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6 | |
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7 | Network Working Group P. Deutsch |
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8 | Request for Comments: 1952 Aladdin Enterprises |
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9 | Category: Informational May 1996 |
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10 | |
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11 | |
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12 | GZIP file format specification version 4.3 |
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13 | |
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14 | Status of This Memo |
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15 | |
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16 | This memo provides information for the Internet community. This memo |
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17 | does not specify an Internet standard of any kind. Distribution of |
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18 | this memo is unlimited. |
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19 | |
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20 | IESG Note: |
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21 | |
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22 | The IESG takes no position on the validity of any Intellectual |
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23 | Property Rights statements contained in this document. |
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24 | |
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25 | Notices |
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26 | |
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27 | Copyright (c) 1996 L. Peter Deutsch |
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28 | |
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29 | Permission is granted to copy and distribute this document for any |
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30 | purpose and without charge, including translations into other |
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31 | languages and incorporation into compilations, provided that the |
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32 | copyright notice and this notice are preserved, and that any |
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33 | substantive changes or deletions from the original are clearly |
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34 | marked. |
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35 | |
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36 | A pointer to the latest version of this and related documentation in |
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37 | HTML format can be found at the URL |
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38 | <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. |
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39 | |
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40 | Abstract |
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41 | |
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42 | This specification defines a lossless compressed data format that is |
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43 | compatible with the widely used GZIP utility. The format includes a |
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44 | cyclic redundancy check value for detecting data corruption. The |
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45 | format presently uses the DEFLATE method of compression but can be |
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46 | easily extended to use other compression methods. The format can be |
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47 | implemented readily in a manner not covered by patents. |
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48 | |
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49 | |
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50 | |
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51 | |
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52 | |
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53 | |
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54 | |
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55 | |
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56 | |
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57 | |
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58 | Deutsch Informational [Page 1] |
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59 | |
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60 | RFC 1952 GZIP File Format Specification May 1996 |
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61 | |
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62 | |
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63 | Table of Contents |
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64 | |
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65 | 1. Introduction ................................................... 2 |
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66 | 1.1. Purpose ................................................... 2 |
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67 | 1.2. Intended audience ......................................... 3 |
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68 | 1.3. Scope ..................................................... 3 |
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69 | 1.4. Compliance ................................................ 3 |
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70 | 1.5. Definitions of terms and conventions used ................. 3 |
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71 | 1.6. Changes from previous versions ............................ 3 |
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72 | 2. Detailed specification ......................................... 4 |
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73 | 2.1. Overall conventions ....................................... 4 |
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74 | 2.2. File format ............................................... 5 |
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75 | 2.3. Member format ............................................. 5 |
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76 | 2.3.1. Member header and trailer ........................... 6 |
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77 | 2.3.1.1. Extra field ................................... 8 |
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78 | 2.3.1.2. Compliance .................................... 9 |
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79 | 3. References .................................................. 9 |
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80 | 4. Security Considerations .................................... 10 |
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81 | 5. Acknowledgements ........................................... 10 |
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82 | 6. Author's Address ........................................... 10 |
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83 | 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11 |
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84 | 8. Appendix: Sample CRC Code .................................. 11 |
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85 | |
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86 | 1. Introduction |
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87 | |
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88 | 1.1. Purpose |
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89 | |
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90 | The purpose of this specification is to define a lossless |
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91 | compressed data format that: |
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92 | |
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93 | * Is independent of CPU type, operating system, file system, |
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94 | and character set, and hence can be used for interchange; |
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95 | * Can compress or decompress a data stream (as opposed to a |
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96 | randomly accessible file) to produce another data stream, |
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97 | using only an a priori bounded amount of intermediate |
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98 | storage, and hence can be used in data communications or |
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99 | similar structures such as Unix filters; |
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100 | * Compresses data with efficiency comparable to the best |
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101 | currently available general-purpose compression methods, |
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102 | and in particular considerably better than the "compress" |
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103 | program; |
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104 | * Can be implemented readily in a manner not covered by |
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105 | patents, and hence can be practiced freely; |
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106 | * Is compatible with the file format produced by the current |
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107 | widely used gzip utility, in that conforming decompressors |
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108 | will be able to read data produced by the existing gzip |
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109 | compressor. |
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110 | |
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111 | |
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112 | |
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113 | |
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114 | Deutsch Informational [Page 2] |
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115 | |
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116 | RFC 1952 GZIP File Format Specification May 1996 |
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117 | |
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118 | |
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119 | The data format defined by this specification does not attempt to: |
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120 | |
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121 | * Provide random access to compressed data; |
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122 | * Compress specialized data (e.g., raster graphics) as well as |
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123 | the best currently available specialized algorithms. |
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124 | |
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125 | 1.2. Intended audience |
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126 | |
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127 | This specification is intended for use by implementors of software |
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128 | to compress data into gzip format and/or decompress data from gzip |
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129 | format. |
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130 | |
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131 | The text of the specification assumes a basic background in |
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132 | programming at the level of bits and other primitive data |
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133 | representations. |
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134 | |
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135 | 1.3. Scope |
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136 | |
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137 | The specification specifies a compression method and a file format |
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138 | (the latter assuming only that a file can store a sequence of |
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139 | arbitrary bytes). It does not specify any particular interface to |
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140 | a file system or anything about character sets or encodings |
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141 | (except for file names and comments, which are optional). |
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142 | |
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143 | 1.4. Compliance |
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144 | |
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145 | Unless otherwise indicated below, a compliant decompressor must be |
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146 | able to accept and decompress any file that conforms to all the |
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147 | specifications presented here; a compliant compressor must produce |
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148 | files that conform to all the specifications presented here. The |
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149 | material in the appendices is not part of the specification per se |
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150 | and is not relevant to compliance. |
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151 | |
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152 | 1.5. Definitions of terms and conventions used |
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153 | |
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154 | byte: 8 bits stored or transmitted as a unit (same as an octet). |
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155 | (For this specification, a byte is exactly 8 bits, even on |
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156 | machines which store a character on a number of bits different |
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157 | from 8.) See below for the numbering of bits within a byte. |
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158 | |
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159 | 1.6. Changes from previous versions |
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160 | |
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161 | There have been no technical changes to the gzip format since |
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162 | version 4.1 of this specification. In version 4.2, some |
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163 | terminology was changed, and the sample CRC code was rewritten for |
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164 | clarity and to eliminate the requirement for the caller to do pre- |
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165 | and post-conditioning. Version 4.3 is a conversion of the |
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166 | specification to RFC style. |
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167 | |
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168 | |
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169 | |
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170 | Deutsch Informational [Page 3] |
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171 | |
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172 | RFC 1952 GZIP File Format Specification May 1996 |
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173 | |
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174 | |
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175 | 2. Detailed specification |
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176 | |
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177 | 2.1. Overall conventions |
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178 | |
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179 | In the diagrams below, a box like this: |
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180 | |
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181 | +---+ |
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182 | | | <-- the vertical bars might be missing |
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183 | +---+ |
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184 | |
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185 | represents one byte; a box like this: |
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186 | |
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187 | +==============+ |
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188 | | | |
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189 | +==============+ |
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190 | |
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191 | represents a variable number of bytes. |
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192 | |
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193 | Bytes stored within a computer do not have a "bit order", since |
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194 | they are always treated as a unit. However, a byte considered as |
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195 | an integer between 0 and 255 does have a most- and least- |
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196 | significant bit, and since we write numbers with the most- |
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197 | significant digit on the left, we also write bytes with the most- |
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198 | significant bit on the left. In the diagrams below, we number the |
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199 | bits of a byte so that bit 0 is the least-significant bit, i.e., |
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200 | the bits are numbered: |
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201 | |
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202 | +--------+ |
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203 | |76543210| |
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204 | +--------+ |
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205 | |
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206 | This document does not address the issue of the order in which |
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207 | bits of a byte are transmitted on a bit-sequential medium, since |
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208 | the data format described here is byte- rather than bit-oriented. |
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209 | |
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210 | Within a computer, a number may occupy multiple bytes. All |
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211 | multi-byte numbers in the format described here are stored with |
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212 | the least-significant byte first (at the lower memory address). |
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213 | For example, the decimal number 520 is stored as: |
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214 | |
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215 | 0 1 |
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216 | +--------+--------+ |
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217 | |00001000|00000010| |
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218 | +--------+--------+ |
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219 | ^ ^ |
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220 | | | |
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221 | | + more significant byte = 2 x 256 |
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222 | + less significant byte = 8 |
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223 | |
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224 | |
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225 | |
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226 | Deutsch Informational [Page 4] |
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227 | |
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228 | RFC 1952 GZIP File Format Specification May 1996 |
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229 | |
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230 | |
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231 | 2.2. File format |
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232 | |
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233 | A gzip file consists of a series of "members" (compressed data |
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234 | sets). The format of each member is specified in the following |
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235 | section. The members simply appear one after another in the file, |
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236 | with no additional information before, between, or after them. |
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237 | |
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238 | 2.3. Member format |
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239 | |
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240 | Each member has the following structure: |
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241 | |
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242 | +---+---+---+---+---+---+---+---+---+---+ |
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243 | |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) |
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244 | +---+---+---+---+---+---+---+---+---+---+ |
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245 | |
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246 | (if FLG.FEXTRA set) |
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247 | |
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248 | +---+---+=================================+ |
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249 | | XLEN |...XLEN bytes of "extra field"...| (more-->) |
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250 | +---+---+=================================+ |
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251 | |
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252 | (if FLG.FNAME set) |
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253 | |
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254 | +=========================================+ |
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255 | |...original file name, zero-terminated...| (more-->) |
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256 | +=========================================+ |
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257 | |
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258 | (if FLG.FCOMMENT set) |
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259 | |
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260 | +===================================+ |
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261 | |...file comment, zero-terminated...| (more-->) |
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262 | +===================================+ |
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263 | |
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264 | (if FLG.FHCRC set) |
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265 | |
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266 | +---+---+ |
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267 | | CRC16 | |
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268 | +---+---+ |
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269 | |
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270 | +=======================+ |
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271 | |...compressed blocks...| (more-->) |
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272 | +=======================+ |
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273 | |
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274 | 0 1 2 3 4 5 6 7 |
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275 | +---+---+---+---+---+---+---+---+ |
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276 | | CRC32 | ISIZE | |
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277 | +---+---+---+---+---+---+---+---+ |
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278 | |
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279 | |
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280 | |
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281 | |
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282 | Deutsch Informational [Page 5] |
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283 | |
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284 | RFC 1952 GZIP File Format Specification May 1996 |
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285 | |
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286 | |
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287 | 2.3.1. Member header and trailer |
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288 | |
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289 | ID1 (IDentification 1) |
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290 | ID2 (IDentification 2) |
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291 | These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 |
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292 | (0x8b, \213), to identify the file as being in gzip format. |
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293 | |
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294 | CM (Compression Method) |
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295 | This identifies the compression method used in the file. CM |
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296 | = 0-7 are reserved. CM = 8 denotes the "deflate" |
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297 | compression method, which is the one customarily used by |
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298 | gzip and which is documented elsewhere. |
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299 | |
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300 | FLG (FLaGs) |
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301 | This flag byte is divided into individual bits as follows: |
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302 | |
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303 | bit 0 FTEXT |
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304 | bit 1 FHCRC |
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305 | bit 2 FEXTRA |
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306 | bit 3 FNAME |
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307 | bit 4 FCOMMENT |
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308 | bit 5 reserved |
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309 | bit 6 reserved |
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310 | bit 7 reserved |
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311 | |
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312 | If FTEXT is set, the file is probably ASCII text. This is |
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313 | an optional indication, which the compressor may set by |
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314 | checking a small amount of the input data to see whether any |
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315 | non-ASCII characters are present. In case of doubt, FTEXT |
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316 | is cleared, indicating binary data. For systems which have |
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317 | different file formats for ascii text and binary data, the |
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318 | decompressor can use FTEXT to choose the appropriate format. |
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319 | We deliberately do not specify the algorithm used to set |
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320 | this bit, since a compressor always has the option of |
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321 | leaving it cleared and a decompressor always has the option |
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322 | of ignoring it and letting some other program handle issues |
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323 | of data conversion. |
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324 | |
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325 | If FHCRC is set, a CRC16 for the gzip header is present, |
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326 | immediately before the compressed data. The CRC16 consists |
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327 | of the two least significant bytes of the CRC32 for all |
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328 | bytes of the gzip header up to and not including the CRC16. |
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329 | [The FHCRC bit was never set by versions of gzip up to |
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330 | 1.2.4, even though it was documented with a different |
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331 | meaning in gzip 1.2.4.] |
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332 | |
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333 | If FEXTRA is set, optional extra fields are present, as |
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334 | described in a following section. |
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335 | |
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336 | |
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337 | |
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338 | Deutsch Informational [Page 6] |
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339 | |
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340 | RFC 1952 GZIP File Format Specification May 1996 |
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341 | |
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342 | |
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343 | If FNAME is set, an original file name is present, |
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344 | terminated by a zero byte. The name must consist of ISO |
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345 | 8859-1 (LATIN-1) characters; on operating systems using |
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346 | EBCDIC or any other character set for file names, the name |
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347 | must be translated to the ISO LATIN-1 character set. This |
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348 | is the original name of the file being compressed, with any |
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349 | directory components removed, and, if the file being |
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350 | compressed is on a file system with case insensitive names, |
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351 | forced to lower case. There is no original file name if the |
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352 | data was compressed from a source other than a named file; |
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353 | for example, if the source was stdin on a Unix system, there |
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354 | is no file name. |
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355 | |
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356 | If FCOMMENT is set, a zero-terminated file comment is |
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357 | present. This comment is not interpreted; it is only |
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358 | intended for human consumption. The comment must consist of |
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359 | ISO 8859-1 (LATIN-1) characters. Line breaks should be |
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360 | denoted by a single line feed character (10 decimal). |
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361 | |
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362 | Reserved FLG bits must be zero. |
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363 | |
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364 | MTIME (Modification TIME) |
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365 | This gives the most recent modification time of the original |
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366 | file being compressed. The time is in Unix format, i.e., |
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367 | seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this |
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368 | may cause problems for MS-DOS and other systems that use |
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369 | local rather than Universal time.) If the compressed data |
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370 | did not come from a file, MTIME is set to the time at which |
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371 | compression started. MTIME = 0 means no time stamp is |
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372 | available. |
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373 | |
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374 | XFL (eXtra FLags) |
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375 | These flags are available for use by specific compression |
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376 | methods. The "deflate" method (CM = 8) sets these flags as |
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377 | follows: |
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378 | |
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379 | XFL = 2 - compressor used maximum compression, |
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380 | slowest algorithm |
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381 | XFL = 4 - compressor used fastest algorithm |
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382 | |
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383 | OS (Operating System) |
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384 | This identifies the type of file system on which compression |
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385 | took place. This may be useful in determining end-of-line |
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386 | convention for text files. The currently defined values are |
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387 | as follows: |
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388 | |
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389 | |
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390 | |
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391 | |
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392 | |
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393 | |
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394 | Deutsch Informational [Page 7] |
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395 | |
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396 | RFC 1952 GZIP File Format Specification May 1996 |
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397 | |
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398 | |
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399 | 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32) |
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400 | 1 - Amiga |
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401 | 2 - VMS (or OpenVMS) |
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402 | 3 - Unix |
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403 | 4 - VM/CMS |
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404 | 5 - Atari TOS |
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405 | 6 - HPFS filesystem (OS/2, NT) |
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406 | 7 - Macintosh |
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407 | 8 - Z-System |
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408 | 9 - CP/M |
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409 | 10 - TOPS-20 |
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410 | 11 - NTFS filesystem (NT) |
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411 | 12 - QDOS |
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412 | 13 - Acorn RISCOS |
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413 | 255 - unknown |
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414 | |
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415 | XLEN (eXtra LENgth) |
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416 | If FLG.FEXTRA is set, this gives the length of the optional |
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417 | extra field. See below for details. |
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418 | |
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419 | CRC32 (CRC-32) |
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420 | This contains a Cyclic Redundancy Check value of the |
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421 | uncompressed data computed according to CRC-32 algorithm |
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422 | used in the ISO 3309 standard and in section 8.1.1.6.2 of |
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423 | ITU-T recommendation V.42. (See http://www.iso.ch for |
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424 | ordering ISO documents. See gopher://info.itu.ch for an |
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425 | online version of ITU-T V.42.) |
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426 | |
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427 | ISIZE (Input SIZE) |
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428 | This contains the size of the original (uncompressed) input |
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429 | data modulo 2^32. |
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430 | |
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431 | 2.3.1.1. Extra field |
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432 | |
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433 | If the FLG.FEXTRA bit is set, an "extra field" is present in |
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434 | the header, with total length XLEN bytes. It consists of a |
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435 | series of subfields, each of the form: |
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436 | |
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437 | +---+---+---+---+==================================+ |
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438 | |SI1|SI2| LEN |... LEN bytes of subfield data ...| |
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439 | +---+---+---+---+==================================+ |
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440 | |
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441 | SI1 and SI2 provide a subfield ID, typically two ASCII letters |
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442 | with some mnemonic value. Jean-Loup Gailly |
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443 | <gzip@prep.ai.mit.edu> is maintaining a registry of subfield |
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444 | IDs; please send him any subfield ID you wish to use. Subfield |
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445 | IDs with SI2 = 0 are reserved for future use. The following |
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446 | IDs are currently defined: |
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447 | |
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448 | |
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449 | |
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450 | Deutsch Informational [Page 8] |
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451 | |
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452 | RFC 1952 GZIP File Format Specification May 1996 |
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453 | |
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454 | |
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455 | SI1 SI2 Data |
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456 | ---------- ---------- ---- |
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457 | 0x41 ('A') 0x70 ('P') Apollo file type information |
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458 | |
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459 | LEN gives the length of the subfield data, excluding the 4 |
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460 | initial bytes. |
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461 | |
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462 | 2.3.1.2. Compliance |
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463 | |
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464 | A compliant compressor must produce files with correct ID1, |
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465 | ID2, CM, CRC32, and ISIZE, but may set all the other fields in |
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466 | the fixed-length part of the header to default values (255 for |
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467 | OS, 0 for all others). The compressor must set all reserved |
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468 | bits to zero. |
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469 | |
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470 | A compliant decompressor must check ID1, ID2, and CM, and |
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471 | provide an error indication if any of these have incorrect |
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472 | values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC |
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473 | at least so it can skip over the optional fields if they are |
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474 | present. It need not examine any other part of the header or |
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475 | trailer; in particular, a decompressor may ignore FTEXT and OS |
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476 | and always produce binary output, and still be compliant. A |
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477 | compliant decompressor must give an error indication if any |
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478 | reserved bit is non-zero, since such a bit could indicate the |
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479 | presence of a new field that would cause subsequent data to be |
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480 | interpreted incorrectly. |
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481 | |
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482 | 3. References |
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483 | |
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484 | [1] "Information Processing - 8-bit single-byte coded graphic |
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485 | character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). |
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486 | The ISO 8859-1 (Latin-1) character set is a superset of 7-bit |
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487 | ASCII. Files defining this character set are available as |
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488 | iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/ |
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489 | |
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490 | [2] ISO 3309 |
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491 | |
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492 | [3] ITU-T recommendation V.42 |
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493 | |
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494 | [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", |
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495 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
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496 | |
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497 | [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in |
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498 | ftp://prep.ai.mit.edu/pub/gnu/ |
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499 | |
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500 | [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table |
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501 | Look-Up", Communications of the ACM, 31(8), pp.1008-1013. |
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502 | |
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503 | |
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504 | |
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505 | |
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506 | Deutsch Informational [Page 9] |
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507 | |
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508 | RFC 1952 GZIP File Format Specification May 1996 |
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509 | |
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510 | |
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511 | [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, |
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512 | pp.118-133. |
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513 | |
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514 | [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt, |
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515 | describing the CRC concept. |
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516 | |
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517 | 4. Security Considerations |
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518 | |
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519 | Any data compression method involves the reduction of redundancy in |
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520 | the data. Consequently, any corruption of the data is likely to have |
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521 | severe effects and be difficult to correct. Uncompressed text, on |
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522 | the other hand, will probably still be readable despite the presence |
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523 | of some corrupted bytes. |
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524 | |
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525 | It is recommended that systems using this data format provide some |
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526 | means of validating the integrity of the compressed data, such as by |
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527 | setting and checking the CRC-32 check value. |
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528 | |
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529 | 5. Acknowledgements |
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530 | |
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531 | Trademarks cited in this document are the property of their |
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532 | respective owners. |
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533 | |
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534 | Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler, |
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535 | the related software described in this specification. Glenn |
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536 | Randers-Pehrson converted this document to RFC and HTML format. |
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537 | |
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538 | 6. Author's Address |
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539 | |
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540 | L. Peter Deutsch |
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541 | Aladdin Enterprises |
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542 | 203 Santa Margarita Ave. |
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543 | Menlo Park, CA 94025 |
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544 | |
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545 | Phone: (415) 322-0103 (AM only) |
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546 | FAX: (415) 322-1734 |
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547 | EMail: <ghost@aladdin.com> |
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548 | |
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549 | Questions about the technical content of this specification can be |
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550 | sent by email to: |
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551 | |
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552 | Jean-Loup Gailly <gzip@prep.ai.mit.edu> and |
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553 | Mark Adler <madler@alumni.caltech.edu> |
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554 | |
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555 | Editorial comments on this specification can be sent by email to: |
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556 | |
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557 | L. Peter Deutsch <ghost@aladdin.com> and |
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558 | Glenn Randers-Pehrson <randeg@alumni.rpi.edu> |
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559 | |
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560 | |
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561 | |
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562 | Deutsch Informational [Page 10] |
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563 | |
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564 | RFC 1952 GZIP File Format Specification May 1996 |
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565 | |
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566 | |
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567 | 7. Appendix: Jean-Loup Gailly's gzip utility |
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568 | |
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569 | The most widely used implementation of gzip compression, and the |
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570 | original documentation on which this specification is based, were |
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571 | created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this |
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572 | implementation is a de facto standard, we mention some more of its |
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573 | features here. Again, the material in this section is not part of |
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574 | the specification per se, and implementations need not follow it to |
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575 | be compliant. |
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576 | |
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577 | When compressing or decompressing a file, gzip preserves the |
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578 | protection, ownership, and modification time attributes on the local |
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579 | file system, since there is no provision for representing protection |
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580 | attributes in the gzip file format itself. Since the file format |
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581 | includes a modification time, the gzip decompressor provides a |
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582 | command line switch that assigns the modification time from the file, |
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583 | rather than the local modification time of the compressed input, to |
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584 | the decompressed output. |
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585 | |
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586 | 8. Appendix: Sample CRC Code |
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587 | |
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588 | The following sample code represents a practical implementation of |
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589 | the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42 |
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590 | for a formal specification.) |
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591 | |
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592 | The sample code is in the ANSI C programming language. Non C users |
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593 | may find it easier to read with these hints: |
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594 | |
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595 | & Bitwise AND operator. |
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596 | ^ Bitwise exclusive-OR operator. |
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597 | >> Bitwise right shift operator. When applied to an |
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598 | unsigned quantity, as here, right shift inserts zero |
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599 | bit(s) at the left. |
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600 | ! Logical NOT operator. |
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601 | ++ "n++" increments the variable n. |
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602 | 0xNNN 0x introduces a hexadecimal (base 16) constant. |
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603 | Suffix L indicates a long value (at least 32 bits). |
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604 | |
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605 | /* Table of CRCs of all 8-bit messages. */ |
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606 | unsigned long crc_table[256]; |
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607 | |
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608 | /* Flag: has the table been computed? Initially false. */ |
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609 | int crc_table_computed = 0; |
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610 | |
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611 | /* Make the table for a fast CRC. */ |
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612 | void make_crc_table(void) |
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613 | { |
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614 | unsigned long c; |
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615 | |
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616 | |
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617 | |
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618 | Deutsch Informational [Page 11] |
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619 | |
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620 | RFC 1952 GZIP File Format Specification May 1996 |
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621 | |
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622 | |
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623 | int n, k; |
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624 | for (n = 0; n < 256; n++) { |
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625 | c = (unsigned long) n; |
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626 | for (k = 0; k < 8; k++) { |
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627 | if (c & 1) { |
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628 | c = 0xedb88320L ^ (c >> 1); |
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629 | } else { |
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630 | c = c >> 1; |
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631 | } |
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632 | } |
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633 | crc_table[n] = c; |
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634 | } |
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635 | crc_table_computed = 1; |
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636 | } |
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637 | |
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638 | /* |
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639 | Update a running crc with the bytes buf[0..len-1] and return |
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640 | the updated crc. The crc should be initialized to zero. Pre- and |
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641 | post-conditioning (one's complement) is performed within this |
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642 | function so it shouldn't be done by the caller. Usage example: |
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643 | |
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644 | unsigned long crc = 0L; |
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645 | |
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646 | while (read_buffer(buffer, length) != EOF) { |
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647 | crc = update_crc(crc, buffer, length); |
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648 | } |
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649 | if (crc != original_crc) error(); |
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650 | */ |
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651 | unsigned long update_crc(unsigned long crc, |
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652 | unsigned char *buf, int len) |
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653 | { |
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654 | unsigned long c = crc ^ 0xffffffffL; |
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655 | int n; |
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656 | |
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657 | if (!crc_table_computed) |
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658 | make_crc_table(); |
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659 | for (n = 0; n < len; n++) { |
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660 | c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8); |
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661 | } |
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662 | return c ^ 0xffffffffL; |
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663 | } |
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664 | |
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665 | /* Return the CRC of the bytes buf[0..len-1]. */ |
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666 | unsigned long crc(unsigned char *buf, int len) |
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667 | { |
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668 | return update_crc(0L, buf, len); |
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669 | } |
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670 | |
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671 | |
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672 | |
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673 | |
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674 | Deutsch Informational [Page 12] |
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675 | |
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