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1 | =head1 LIBECB - e-C-Builtins |
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2 | |
1 | =head1 LIBECB |
3 | =head2 ABOUT LIBECB |
2 | |
4 | |
3 | You suck, we don't(tm) |
5 | Libecb is currently a simple header file that doesn't require any |
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6 | configuration to use or include in your project. |
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7 | |
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8 | It's part of the e-suite of libraries, other members of which include |
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9 | libev and libeio. |
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10 | |
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11 | Its homepage can be found here: |
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12 | |
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13 | http://software.schmorp.de/pkg/libecb |
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14 | |
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15 | It mainly provides a number of wrappers around GCC built-ins, together |
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16 | with replacement functions for other compilers. In addition to this, |
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17 | it provides a number of other lowlevel C utilities, such as endianness |
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18 | detection, byte swapping or bit rotations. |
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19 | |
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20 | Or in other words, things that should be built into any standard C system, |
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21 | but aren't, implemented as efficient as possible with GCC, and still |
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22 | correct with other compilers. |
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23 | |
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24 | More might come. |
4 | |
25 | |
5 | =head2 ABOUT THE HEADER |
26 | =head2 ABOUT THE HEADER |
6 | |
27 | |
7 | - how to include it |
28 | At the moment, all you have to do is copy F<ecb.h> somewhere where your |
8 | - it includes inttypes.h |
29 | compiler can find it and include it: |
9 | - no .a |
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10 | - whats a bool |
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11 | - function mean macro or function |
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12 | - macro means untyped |
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13 | |
30 | |
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31 | #include <ecb.h> |
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32 | |
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33 | The header should work fine for both C and C++ compilation, and gives you |
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34 | all of F<inttypes.h> in addition to the ECB symbols. |
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35 | |
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36 | There are currently no object files to link to - future versions might |
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37 | come with an (optional) object code library to link against, to reduce |
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38 | code size or gain access to additional features. |
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39 | |
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40 | It also currently includes everything from F<inttypes.h>. |
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41 | |
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42 | =head2 ABOUT THIS MANUAL / CONVENTIONS |
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43 | |
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44 | This manual mainly describes each (public) function available after |
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45 | including the F<ecb.h> header. The header might define other symbols than |
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46 | these, but these are not part of the public API, and not supported in any |
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47 | way. |
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48 | |
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49 | When the manual mentions a "function" then this could be defined either as |
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50 | as inline function, a macro, or an external symbol. |
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51 | |
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52 | When functions use a concrete standard type, such as C<int> or |
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53 | C<uint32_t>, then the corresponding function works only with that type. If |
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54 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
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55 | the corresponding function relies on C to implement the correct types, and |
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56 | is usually implemented as a macro. Specifically, a "bool" in this manual |
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57 | refers to any kind of boolean value, not a specific type. |
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58 | |
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59 | =head2 TYPES / TYPE SUPPORT |
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60 | |
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61 | ecb.h makes sure that the following types are defined (in the expected way): |
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62 | |
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63 | int8_t uint8_ |
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64 | int16_t uint16_t |
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65 | int32_t uint32_ |
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66 | int64_t uint64_t |
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67 | int_fast8_t uint_fast8_t |
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68 | int_fast16_t uint_fast16_t |
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69 | int_fast32_t uint_fast32_t |
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70 | int_fast64_t uint_fast64_t |
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71 | intptr_t uintptr_t |
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72 | |
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73 | The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this |
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74 | platform (currently C<4> or C<8>) and can be used in preprocessor |
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75 | expressions. |
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76 | |
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77 | For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>. |
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78 | |
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79 | =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS |
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80 | |
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81 | All the following symbols expand to an expression that can be tested in |
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82 | preprocessor instructions as well as treated as a boolean (use C<!!> to |
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83 | ensure it's either C<0> or C<1> if you need that). |
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84 | |
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85 | =over 4 |
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86 | |
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87 | =item ECB_C |
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88 | |
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89 | True if the implementation defines the C<__STDC__> macro to a true value, |
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90 | while not claiming to be C++. |
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91 | |
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92 | =item ECB_C99 |
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93 | |
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94 | True if the implementation claims to be compliant to C99 (ISO/IEC |
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95 | 9899:1999) or any later version, while not claiming to be C++. |
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96 | |
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97 | Note that later versions (ECB_C11) remove core features again (for |
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98 | example, variable length arrays). |
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99 | |
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100 | =item ECB_C11, ECB_C17 |
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101 | |
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102 | True if the implementation claims to be compliant to C11/C17 (ISO/IEC |
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103 | 9899:2011, :20187) or any later version, while not claiming to be C++. |
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104 | |
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105 | =item ECB_CPP |
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106 | |
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107 | True if the implementation defines the C<__cplusplus__> macro to a true |
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108 | value, which is typically true for C++ compilers. |
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109 | |
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110 | =item ECB_CPP11, ECB_CPP14, ECB_CPP17 |
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111 | |
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112 | True if the implementation claims to be compliant to C++11/C++14/C++17 |
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113 | (ISO/IEC 14882:2011, :2014, :2017) or any later version. |
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114 | |
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115 | =item ECB_GCC_VERSION (major, minor) |
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116 | |
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117 | Expands to a true value (suitable for testing in by the preprocessor) |
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118 | if the compiler used is GNU C and the version is the given version, or |
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119 | higher. |
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120 | |
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121 | This macro tries to return false on compilers that claim to be GCC |
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122 | compatible but aren't. |
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123 | |
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124 | =item ECB_EXTERN_C |
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125 | |
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126 | Expands to C<extern "C"> in C++, and a simple C<extern> in C. |
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127 | |
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128 | This can be used to declare a single external C function: |
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129 | |
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130 | ECB_EXTERN_C int printf (const char *format, ...); |
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131 | |
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132 | =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END |
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133 | |
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134 | These two macros can be used to wrap multiple C<extern "C"> definitions - |
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135 | they expand to nothing in C. |
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136 | |
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137 | They are most useful in header files: |
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138 | |
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139 | ECB_EXTERN_C_BEG |
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140 | |
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141 | int mycfun1 (int x); |
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142 | int mycfun2 (int x); |
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143 | |
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144 | ECB_EXTERN_C_END |
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145 | |
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146 | =item ECB_STDFP |
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147 | |
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148 | If this evaluates to a true value (suitable for testing in by the |
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149 | preprocessor), then C<float> and C<double> use IEEE 754 single/binary32 |
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150 | and double/binary64 representations internally I<and> the endianness of |
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151 | both types match the endianness of C<uint32_t> and C<uint64_t>. |
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152 | |
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153 | This means you can just copy the bits of a C<float> (or C<double>) to an |
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154 | C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation |
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155 | without having to think about format or endianness. |
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156 | |
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157 | This is true for basically all modern platforms, although F<ecb.h> might |
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158 | not be able to deduce this correctly everywhere and might err on the safe |
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159 | side. |
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160 | |
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161 | =item ECB_AMD64, ECB_AMD64_X32 |
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162 | |
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163 | These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32 |
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164 | ABI, respectively, and undefined elsewhere. |
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165 | |
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166 | The designers of the new X32 ABI for some inexplicable reason decided to |
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167 | make it look exactly like amd64, even though it's completely incompatible |
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168 | to that ABI, breaking about every piece of software that assumed that |
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169 | C<__x86_64> stands for, well, the x86-64 ABI, making these macros |
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170 | necessary. |
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171 | |
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172 | =back |
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173 | |
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174 | =head2 MACRO TRICKERY |
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175 | |
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176 | =over 4 |
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177 | |
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178 | =item ECB_CONCAT (a, b) |
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179 | |
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180 | Expands any macros in C<a> and C<b>, then concatenates the result to form |
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181 | a single token. This is mainly useful to form identifiers from components, |
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182 | e.g.: |
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183 | |
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184 | #define S1 str |
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185 | #define S2 cpy |
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186 | |
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187 | ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src); |
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188 | |
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189 | =item ECB_STRINGIFY (arg) |
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190 | |
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191 | Expands any macros in C<arg> and returns the stringified version of |
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192 | it. This is mainly useful to get the contents of a macro in string form, |
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193 | e.g.: |
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194 | |
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195 | #define SQL_LIMIT 100 |
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196 | sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT)); |
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197 | |
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198 | =item ECB_STRINGIFY_EXPR (expr) |
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199 | |
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200 | Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it |
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201 | is a valid expression. This is useful to catch typos or cases where the |
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202 | macro isn't available: |
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203 | |
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204 | #include <errno.h> |
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205 | |
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206 | ECB_STRINGIFY (EDOM); // "33" (on my system at least) |
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207 | ECB_STRINGIFY_EXPR (EDOM); // "33" |
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208 | |
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209 | // now imagine we had a typo: |
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210 | |
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211 | ECB_STRINGIFY (EDAM); // "EDAM" |
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212 | ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined |
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213 | |
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214 | =back |
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215 | |
14 | =head2 GCC ATTRIBUTES |
216 | =head2 ATTRIBUTES |
15 | |
217 | |
16 | blabla where to put, what others |
218 | A major part of libecb deals with additional attributes that can be |
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219 | assigned to functions, variables and sometimes even types - much like |
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220 | C<const> or C<volatile> in C. They are implemented using either GCC |
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221 | attributes or other compiler/language specific features. Attributes |
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222 | declarations must be put before the whole declaration: |
17 | |
223 | |
18 | =over 4 |
224 | ecb_const int mysqrt (int a); |
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225 | ecb_unused int i; |
19 | |
226 | |
20 | =item ecb_attribute ((attrs...)) |
227 | =over 4 |
21 | |
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22 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
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23 | to nothing on other compilers, so the effect is that only GCC sees these. |
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24 | |
228 | |
25 | =item ecb_unused |
229 | =item ecb_unused |
26 | |
230 | |
27 | Marks a function or a variable as "unused", which simply suppresses a |
231 | Marks a function or a variable as "unused", which simply suppresses a |
28 | warning by GCC when it detects it as unused. This is useful when you e.g. |
232 | warning by GCC when it detects it as unused. This is useful when you e.g. |
29 | declare a variable but do not always use it: |
233 | declare a variable but do not always use it: |
30 | |
234 | |
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235 | { |
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236 | ecb_unused int var; |
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237 | |
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238 | #ifdef SOMECONDITION |
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239 | var = ...; |
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240 | return var; |
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241 | #else |
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242 | return 0; |
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243 | #endif |
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244 | } |
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245 | |
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246 | =item ecb_deprecated |
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247 | |
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248 | Similar to C<ecb_unused>, but marks a function, variable or type as |
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249 | deprecated. This makes some compilers warn when the type is used. |
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250 | |
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251 | =item ecb_deprecated_message (message) |
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252 | |
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253 | Same as C<ecb_deprecated>, but if possible, the specified diagnostic is |
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254 | used instead of a generic depreciation message when the object is being |
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255 | used. |
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256 | |
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257 | =item ecb_inline |
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258 | |
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259 | Expands either to (a compiler-specific equivalent of) C<static inline> or |
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260 | to just C<static>, if inline isn't supported. It should be used to declare |
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261 | functions that should be inlined, for code size or speed reasons. |
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262 | |
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263 | Example: inline this function, it surely will reduce codesize. |
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264 | |
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265 | ecb_inline int |
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266 | negmul (int a, int b) |
31 | { |
267 | { |
32 | int var ecb_unused; |
268 | return - (a * b); |
33 | |
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34 | #ifdef SOMECONDITION |
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35 | var = ...; |
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36 | return var; |
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37 | #else |
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38 | return 0; |
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39 | #endif |
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40 | } |
269 | } |
41 | |
270 | |
42 | =item ecb_noinline |
271 | =item ecb_noinline |
43 | |
272 | |
44 | Prevent a function from being inlined - it might be optimised away, but |
273 | Prevents a function from being inlined - it might be optimised away, but |
45 | not inlined into other functions. This is useful if you know your function |
274 | not inlined into other functions. This is useful if you know your function |
46 | is rarely called and large enough for inlining not to be helpful. |
275 | is rarely called and large enough for inlining not to be helpful. |
47 | |
276 | |
48 | =item ecb_noreturn |
277 | =item ecb_noreturn |
49 | |
278 | |
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279 | Marks a function as "not returning, ever". Some typical functions that |
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280 | don't return are C<exit> or C<abort> (which really works hard to not |
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281 | return), and now you can make your own: |
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282 | |
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283 | ecb_noreturn void |
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284 | my_abort (const char *errline) |
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285 | { |
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286 | puts (errline); |
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287 | abort (); |
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288 | } |
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289 | |
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290 | In this case, the compiler would probably be smart enough to deduce it on |
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291 | its own, so this is mainly useful for declarations. |
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292 | |
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293 | =item ecb_restrict |
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294 | |
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295 | Expands to the C<restrict> keyword or equivalent on compilers that support |
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296 | them, and to nothing on others. Must be specified on a pointer type or |
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297 | an array index to indicate that the memory doesn't alias with any other |
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298 | restricted pointer in the same scope. |
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299 | |
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300 | Example: multiply a vector, and allow the compiler to parallelise the |
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301 | loop, because it knows it doesn't overwrite input values. |
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302 | |
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303 | void |
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304 | multiply (ecb_restrict float *src, |
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305 | ecb_restrict float *dst, |
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306 | int len, float factor) |
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307 | { |
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308 | int i; |
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309 | |
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310 | for (i = 0; i < len; ++i) |
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311 | dst [i] = src [i] * factor; |
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312 | } |
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313 | |
50 | =item ecb_const |
314 | =item ecb_const |
51 | |
315 | |
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316 | Declares that the function only depends on the values of its arguments, |
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317 | much like a mathematical function. It specifically does not read or write |
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318 | any memory any arguments might point to, global variables, or call any |
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319 | non-const functions. It also must not have any side effects. |
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320 | |
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321 | Such a function can be optimised much more aggressively by the compiler - |
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322 | for example, multiple calls with the same arguments can be optimised into |
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323 | a single call, which wouldn't be possible if the compiler would have to |
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324 | expect any side effects. |
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325 | |
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326 | It is best suited for functions in the sense of mathematical functions, |
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327 | such as a function returning the square root of its input argument. |
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328 | |
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329 | Not suited would be a function that calculates the hash of some memory |
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330 | area you pass in, prints some messages or looks at a global variable to |
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331 | decide on rounding. |
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332 | |
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333 | See C<ecb_pure> for a slightly less restrictive class of functions. |
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334 | |
52 | =item ecb_pure |
335 | =item ecb_pure |
53 | |
336 | |
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337 | Similar to C<ecb_const>, declares a function that has no side |
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338 | effects. Unlike C<ecb_const>, the function is allowed to examine global |
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339 | variables and any other memory areas (such as the ones passed to it via |
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340 | pointers). |
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341 | |
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342 | While these functions cannot be optimised as aggressively as C<ecb_const> |
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343 | functions, they can still be optimised away in many occasions, and the |
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344 | compiler has more freedom in moving calls to them around. |
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345 | |
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346 | Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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347 | function that calculates the MD5 sum of some input and updates some MD5 |
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348 | state passed as argument would I<NOT> be pure, however, as it would modify |
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349 | some memory area that is not the return value. |
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350 | |
54 | =item ecb_hot |
351 | =item ecb_hot |
55 | |
352 | |
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353 | This declares a function as "hot" with regards to the cache - the function |
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354 | is used so often, that it is very beneficial to keep it in the cache if |
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355 | possible. |
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356 | |
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357 | The compiler reacts by trying to place hot functions near to each other in |
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358 | memory. |
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359 | |
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360 | Whether a function is hot or not often depends on the whole program, |
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361 | and less on the function itself. C<ecb_cold> is likely more useful in |
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362 | practise. |
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363 | |
56 | =item ecb_cold |
364 | =item ecb_cold |
57 | |
365 | |
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366 | The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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367 | the cache, or in other words, this function is not called often, or not at |
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368 | speed-critical times, and keeping it in the cache might be a waste of said |
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369 | cache. |
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370 | |
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371 | In addition to placing cold functions together (or at least away from hot |
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372 | functions), this knowledge can be used in other ways, for example, the |
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373 | function will be optimised for size, as opposed to speed, and codepaths |
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374 | leading to calls to those functions can automatically be marked as if |
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375 | C<ecb_expect_false> had been used to reach them. |
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376 | |
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377 | Good examples for such functions would be error reporting functions, or |
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378 | functions only called in exceptional or rare cases. |
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379 | |
58 | =item ecb_artificial |
380 | =item ecb_artificial |
59 | |
381 | |
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382 | Declares the function as "artificial", in this case meaning that this |
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383 | function is not really meant to be a function, but more like an accessor |
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384 | - many methods in C++ classes are mere accessor functions, and having a |
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385 | crash reported in such a method, or single-stepping through them, is not |
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386 | usually so helpful, especially when it's inlined to just a few instructions. |
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387 | |
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388 | Marking them as artificial will instruct the debugger about just this, |
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389 | leading to happier debugging and thus happier lives. |
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390 | |
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391 | Example: in some kind of smart-pointer class, mark the pointer accessor as |
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392 | artificial, so that the whole class acts more like a pointer and less like |
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393 | some C++ abstraction monster. |
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394 | |
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395 | template<typename T> |
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396 | struct my_smart_ptr |
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397 | { |
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398 | T *value; |
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399 | |
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400 | ecb_artificial |
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401 | operator T *() |
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402 | { |
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403 | return value; |
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404 | } |
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405 | }; |
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406 | |
60 | =back |
407 | =back |
61 | |
408 | |
62 | =head2 OPTIMISATION HINTS |
409 | =head2 OPTIMISATION HINTS |
63 | |
410 | |
64 | =over 4 |
411 | =over 4 |
65 | |
412 | |
|
|
413 | =item ECB_OPTIMIZE_SIZE |
|
|
414 | |
|
|
415 | Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol |
|
|
416 | can also be defined before including F<ecb.h>, in which case it will be |
|
|
417 | unchanged. |
|
|
418 | |
66 | =item bool ecb_is_constant(expr) [MACRO] |
419 | =item bool ecb_is_constant (expr) |
67 | |
420 | |
68 | Returns true iff the expression can be deduced to be a compile-time |
421 | Returns true iff the expression can be deduced to be a compile-time |
69 | constant, and false otherwise. |
422 | constant, and false otherwise. |
70 | |
423 | |
71 | For example, when you have a C<rndm16> function that returns a 16 bit |
424 | For example, when you have a C<rndm16> function that returns a 16 bit |
… | |
… | |
89 | return is_constant (n) && !(n & (n - 1)) |
442 | return is_constant (n) && !(n & (n - 1)) |
90 | ? rndm16 () & (num - 1) |
443 | ? rndm16 () & (num - 1) |
91 | : (n * (uint32_t)rndm16 ()) >> 16; |
444 | : (n * (uint32_t)rndm16 ()) >> 16; |
92 | } |
445 | } |
93 | |
446 | |
94 | =item bool ecb_expect (expr, value) [MACRO] |
447 | =item ecb_expect (expr, value) |
95 | |
448 | |
96 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
449 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
97 | the C<expr> evaluates to C<value> a lot, which can be used for static |
450 | the C<expr> evaluates to C<value> a lot, which can be used for static |
98 | branch optimisations. |
451 | branch optimisations. |
99 | |
452 | |
100 | Usually, you want to use the more intuitive C<ecb_likely> and |
453 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
101 | C<ecb_unlikely> functions instead. |
454 | C<ecb_expect_false> functions instead. |
102 | |
455 | |
103 | =item bool ecb_likely (bool) [MACRO] |
456 | =item bool ecb_expect_true (cond) |
104 | |
457 | |
105 | =item bool ecb_unlikely (bool) [MACRO] |
458 | =item bool ecb_expect_false (cond) |
106 | |
459 | |
107 | These two functions expect a expression that is true or false and return |
460 | These two functions expect a expression that is true or false and return |
108 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
461 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
109 | other conditional statement, it will not change the program: |
462 | other conditional statement, it will not change the program: |
110 | |
463 | |
111 | /* these two do the same thing */ |
464 | /* these two do the same thing */ |
112 | if (some_condition) ...; |
465 | if (some_condition) ...; |
113 | if (ecb_likely (some_condition)) ...; |
466 | if (ecb_expect_true (some_condition)) ...; |
114 | |
467 | |
115 | However, by using C<ecb_likely>, you tell the compiler that the condition |
468 | However, by using C<ecb_expect_true>, you tell the compiler that the |
116 | is likely to be true (and for C<ecb_unlikel>, that it is unlikely to be |
469 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
117 | true). |
470 | unlikely to be true). |
118 | |
471 | |
119 | For example, when you check for a null pointer and expect this to be a |
472 | For example, when you check for a null pointer and expect this to be a |
120 | rare, exceptional, case, then use C<ecb_unlikely>: |
473 | rare, exceptional, case, then use C<ecb_expect_false>: |
121 | |
474 | |
122 | void my_free (void *ptr) |
475 | void my_free (void *ptr) |
123 | { |
476 | { |
124 | if (ecb_unlikely (ptr == 0)) |
477 | if (ecb_expect_false (ptr == 0)) |
125 | return; |
478 | return; |
126 | } |
479 | } |
127 | |
480 | |
128 | Consequent use of these functions to mark away exceptional cases or to |
481 | Consequent use of these functions to mark away exceptional cases or to |
129 | tell the compiler what the hot path through a function is can increase |
482 | tell the compiler what the hot path through a function is can increase |
130 | performance considerably. |
483 | performance considerably. |
|
|
484 | |
|
|
485 | You might know these functions under the name C<likely> and C<unlikely> |
|
|
486 | - while these are common aliases, we find that the expect name is easier |
|
|
487 | to understand when quickly skimming code. If you wish, you can use |
|
|
488 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
|
|
489 | C<ecb_expect_false> - these are simply aliases. |
131 | |
490 | |
132 | A very good example is in a function that reserves more space for some |
491 | A very good example is in a function that reserves more space for some |
133 | memory block (for example, inside an implementation of a string stream) - |
492 | memory block (for example, inside an implementation of a string stream) - |
134 | each time something is added, you have to check for a buffer overrun, but |
493 | each time something is added, you have to check for a buffer overrun, but |
135 | you expect that most checks will turn out to be false: |
494 | you expect that most checks will turn out to be false: |
136 | |
495 | |
137 | /* make sure we have "size" extra room in our buffer */ |
496 | /* make sure we have "size" extra room in our buffer */ |
138 | ecb_inline void |
497 | ecb_inline void |
139 | reserve (int size) |
498 | reserve (int size) |
140 | { |
499 | { |
141 | if (ecb_unlikely (current + size > end)) |
500 | if (ecb_expect_false (current + size > end)) |
142 | real_reserve_method (size); /* presumably noinline */ |
501 | real_reserve_method (size); /* presumably noinline */ |
143 | } |
502 | } |
144 | |
503 | |
145 | =item bool ecb_assume (cond) [MACRO] |
504 | =item ecb_assume (cond) |
146 | |
505 | |
147 | Try to tell the compiler that some condition is true, even if it's not |
506 | Tries to tell the compiler that some condition is true, even if it's not |
148 | obvious. |
507 | obvious. This is not a function, but a statement: it cannot be used in |
|
|
508 | another expression. |
149 | |
509 | |
150 | This can be used to teach the compiler about invariants or other |
510 | This can be used to teach the compiler about invariants or other |
151 | conditions that might improve code generation, but which are impossible to |
511 | conditions that might improve code generation, but which are impossible to |
152 | deduce form the code itself. |
512 | deduce form the code itself. |
153 | |
513 | |
154 | For example, the example reservation function from the C<ecb_unlikely> |
514 | For example, the example reservation function from the C<ecb_expect_false> |
155 | description could be written thus (only C<ecb_assume> was added): |
515 | description could be written thus (only C<ecb_assume> was added): |
156 | |
516 | |
157 | ecb_inline void |
517 | ecb_inline void |
158 | reserve (int size) |
518 | reserve (int size) |
159 | { |
519 | { |
160 | if (ecb_unlikely (current + size > end)) |
520 | if (ecb_expect_false (current + size > end)) |
161 | real_reserve_method (size); /* presumably noinline */ |
521 | real_reserve_method (size); /* presumably noinline */ |
162 | |
522 | |
163 | ecb_assume (current + size <= end); |
523 | ecb_assume (current + size <= end); |
164 | } |
524 | } |
165 | |
525 | |
… | |
… | |
170 | |
530 | |
171 | Then the compiler I<might> be able to optimise out the second call |
531 | Then the compiler I<might> be able to optimise out the second call |
172 | completely, as it knows that C<< current + 1 > end >> is false and the |
532 | completely, as it knows that C<< current + 1 > end >> is false and the |
173 | call will never be executed. |
533 | call will never be executed. |
174 | |
534 | |
175 | =item bool ecb_unreachable () |
535 | =item ecb_unreachable () |
176 | |
536 | |
177 | This function does nothing itself, except tell the compiler that it will |
537 | This function does nothing itself, except tell the compiler that it will |
178 | never be executed. Apart from suppressing a warning in some cases, this |
538 | never be executed. Apart from suppressing a warning in some cases, this |
179 | function can be used to implement C<ecb_assume> or similar functions. |
539 | function can be used to implement C<ecb_assume> or similar functionality. |
180 | |
540 | |
181 | =item bool ecb_prefetch (addr, rw, locality) [MACRO] |
541 | =item ecb_prefetch (addr, rw, locality) |
182 | |
542 | |
183 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
543 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
184 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
544 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
185 | C<0> means that there will only be one access later, C<3> means that |
545 | C<0> means that there will only be one access later, C<3> means that |
186 | the data will likely be accessed very often, and values in between mean |
546 | the data will likely be accessed very often, and values in between mean |
187 | something... in between. The memory pointed to by the address does not |
547 | something... in between. The memory pointed to by the address does not |
188 | need to be accessible (it could be a null pointer for example), but C<rw> |
548 | need to be accessible (it could be a null pointer for example), but C<rw> |
189 | and C<locality> must be compile-time constants. |
549 | and C<locality> must be compile-time constants. |
190 | |
550 | |
|
|
551 | This is a statement, not a function: you cannot use it as part of an |
|
|
552 | expression. |
|
|
553 | |
191 | An obvious way to use this is to prefetch some data far away, in a big |
554 | An obvious way to use this is to prefetch some data far away, in a big |
192 | array you loop over. This prefetches memory some 128 array elements later, |
555 | array you loop over. This prefetches memory some 128 array elements later, |
193 | in the hope that it will be ready when the CPU arrives at that location. |
556 | in the hope that it will be ready when the CPU arrives at that location. |
194 | |
557 | |
195 | int sum = 0; |
558 | int sum = 0; |
… | |
… | |
214 | After processing the node, (part of) the next node might already be in |
577 | After processing the node, (part of) the next node might already be in |
215 | cache. |
578 | cache. |
216 | |
579 | |
217 | =back |
580 | =back |
218 | |
581 | |
219 | =head2 BIT FIDDLING / BITSTUFFS |
582 | =head2 BIT FIDDLING / BIT WIZARDRY |
220 | |
583 | |
221 | =over 4 |
584 | =over 4 |
222 | |
585 | |
223 | =item bool ecb_big_endian () |
586 | =item bool ecb_big_endian () |
224 | |
587 | |
225 | =item bool ecb_little_endian () |
588 | =item bool ecb_little_endian () |
226 | |
589 | |
|
|
590 | These two functions return true if the byte order is big endian |
|
|
591 | (most-significant byte first) or little endian (least-significant byte |
|
|
592 | first) respectively. |
|
|
593 | |
|
|
594 | On systems that are neither, their return values are unspecified. |
|
|
595 | |
227 | =item int ecb_ctz32 (uint32_t x) |
596 | =item int ecb_ctz32 (uint32_t x) |
228 | |
597 | |
|
|
598 | =item int ecb_ctz64 (uint64_t x) |
|
|
599 | |
|
|
600 | Returns the index of the least significant bit set in C<x> (or |
|
|
601 | equivalently the number of bits set to 0 before the least significant bit |
|
|
602 | set), starting from 0. If C<x> is 0 the result is undefined. |
|
|
603 | |
|
|
604 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
|
|
605 | |
|
|
606 | For example: |
|
|
607 | |
|
|
608 | ecb_ctz32 (3) = 0 |
|
|
609 | ecb_ctz32 (6) = 1 |
|
|
610 | |
|
|
611 | =item bool ecb_is_pot32 (uint32_t x) |
|
|
612 | |
|
|
613 | =item bool ecb_is_pot64 (uint32_t x) |
|
|
614 | |
|
|
615 | Returns true iff C<x> is a power of two or C<x == 0>. |
|
|
616 | |
|
|
617 | For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>. |
|
|
618 | |
|
|
619 | =item int ecb_ld32 (uint32_t x) |
|
|
620 | |
|
|
621 | =item int ecb_ld64 (uint64_t x) |
|
|
622 | |
|
|
623 | Returns the index of the most significant bit set in C<x>, or the number |
|
|
624 | of digits the number requires in binary (so that C<< 2**ld <= x < |
|
|
625 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
|
|
626 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
|
|
627 | example to see how many bits a certain number requires to be encoded. |
|
|
628 | |
|
|
629 | This function is similar to the "count leading zero bits" function, except |
|
|
630 | that that one returns how many zero bits are "in front" of the number (in |
|
|
631 | the given data type), while C<ecb_ld> returns how many bits the number |
|
|
632 | itself requires. |
|
|
633 | |
|
|
634 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
|
|
635 | |
229 | =item int ecb_popcount32 (uint32_t x) |
636 | =item int ecb_popcount32 (uint32_t x) |
230 | |
637 | |
|
|
638 | =item int ecb_popcount64 (uint64_t x) |
|
|
639 | |
|
|
640 | Returns the number of bits set to 1 in C<x>. |
|
|
641 | |
|
|
642 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
|
|
643 | |
|
|
644 | For example: |
|
|
645 | |
|
|
646 | ecb_popcount32 (7) = 3 |
|
|
647 | ecb_popcount32 (255) = 8 |
|
|
648 | |
|
|
649 | =item uint8_t ecb_bitrev8 (uint8_t x) |
|
|
650 | |
|
|
651 | =item uint16_t ecb_bitrev16 (uint16_t x) |
|
|
652 | |
|
|
653 | =item uint32_t ecb_bitrev32 (uint32_t x) |
|
|
654 | |
|
|
655 | Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
|
|
656 | and so on. |
|
|
657 | |
|
|
658 | Example: |
|
|
659 | |
|
|
660 | ecb_bitrev8 (0xa7) = 0xea |
|
|
661 | ecb_bitrev32 (0xffcc4411) = 0x882233ff |
|
|
662 | |
231 | =item uint32_t ecb_bswap16 (uint32_t x) |
663 | =item uint32_t ecb_bswap16 (uint32_t x) |
232 | |
664 | |
233 | =item uint32_t ecb_bswap32 (uint32_t x) |
665 | =item uint32_t ecb_bswap32 (uint32_t x) |
234 | |
666 | |
|
|
667 | =item uint64_t ecb_bswap64 (uint64_t x) |
|
|
668 | |
|
|
669 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
|
|
670 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
|
|
671 | C<ecb_bswap32>). |
|
|
672 | |
|
|
673 | =item T ecb_bswap (T x) [C++] |
|
|
674 | |
|
|
675 | For C++, an additional generic bswap function is provided. It supports |
|
|
676 | C<uint8_t>, C<uint16_t>, C<uint32_t> and C<uint64_t>. |
|
|
677 | |
|
|
678 | =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
|
|
679 | |
|
|
680 | =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
|
|
681 | |
|
|
682 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
|
|
683 | |
|
|
684 | =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
|
|
685 | |
|
|
686 | =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
|
|
687 | |
|
|
688 | =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
|
|
689 | |
235 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
690 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
236 | |
691 | |
237 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
692 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
|
|
693 | |
|
|
694 | These two families of functions return the value of C<x> after rotating |
|
|
695 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
|
|
696 | (C<ecb_rotl>). |
|
|
697 | |
|
|
698 | Current GCC versions understand these functions and usually compile them |
|
|
699 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
|
|
700 | x86). |
|
|
701 | |
|
|
702 | =back |
|
|
703 | |
|
|
704 | =head2 HOST ENDIANNESS CONVERSION |
|
|
705 | |
|
|
706 | =over 4 |
|
|
707 | |
|
|
708 | =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v) |
|
|
709 | |
|
|
710 | =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v) |
|
|
711 | |
|
|
712 | =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v) |
|
|
713 | |
|
|
714 | =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v) |
|
|
715 | |
|
|
716 | =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v) |
|
|
717 | |
|
|
718 | =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v) |
|
|
719 | |
|
|
720 | Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order. |
|
|
721 | |
|
|
722 | The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>, |
|
|
723 | where be and le stand for big endian and little endian, respectively. |
|
|
724 | |
|
|
725 | =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v) |
|
|
726 | |
|
|
727 | =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v) |
|
|
728 | |
|
|
729 | =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v) |
|
|
730 | |
|
|
731 | =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v) |
|
|
732 | |
|
|
733 | =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v) |
|
|
734 | |
|
|
735 | =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v) |
|
|
736 | |
|
|
737 | Like above, but converts I<from> host byte order to the specified |
|
|
738 | endianness. |
|
|
739 | |
|
|
740 | =back |
|
|
741 | |
|
|
742 | In C++ the following additional functions are supported: |
|
|
743 | |
|
|
744 | =over 4 |
|
|
745 | |
|
|
746 | =item T ecb_be_to_host (T v) |
|
|
747 | |
|
|
748 | =item T ecb_le_to_host (T v) |
|
|
749 | |
|
|
750 | =item T ecb_host_to_be (T v) |
|
|
751 | |
|
|
752 | =item T ecb_host_to_le (T v) |
|
|
753 | |
|
|
754 | These work like their C counterparts, above, but use templates for the |
|
|
755 | type, which make them useful in generic code. |
|
|
756 | |
|
|
757 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t> |
|
|
758 | (so unlike their C counterparts, there is a version for C<uint8_t>, which |
|
|
759 | again can be useful in generic code). |
|
|
760 | |
|
|
761 | =head2 UNALIGNED LOAD/STORE |
|
|
762 | |
|
|
763 | These function load or store unaligned multi-byte values. |
|
|
764 | |
|
|
765 | =over 4 |
|
|
766 | |
|
|
767 | =item uint_fast16_t ecb_peek_u16_u (const void *ptr) |
|
|
768 | |
|
|
769 | =item uint_fast32_t ecb_peek_u32_u (const void *ptr) |
|
|
770 | |
|
|
771 | =item uint_fast64_t ecb_peek_u64_u (const void *ptr) |
|
|
772 | |
|
|
773 | These functions load an unaligned, unsigned 16, 32 or 64 bit value from |
|
|
774 | memory. |
|
|
775 | |
|
|
776 | =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr) |
|
|
777 | |
|
|
778 | =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr) |
|
|
779 | |
|
|
780 | =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr) |
|
|
781 | |
|
|
782 | =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr) |
|
|
783 | |
|
|
784 | =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr) |
|
|
785 | |
|
|
786 | =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr) |
|
|
787 | |
|
|
788 | Like above, but additionally convert from big endian (C<be>) or little |
|
|
789 | endian (C<le>) byte order to host byte order while doing so. |
|
|
790 | |
|
|
791 | =item ecb_poke_u16_u (void *ptr, uint16_t v) |
|
|
792 | |
|
|
793 | =item ecb_poke_u32_u (void *ptr, uint32_t v) |
|
|
794 | |
|
|
795 | =item ecb_poke_u64_u (void *ptr, uint64_t v) |
|
|
796 | |
|
|
797 | These functions store an unaligned, unsigned 16, 32 or 64 bit value to |
|
|
798 | memory. |
|
|
799 | |
|
|
800 | =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v) |
|
|
801 | |
|
|
802 | =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v) |
|
|
803 | |
|
|
804 | =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v) |
|
|
805 | |
|
|
806 | =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v) |
|
|
807 | |
|
|
808 | =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v) |
|
|
809 | |
|
|
810 | =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v) |
|
|
811 | |
|
|
812 | Like above, but additionally convert from host byte order to big endian |
|
|
813 | (C<be>) or little endian (C<le>) byte order while doing so. |
|
|
814 | |
|
|
815 | =back |
|
|
816 | |
|
|
817 | In C++ the following additional functions are supported: |
|
|
818 | |
|
|
819 | =over 4 |
|
|
820 | |
|
|
821 | =item T ecb_peek (const void *ptr) |
|
|
822 | |
|
|
823 | =item T ecb_peek_be (const void *ptr) |
|
|
824 | |
|
|
825 | =item T ecb_peek_le (const void *ptr) |
|
|
826 | |
|
|
827 | =item T ecb_peek_u (const void *ptr) |
|
|
828 | |
|
|
829 | =item T ecb_peek_be_u (const void *ptr) |
|
|
830 | |
|
|
831 | =item T ecb_peek_le_u (const void *ptr) |
|
|
832 | |
|
|
833 | Similarly to their C counterparts, these functions load an unsigned 8, 16, |
|
|
834 | 32 or 64 bit value from memory, with optional conversion from big/little |
|
|
835 | endian. |
|
|
836 | |
|
|
837 | Since the type cannot be deduced, it has top be specified explicitly, e.g. |
|
|
838 | |
|
|
839 | uint_fast16_t v = ecb_peek<uint16_t> (ptr); |
|
|
840 | |
|
|
841 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>. |
|
|
842 | |
|
|
843 | Unlike their C counterparts, these functions support 8 bit quantities |
|
|
844 | (C<uint8_t>) and also have an aligned version (without the C<_u> prefix), |
|
|
845 | all of which hopefully makes them more useful in generic code. |
|
|
846 | |
|
|
847 | =item ecb_poke (void *ptr, T v) |
|
|
848 | |
|
|
849 | =item ecb_poke_be (void *ptr, T v) |
|
|
850 | |
|
|
851 | =item ecb_poke_le (void *ptr, T v) |
|
|
852 | |
|
|
853 | =item ecb_poke_u (void *ptr, T v) |
|
|
854 | |
|
|
855 | =item ecb_poke_be_u (void *ptr, T v) |
|
|
856 | |
|
|
857 | =item ecb_poke_le_u (void *ptr, T v) |
|
|
858 | |
|
|
859 | Again, similarly to their C counterparts, these functions store an |
|
|
860 | unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to |
|
|
861 | big/little endian. |
|
|
862 | |
|
|
863 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>. |
|
|
864 | |
|
|
865 | Unlike their C counterparts, these functions support 8 bit quantities |
|
|
866 | (C<uint8_t>) and also have an aligned version (without the C<_u> prefix), |
|
|
867 | all of which hopefully makes them more useful in generic code. |
|
|
868 | |
|
|
869 | =back |
|
|
870 | |
|
|
871 | =head2 FLOATING POINT FIDDLING |
|
|
872 | |
|
|
873 | =over 4 |
|
|
874 | |
|
|
875 | =item ECB_INFINITY [-UECB_NO_LIBM] |
|
|
876 | |
|
|
877 | Evaluates to positive infinity if supported by the platform, otherwise to |
|
|
878 | a truly huge number. |
|
|
879 | |
|
|
880 | =item ECB_NAN [-UECB_NO_LIBM] |
|
|
881 | |
|
|
882 | Evaluates to a quiet NAN if supported by the platform, otherwise to |
|
|
883 | C<ECB_INFINITY>. |
|
|
884 | |
|
|
885 | =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM] |
|
|
886 | |
|
|
887 | Same as C<ldexpf>, but always available. |
|
|
888 | |
|
|
889 | =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM] |
|
|
890 | |
|
|
891 | =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM] |
|
|
892 | |
|
|
893 | =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM] |
|
|
894 | |
|
|
895 | These functions each take an argument in the native C<float> or C<double> |
|
|
896 | type and return the IEEE 754 bit representation of it (binary16/half, |
|
|
897 | binary32/single or binary64/double precision). |
|
|
898 | |
|
|
899 | The bit representation is just as IEEE 754 defines it, i.e. the sign bit |
|
|
900 | will be the most significant bit, followed by exponent and mantissa. |
|
|
901 | |
|
|
902 | This function should work even when the native floating point format isn't |
|
|
903 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
904 | also within reasonable limits (it tries to convert NaNs, infinities and |
|
|
905 | denormals, but will likely convert negative zero to positive zero). |
|
|
906 | |
|
|
907 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
908 | be able to optimise away this function completely. |
|
|
909 | |
|
|
910 | These functions can be helpful when serialising floats to the network - you |
|
|
911 | can serialise the return value like a normal uint16_t/uint32_t/uint64_t. |
|
|
912 | |
|
|
913 | Another use for these functions is to manipulate floating point values |
|
|
914 | directly. |
|
|
915 | |
|
|
916 | Silly example: toggle the sign bit of a float. |
|
|
917 | |
|
|
918 | /* On gcc-4.7 on amd64, */ |
|
|
919 | /* this results in a single add instruction to toggle the bit, and 4 extra */ |
|
|
920 | /* instructions to move the float value to an integer register and back. */ |
|
|
921 | |
|
|
922 | x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U) |
|
|
923 | |
|
|
924 | =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM] |
|
|
925 | |
|
|
926 | =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM] |
|
|
927 | |
|
|
928 | =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM] |
|
|
929 | |
|
|
930 | The reverse operation of the previous function - takes the bit |
|
|
931 | representation of an IEEE binary16, binary32 or binary64 number (half, |
|
|
932 | single or double precision) and converts it to the native C<float> or |
|
|
933 | C<double> format. |
|
|
934 | |
|
|
935 | This function should work even when the native floating point format isn't |
|
|
936 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
937 | also within reasonable limits (it tries to convert normals and denormals, |
|
|
938 | and might be lucky for infinities, and with extraordinary luck, also for |
|
|
939 | negative zero). |
|
|
940 | |
|
|
941 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
942 | be able to optimise away this function completely. |
|
|
943 | |
|
|
944 | =item uint16_t ecb_binary32_to_binary16 (uint32_t x) |
|
|
945 | |
|
|
946 | =item uint32_t ecb_binary16_to_binary32 (uint16_t x) |
|
|
947 | |
|
|
948 | Convert a IEEE binary32/single precision to binary16/half format, and vice |
|
|
949 | versa, handling all details (round-to-nearest-even, subnormals, infinity |
|
|
950 | and NaNs) correctly. |
|
|
951 | |
|
|
952 | These are functions are available under C<-DECB_NO_LIBM>, since |
|
|
953 | they do not rely on the platform floating point format. The |
|
|
954 | C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are |
|
|
955 | usually what you want. |
238 | |
956 | |
239 | =back |
957 | =back |
240 | |
958 | |
241 | =head2 ARITHMETIC |
959 | =head2 ARITHMETIC |
242 | |
960 | |
243 | =over 4 |
961 | =over 4 |
244 | |
962 | |
245 | =item x = ecb_mod (m, n) [MACRO] |
963 | =item x = ecb_mod (m, n) |
|
|
964 | |
|
|
965 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
|
|
966 | of the division operation between C<m> and C<n>, using floored |
|
|
967 | division. Unlike the C remainder operator C<%>, this function ensures that |
|
|
968 | the return value is always positive and that the two numbers I<m> and |
|
|
969 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
|
|
970 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
|
|
971 | in the language. |
|
|
972 | |
|
|
973 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
|
|
974 | negatable, that is, both C<m> and C<-m> must be representable in its |
|
|
975 | type (this typically excludes the minimum signed integer value, the same |
|
|
976 | limitation as for C</> and C<%> in C). |
|
|
977 | |
|
|
978 | Current GCC versions compile this into an efficient branchless sequence on |
|
|
979 | almost all CPUs. |
|
|
980 | |
|
|
981 | For example, when you want to rotate forward through the members of an |
|
|
982 | array for increasing C<m> (which might be negative), then you should use |
|
|
983 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
|
|
984 | change direction for negative values: |
|
|
985 | |
|
|
986 | for (m = -100; m <= 100; ++m) |
|
|
987 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
|
|
988 | |
|
|
989 | =item x = ecb_div_rd (val, div) |
|
|
990 | |
|
|
991 | =item x = ecb_div_ru (val, div) |
|
|
992 | |
|
|
993 | Returns C<val> divided by C<div> rounded down or up, respectively. |
|
|
994 | C<val> and C<div> must have integer types and C<div> must be strictly |
|
|
995 | positive. Note that these functions are implemented with macros in C |
|
|
996 | and with function templates in C++. |
246 | |
997 | |
247 | =back |
998 | =back |
248 | |
999 | |
249 | =head2 UTILITY |
1000 | =head2 UTILITY |
250 | |
1001 | |
251 | =over 4 |
1002 | =over 4 |
252 | |
1003 | |
253 | =item element_count = ecb_array_length (name) [MACRO] |
1004 | =item element_count = ecb_array_length (name) |
254 | |
1005 | |
255 | =back |
1006 | Returns the number of elements in the array C<name>. For example: |
256 | |
1007 | |
|
|
1008 | int primes[] = { 2, 3, 5, 7, 11 }; |
|
|
1009 | int sum = 0; |
257 | |
1010 | |
|
|
1011 | for (i = 0; i < ecb_array_length (primes); i++) |
|
|
1012 | sum += primes [i]; |
|
|
1013 | |
|
|
1014 | =back |
|
|
1015 | |
|
|
1016 | =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
|
|
1017 | |
|
|
1018 | These symbols need to be defined before including F<ecb.h> the first time. |
|
|
1019 | |
|
|
1020 | =over 4 |
|
|
1021 | |
|
|
1022 | =item ECB_NO_THREADS |
|
|
1023 | |
|
|
1024 | If F<ecb.h> is never used from multiple threads, then this symbol can |
|
|
1025 | be defined, in which case memory fences (and similar constructs) are |
|
|
1026 | completely removed, leading to more efficient code and fewer dependencies. |
|
|
1027 | |
|
|
1028 | Setting this symbol to a true value implies C<ECB_NO_SMP>. |
|
|
1029 | |
|
|
1030 | =item ECB_NO_SMP |
|
|
1031 | |
|
|
1032 | The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
|
|
1033 | multiple threads, but never concurrently (e.g. if the system the program |
|
|
1034 | runs on has only a single CPU with a single core, no hyperthreading and so |
|
|
1035 | on), then this symbol can be defined, leading to more efficient code and |
|
|
1036 | fewer dependencies. |
|
|
1037 | |
|
|
1038 | =item ECB_NO_LIBM |
|
|
1039 | |
|
|
1040 | When defined to C<1>, do not export any functions that might introduce |
|
|
1041 | dependencies on the math library (usually called F<-lm>) - these are |
|
|
1042 | marked with [-UECB_NO_LIBM]. |
|
|
1043 | |
|
|
1044 | =back |
|
|
1045 | |
|
|
1046 | =head1 UNDOCUMENTED FUNCTIONALITY |
|
|
1047 | |
|
|
1048 | F<ecb.h> is full of undocumented functionality as well, some of which is |
|
|
1049 | intended to be internal-use only, some of which we forgot to document, and |
|
|
1050 | some of which we hide because we are not sure we will keep the interface |
|
|
1051 | stable. |
|
|
1052 | |
|
|
1053 | While you are welcome to rummage around and use whatever you find useful |
|
|
1054 | (we can't stop you), keep in mind that we will change undocumented |
|
|
1055 | functionality in incompatible ways without thinking twice, while we are |
|
|
1056 | considerably more conservative with documented things. |
|
|
1057 | |
|
|
1058 | =head1 AUTHORS |
|
|
1059 | |
|
|
1060 | C<libecb> is designed and maintained by: |
|
|
1061 | |
|
|
1062 | Emanuele Giaquinta <e.giaquinta@glauco.it> |
|
|
1063 | Marc Alexander Lehmann <schmorp@schmorp.de> |
|
|
1064 | |
|
|
1065 | |