99 7.18.2, C99 7.18.3, C11 7.20.2, C11 7.20.3).}
|
|
0
|
694
|
|
|
695 Determined by ABI@.
|
|
|
696
|
|
|
697 @item
|
|
111
|
698 @cite{The result of attempting to indirectly access an object with
|
|
|
699 automatic or thread storage duration from a thread other than the one
|
|
|
700 with which it is associated (C11 6.2.4).}
|
|
|
701
|
|
|
702 Such accesses are supported, subject to the same requirements for
|
|
|
703 synchronization for concurrent accesses as for concurrent accesses to
|
|
|
704 any object.
|
|
|
705
|
|
|
706 @item
|
|
0
|
707 @cite{The number, order, and encoding of bytes in any object
|
|
111
|
708 (when not explicitly specified in this International Standard) (C99
|
|
|
709 and C11 6.2.6.1).}
|
|
0
|
710
|
|
|
711 Determined by ABI@.
|
|
|
712
|
|
|
713 @item
|
|
111
|
714 @cite{Whether any extended alignments are supported and the contexts
|
|
|
715 in which they are supported (C11 6.2.8).}
|
|
|
716
|
|
|
717 Extended alignments up to @math{2^{28}} (bytes) are supported for
|
|
|
718 objects of automatic storage duration. Alignments supported for
|
|
|
719 objects of static and thread storage duration are determined by the
|
|
|
720 ABI.
|
|
|
721
|
|
|
722 @item
|
|
|
723 @cite{Valid alignment values other than those returned by an _Alignof
|
|
|
724 expression for fundamental types, if any (C11 6.2.8).}
|
|
|
725
|
|
|
726 Valid alignments are powers of 2 up to and including @math{2^{28}}.
|
|
|
727
|
|
|
728 @item
|
|
|
729 @cite{The value of the result of the @code{sizeof} and @code{_Alignof}
|
|
|
730 operators (C90 6.3.3.4, C99 and C11 6.5.3.4).}
|
|
0
|
731
|
|
|
732 Determined by ABI@.
|
|
|
733
|
|
|
734 @end itemize
|
|
|
735
|
|
|
736 @node Locale-specific behavior implementation
|
|
111
|
737 @section Locale-Specific Behavior
|
|
0
|
738
|
|
|
739 The behavior of these points are dependent on the implementation
|
|
|
740 of the C library, and are not defined by GCC itself.
|
