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2 | Frequently Asked Questions about zlib |
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3 | |
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4 | |
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5 | If your question is not there, please check the zlib home page |
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6 | http://zlib.net/ which may have more recent information. |
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7 | The lastest zlib FAQ is at http://zlib.net/zlib_faq.html |
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8 | |
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9 | |
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10 | 1. Is zlib Y2K-compliant? |
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11 | |
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12 | Yes. zlib doesn't handle dates. |
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13 | |
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14 | 2. Where can I get a Windows DLL version? |
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15 | |
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16 | The zlib sources can be compiled without change to produce a DLL. See the |
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17 | file win32/DLL_FAQ.txt in the zlib distribution. Pointers to the |
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18 | precompiled DLL are found in the zlib web site at http://zlib.net/ . |
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19 | |
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20 | 3. Where can I get a Visual Basic interface to zlib? |
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21 | |
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22 | See |
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23 | * http://marknelson.us/1997/01/01/zlib-engine/ |
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24 | * win32/DLL_FAQ.txt in the zlib distribution |
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25 | |
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26 | 4. compress() returns Z_BUF_ERROR. |
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27 | |
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28 | Make sure that before the call of compress(), the length of the compressed |
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29 | buffer is equal to the available size of the compressed buffer and not |
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30 | zero. For Visual Basic, check that this parameter is passed by reference |
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31 | ("as any"), not by value ("as long"). |
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32 | |
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33 | 5. deflate() or inflate() returns Z_BUF_ERROR. |
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34 | |
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35 | Before making the call, make sure that avail_in and avail_out are not zero. |
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36 | When setting the parameter flush equal to Z_FINISH, also make sure that |
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37 | avail_out is big enough to allow processing all pending input. Note that a |
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38 | Z_BUF_ERROR is not fatal--another call to deflate() or inflate() can be |
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39 | made with more input or output space. A Z_BUF_ERROR may in fact be |
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40 | unavoidable depending on how the functions are used, since it is not |
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41 | possible to tell whether or not there is more output pending when |
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42 | strm.avail_out returns with zero. See http://zlib.net/zlib_how.html for a |
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43 | heavily annotated example. |
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44 | |
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45 | 6. Where's the zlib documentation (man pages, etc.)? |
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46 | |
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47 | It's in zlib.h . Examples of zlib usage are in the files example.c and |
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48 | minigzip.c, with more in examples/ . |
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49 | |
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50 | 7. Why don't you use GNU autoconf or libtool or ...? |
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51 | |
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52 | Because we would like to keep zlib as a very small and simple package. |
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53 | zlib is rather portable and doesn't need much configuration. |
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54 | |
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55 | 8. I found a bug in zlib. |
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56 | |
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57 | Most of the time, such problems are due to an incorrect usage of zlib. |
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58 | Please try to reproduce the problem with a small program and send the |
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59 | corresponding source to us at zlib@gzip.org . Do not send multi-megabyte |
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60 | data files without prior agreement. |
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61 | |
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62 | 9. Why do I get "undefined reference to gzputc"? |
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63 | |
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64 | If "make test" produces something like |
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65 | |
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66 | example.o(.text+0x154): undefined reference to `gzputc' |
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67 | |
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68 | check that you don't have old files libz.* in /usr/lib, /usr/local/lib or |
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69 | /usr/X11R6/lib. Remove any old versions, then do "make install". |
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70 | |
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71 | 10. I need a Delphi interface to zlib. |
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72 | |
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73 | See the contrib/delphi directory in the zlib distribution. |
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74 | |
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75 | 11. Can zlib handle .zip archives? |
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76 | |
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77 | Not by itself, no. See the directory contrib/minizip in the zlib |
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78 | distribution. |
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79 | |
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80 | 12. Can zlib handle .Z files? |
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81 | |
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82 | No, sorry. You have to spawn an uncompress or gunzip subprocess, or adapt |
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83 | the code of uncompress on your own. |
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84 | |
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85 | 13. How can I make a Unix shared library? |
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86 | |
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87 | make clean |
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88 | ./configure -s |
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89 | make |
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90 | |
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91 | 14. How do I install a shared zlib library on Unix? |
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92 | |
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93 | After the above, then: |
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94 | |
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95 | make install |
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96 | |
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97 | However, many flavors of Unix come with a shared zlib already installed. |
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98 | Before going to the trouble of compiling a shared version of zlib and |
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99 | trying to install it, you may want to check if it's already there! If you |
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100 | can #include <zlib.h>, it's there. The -lz option will probably link to |
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101 | it. You can check the version at the top of zlib.h or with the |
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102 | ZLIB_VERSION symbol defined in zlib.h . |
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103 | |
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104 | 15. I have a question about OttoPDF. |
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105 | |
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106 | We are not the authors of OttoPDF. The real author is on the OttoPDF web |
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107 | site: Joel Hainley, jhainley@myndkryme.com. |
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108 | |
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109 | 16. Can zlib decode Flate data in an Adobe PDF file? |
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110 | |
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111 | Yes. See http://www.pdflib.com/ . To modify PDF forms, see |
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112 | http://sourceforge.net/projects/acroformtool/ . |
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113 | |
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114 | 17. Why am I getting this "register_frame_info not found" error on Solaris? |
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115 | |
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116 | After installing zlib 1.1.4 on Solaris 2.6, running applications using zlib |
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117 | generates an error such as: |
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118 | |
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119 | ld.so.1: rpm: fatal: relocation error: file /usr/local/lib/libz.so: |
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120 | symbol __register_frame_info: referenced symbol not found |
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121 | |
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122 | The symbol __register_frame_info is not part of zlib, it is generated by |
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123 | the C compiler (cc or gcc). You must recompile applications using zlib |
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124 | which have this problem. This problem is specific to Solaris. See |
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125 | http://www.sunfreeware.com for Solaris versions of zlib and applications |
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126 | using zlib. |
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127 | |
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128 | 18. Why does gzip give an error on a file I make with compress/deflate? |
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129 | |
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130 | The compress and deflate functions produce data in the zlib format, which |
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131 | is different and incompatible with the gzip format. The gz* functions in |
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132 | zlib on the other hand use the gzip format. Both the zlib and gzip formats |
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133 | use the same compressed data format internally, but have different headers |
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134 | and trailers around the compressed data. |
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135 | |
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136 | 19. Ok, so why are there two different formats? |
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137 | |
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138 | The gzip format was designed to retain the directory information about a |
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139 | single file, such as the name and last modification date. The zlib format |
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140 | on the other hand was designed for in-memory and communication channel |
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141 | applications, and has a much more compact header and trailer and uses a |
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142 | faster integrity check than gzip. |
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143 | |
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144 | 20. Well that's nice, but how do I make a gzip file in memory? |
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145 | |
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146 | You can request that deflate write the gzip format instead of the zlib |
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147 | format using deflateInit2(). You can also request that inflate decode the |
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148 | gzip format using inflateInit2(). Read zlib.h for more details. |
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149 | |
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150 | 21. Is zlib thread-safe? |
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151 | |
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152 | Yes. However any library routines that zlib uses and any application- |
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153 | provided memory allocation routines must also be thread-safe. zlib's gz* |
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154 | functions use stdio library routines, and most of zlib's functions use the |
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155 | library memory allocation routines by default. zlib's *Init* functions |
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156 | allow for the application to provide custom memory allocation routines. |
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157 | |
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158 | Of course, you should only operate on any given zlib or gzip stream from a |
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159 | single thread at a time. |
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160 | |
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161 | 22. Can I use zlib in my commercial application? |
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162 | |
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163 | Yes. Please read the license in zlib.h. |
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164 | |
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165 | 23. Is zlib under the GNU license? |
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166 | |
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167 | No. Please read the license in zlib.h. |
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168 | |
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169 | 24. The license says that altered source versions must be "plainly marked". So |
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170 | what exactly do I need to do to meet that requirement? |
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171 | |
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172 | You need to change the ZLIB_VERSION and ZLIB_VERNUM #defines in zlib.h. In |
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173 | particular, the final version number needs to be changed to "f", and an |
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174 | identification string should be appended to ZLIB_VERSION. Version numbers |
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175 | x.x.x.f are reserved for modifications to zlib by others than the zlib |
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176 | maintainers. For example, if the version of the base zlib you are altering |
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177 | is "1.2.3.4", then in zlib.h you should change ZLIB_VERNUM to 0x123f, and |
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178 | ZLIB_VERSION to something like "1.2.3.f-zachary-mods-v3". You can also |
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179 | update the version strings in deflate.c and inftrees.c. |
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180 | |
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181 | For altered source distributions, you should also note the origin and |
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182 | nature of the changes in zlib.h, as well as in ChangeLog and README, along |
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183 | with the dates of the alterations. The origin should include at least your |
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184 | name (or your company's name), and an email address to contact for help or |
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185 | issues with the library. |
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186 | |
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187 | Note that distributing a compiled zlib library along with zlib.h and |
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188 | zconf.h is also a source distribution, and so you should change |
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189 | ZLIB_VERSION and ZLIB_VERNUM and note the origin and nature of the changes |
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190 | in zlib.h as you would for a full source distribution. |
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191 | |
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192 | 25. Will zlib work on a big-endian or little-endian architecture, and can I |
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193 | exchange compressed data between them? |
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194 | |
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195 | Yes and yes. |
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196 | |
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197 | 26. Will zlib work on a 64-bit machine? |
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198 | |
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199 | Yes. It has been tested on 64-bit machines, and has no dependence on any |
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200 | data types being limited to 32-bits in length. If you have any |
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201 | difficulties, please provide a complete problem report to zlib@gzip.org |
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202 | |
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203 | 27. Will zlib decompress data from the PKWare Data Compression Library? |
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204 | |
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205 | No. The PKWare DCL uses a completely different compressed data format than |
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206 | does PKZIP and zlib. However, you can look in zlib's contrib/blast |
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207 | directory for a possible solution to your problem. |
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208 | |
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209 | 28. Can I access data randomly in a compressed stream? |
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210 | |
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211 | No, not without some preparation. If when compressing you periodically use |
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212 | Z_FULL_FLUSH, carefully write all the pending data at those points, and |
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213 | keep an index of those locations, then you can start decompression at those |
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214 | points. You have to be careful to not use Z_FULL_FLUSH too often, since it |
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215 | can significantly degrade compression. Alternatively, you can scan a |
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216 | deflate stream once to generate an index, and then use that index for |
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217 | random access. See examples/zran.c . |
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218 | |
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219 | 29. Does zlib work on MVS, OS/390, CICS, etc.? |
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220 | |
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221 | It has in the past, but we have not heard of any recent evidence. There |
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222 | were working ports of zlib 1.1.4 to MVS, but those links no longer work. |
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223 | If you know of recent, successful applications of zlib on these operating |
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224 | systems, please let us know. Thanks. |
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225 | |
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226 | 30. Is there some simpler, easier to read version of inflate I can look at to |
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227 | understand the deflate format? |
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228 | |
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229 | First off, you should read RFC 1951. Second, yes. Look in zlib's |
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230 | contrib/puff directory. |
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231 | |
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232 | 31. Does zlib infringe on any patents? |
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233 | |
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234 | As far as we know, no. In fact, that was originally the whole point behind |
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235 | zlib. Look here for some more information: |
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236 | |
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237 | http://www.gzip.org/#faq11 |
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238 | |
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239 | 32. Can zlib work with greater than 4 GB of data? |
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240 | |
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241 | Yes. inflate() and deflate() will process any amount of data correctly. |
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242 | Each call of inflate() or deflate() is limited to input and output chunks |
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243 | of the maximum value that can be stored in the compiler's "unsigned int" |
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244 | type, but there is no limit to the number of chunks. Note however that the |
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245 | strm.total_in and strm_total_out counters may be limited to 4 GB. These |
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246 | counters are provided as a convenience and are not used internally by |
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247 | inflate() or deflate(). The application can easily set up its own counters |
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248 | updated after each call of inflate() or deflate() to count beyond 4 GB. |
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249 | compress() and uncompress() may be limited to 4 GB, since they operate in a |
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250 | single call. gzseek() and gztell() may be limited to 4 GB depending on how |
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251 | zlib is compiled. See the zlibCompileFlags() function in zlib.h. |
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252 | |
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253 | The word "may" appears several times above since there is a 4 GB limit only |
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254 | if the compiler's "long" type is 32 bits. If the compiler's "long" type is |
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255 | 64 bits, then the limit is 16 exabytes. |
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256 | |
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257 | 33. Does zlib have any security vulnerabilities? |
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258 | |
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259 | The only one that we are aware of is potentially in gzprintf(). If zlib is |
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260 | compiled to use sprintf() or vsprintf(), then there is no protection |
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261 | against a buffer overflow of an 8K string space (or other value as set by |
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262 | gzbuffer()), other than the caller of gzprintf() assuring that the output |
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263 | will not exceed 8K. On the other hand, if zlib is compiled to use |
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264 | snprintf() or vsnprintf(), which should normally be the case, then there is |
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265 | no vulnerability. The ./configure script will display warnings if an |
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266 | insecure variation of sprintf() will be used by gzprintf(). Also the |
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267 | zlibCompileFlags() function will return information on what variant of |
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268 | sprintf() is used by gzprintf(). |
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269 | |
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270 | If you don't have snprintf() or vsnprintf() and would like one, you can |
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271 | find a portable implementation here: |
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272 | |
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273 | http://www.ijs.si/software/snprintf/ |
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274 | |
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275 | Note that you should be using the most recent version of zlib. Versions |
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276 | 1.1.3 and before were subject to a double-free vulnerability, and versions |
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277 | 1.2.1 and 1.2.2 were subject to an access exception when decompressing |
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278 | invalid compressed data. |
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279 | |
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280 | 34. Is there a Java version of zlib? |
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281 | |
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282 | Probably what you want is to use zlib in Java. zlib is already included |
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283 | as part of the Java SDK in the java.util.zip package. If you really want |
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284 | a version of zlib written in the Java language, look on the zlib home |
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285 | page for links: http://zlib.net/ . |
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286 | |
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287 | 35. I get this or that compiler or source-code scanner warning when I crank it |
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288 | up to maximally-pedantic. Can't you guys write proper code? |
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289 | |
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290 | Many years ago, we gave up attempting to avoid warnings on every compiler |
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291 | in the universe. It just got to be a waste of time, and some compilers |
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292 | were downright silly as well as contradicted each other. So now, we simply |
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293 | make sure that the code always works. |
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294 | |
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295 | 36. Valgrind (or some similar memory access checker) says that deflate is |
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296 | performing a conditional jump that depends on an uninitialized value. |
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297 | Isn't that a bug? |
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298 | |
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299 | No. That is intentional for performance reasons, and the output of deflate |
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300 | is not affected. This only started showing up recently since zlib 1.2.x |
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301 | uses malloc() by default for allocations, whereas earlier versions used |
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302 | calloc(), which zeros out the allocated memory. Even though the code was |
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303 | correct, versions 1.2.4 and later was changed to not stimulate these |
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304 | checkers. |
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305 | |
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306 | 37. Will zlib read the (insert any ancient or arcane format here) compressed |
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307 | data format? |
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308 | |
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309 | Probably not. Look in the comp.compression FAQ for pointers to various |
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310 | formats and associated software. |
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311 | |
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312 | 38. How can I encrypt/decrypt zip files with zlib? |
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313 | |
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314 | zlib doesn't support encryption. The original PKZIP encryption is very |
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315 | weak and can be broken with freely available programs. To get strong |
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316 | encryption, use GnuPG, http://www.gnupg.org/ , which already includes zlib |
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317 | compression. For PKZIP compatible "encryption", look at |
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318 | http://www.info-zip.org/ |
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319 | |
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320 | 39. What's the difference between the "gzip" and "deflate" HTTP 1.1 encodings? |
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321 | |
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322 | "gzip" is the gzip format, and "deflate" is the zlib format. They should |
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323 | probably have called the second one "zlib" instead to avoid confusion with |
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324 | the raw deflate compressed data format. While the HTTP 1.1 RFC 2616 |
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325 | correctly points to the zlib specification in RFC 1950 for the "deflate" |
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326 | transfer encoding, there have been reports of servers and browsers that |
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327 | incorrectly produce or expect raw deflate data per the deflate |
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328 | specficiation in RFC 1951, most notably Microsoft. So even though the |
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329 | "deflate" transfer encoding using the zlib format would be the more |
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330 | efficient approach (and in fact exactly what the zlib format was designed |
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331 | for), using the "gzip" transfer encoding is probably more reliable due to |
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332 | an unfortunate choice of name on the part of the HTTP 1.1 authors. |
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333 | |
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334 | Bottom line: use the gzip format for HTTP 1.1 encoding. |
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335 | |
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336 | 40. Does zlib support the new "Deflate64" format introduced by PKWare? |
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337 | |
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338 | No. PKWare has apparently decided to keep that format proprietary, since |
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339 | they have not documented it as they have previous compression formats. In |
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340 | any case, the compression improvements are so modest compared to other more |
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341 | modern approaches, that it's not worth the effort to implement. |
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342 | |
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343 | 41. I'm having a problem with the zip functions in zlib, can you help? |
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344 | |
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345 | There are no zip functions in zlib. You are probably using minizip by |
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346 | Giles Vollant, which is found in the contrib directory of zlib. It is not |
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347 | part of zlib. In fact none of the stuff in contrib is part of zlib. The |
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348 | files in there are not supported by the zlib authors. You need to contact |
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349 | the authors of the respective contribution for help. |
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350 | |
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351 | 42. The match.asm code in contrib is under the GNU General Public License. |
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352 | Since it's part of zlib, doesn't that mean that all of zlib falls under the |
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353 | GNU GPL? |
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354 | |
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355 | No. The files in contrib are not part of zlib. They were contributed by |
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356 | other authors and are provided as a convenience to the user within the zlib |
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357 | distribution. Each item in contrib has its own license. |
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358 | |
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359 | 43. Is zlib subject to export controls? What is its ECCN? |
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360 | |
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361 | zlib is not subject to export controls, and so is classified as EAR99. |
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362 | |
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363 | 44. Can you please sign these lengthy legal documents and fax them back to us |
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364 | so that we can use your software in our product? |
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365 | |
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366 | No. Go away. Shoo. |
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