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