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6 | <title>zlib Usage Example</title> |
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7 | <!-- Copyright (c) 2004 Mark Adler. --> |
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9 | <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#00A000"> |
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10 | <h2 align="center"> zlib Usage Example </h2> |
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11 | We often get questions about how the <tt>deflate()</tt> and <tt>inflate()</tt> functions should be used. |
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12 | Users wonder when they should provide more input, when they should use more output, |
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13 | what to do with a <tt>Z_BUF_ERROR</tt>, how to make sure the process terminates properly, and |
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14 | so on. So for those who have read <tt>zlib.h</tt> (a few times), and |
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15 | would like further edification, below is an annotated example in C of simple routines to compress and decompress |
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16 | from an input file to an output file using <tt>deflate()</tt> and <tt>inflate()</tt> respectively. The |
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17 | annotations are interspersed between lines of the code. So please read between the lines. |
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18 | We hope this helps explain some of the intricacies of <em>zlib</em>. |
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19 | <p> |
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20 | Without further adieu, here is the program <a href="zpipe.c"><tt>zpipe.c</tt></a>: |
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21 | <pre><b> |
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22 | /* zpipe.c: example of proper use of zlib's inflate() and deflate() |
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23 | Not copyrighted -- provided to the public domain |
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24 | Version 1.2 9 November 2004 Mark Adler */ |
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25 | |
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26 | /* Version history: |
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27 | 1.0 30 Oct 2004 First version |
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28 | 1.1 8 Nov 2004 Add void casting for unused return values |
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29 | Use switch statement for inflate() return values |
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30 | 1.2 9 Nov 2004 Add assertions to document zlib guarantees |
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31 | */ |
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32 | </b></pre><!-- --> |
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33 | We now include the header files for the required definitions. From |
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34 | <tt>stdio.h</tt> we use <tt>fopen()</tt>, <tt>fread()</tt>, <tt>fwrite()</tt>, |
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35 | <tt>feof()</tt>, <tt>ferror()</tt>, and <tt>fclose()</tt> for file i/o, and |
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36 | <tt>fputs()</tt> for error messages. From <tt>string.h</tt> we use |
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37 | <tt>strcmp()</tt> for command line argument processing. |
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38 | From <tt>assert.h</tt> we use the <tt>assert()</tt> macro. |
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39 | From <tt>zlib.h</tt> |
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40 | we use the basic compression functions <tt>deflateInit()</tt>, |
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41 | <tt>deflate()</tt>, and <tt>deflateEnd()</tt>, and the basic decompression |
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42 | functions <tt>inflateInit()</tt>, <tt>inflate()</tt>, and |
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43 | <tt>inflateEnd()</tt>. |
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44 | <pre><b> |
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45 | #include <stdio.h> |
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46 | #include <string.h> |
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47 | #include <assert.h> |
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48 | #include "zlib.h" |
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49 | </b></pre><!-- --> |
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50 | <tt>CHUNK</tt> is simply the buffer size for feeding data to and pulling data |
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51 | from the <em>zlib</em> routines. Larger buffer sizes would be more efficient, |
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52 | especially for <tt>inflate()</tt>. If the memory is available, buffers sizes |
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53 | on the order of 128K or 256K bytes should be used. |
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54 | <pre><b> |
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55 | #define CHUNK 16384 |
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56 | </b></pre><!-- --> |
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57 | The <tt>def()</tt> routine compresses data from an input file to an output file. The output data |
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58 | will be in the <em>zlib</em> format, which is different from the <em>gzip</em> or <em>zip</em> |
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59 | formats. The <em>zlib</em> format has a very small header of only two bytes to identify it as |
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60 | a <em>zlib</em> stream and to provide decoding information, and a four-byte trailer with a fast |
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61 | check value to verify the integrity of the uncompressed data after decoding. |
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62 | <pre><b> |
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63 | /* Compress from file source to file dest until EOF on source. |
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64 | def() returns Z_OK on success, Z_MEM_ERROR if memory could not be |
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65 | allocated for processing, Z_STREAM_ERROR if an invalid compression |
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66 | level is supplied, Z_VERSION_ERROR if the version of zlib.h and the |
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67 | version of the library linked do not match, or Z_ERRNO if there is |
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68 | an error reading or writing the files. */ |
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69 | int def(FILE *source, FILE *dest, int level) |
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70 | { |
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71 | </b></pre> |
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72 | Here are the local variables for <tt>def()</tt>. <tt>ret</tt> will be used for <em>zlib</em> |
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73 | return codes. <tt>flush</tt> will keep track of the current flushing state for <tt>deflate()</tt>, |
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74 | which is either no flushing, or flush to completion after the end of the input file is reached. |
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75 | <tt>have</tt> is the amount of data returned from <tt>deflate()</tt>. The <tt>strm</tt> structure |
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76 | is used to pass information to and from the <em>zlib</em> routines, and to maintain the |
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77 | <tt>deflate()</tt> state. <tt>in</tt> and <tt>out</tt> are the input and output buffers for |
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78 | <tt>deflate()</tt>. |
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79 | <pre><b> |
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80 | int ret, flush; |
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81 | unsigned have; |
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82 | z_stream strm; |
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83 | char in[CHUNK]; |
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84 | char out[CHUNK]; |
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85 | </b></pre><!-- --> |
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86 | The first thing we do is to initialize the <em>zlib</em> state for compression using |
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87 | <tt>deflateInit()</tt>. This must be done before the first use of <tt>deflate()</tt>. |
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88 | The <tt>zalloc</tt>, <tt>zfree</tt>, and <tt>opaque</tt> fields in the <tt>strm</tt> |
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89 | structure must be initialized before calling <tt>deflateInit()</tt>. Here they are |
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90 | set to the <em>zlib</em> constant <tt>Z_NULL</tt> to request that <em>zlib</em> use |
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91 | the default memory allocation routines. An application may also choose to provide |
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92 | custom memory allocation routines here. <tt>deflateInit()</tt> will allocate on the |
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93 | order of 256K bytes for the internal state. |
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94 | (See <a href="zlib_tech.html"><em>zlib Technical Details</em></a>.) |
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95 | <p> |
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96 | <tt>deflateInit()</tt> is called with a pointer to the structure to be initialized and |
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97 | the compression level, which is an integer in the range of -1 to 9. Lower compression |
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98 | levels result in faster execution, but less compression. Higher levels result in |
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99 | greater compression, but slower execution. The <em>zlib</em> constant Z_DEFAULT_COMPRESSION, |
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100 | equal to -1, |
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101 | provides a good compromise between compression and speed and is equivalent to level 6. |
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102 | Level 0 actually does no compression at all, and in fact expands the data slightly to produce |
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103 | the <em>zlib</em> format (it is not a byte-for-byte copy of the input). |
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104 | More advanced applications of <em>zlib</em> |
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105 | may use <tt>deflateInit2()</tt> here instead. Such an application may want to reduce how |
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106 | much memory will be used, at some price in compression. Or it may need to request a |
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107 | <em>gzip</em> header and trailer instead of a <em>zlib</em> header and trailer, or raw |
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108 | encoding with no header or trailer at all. |
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109 | <p> |
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110 | We must check the return value of <tt>deflateInit()</tt> against the <em>zlib</em> constant |
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111 | <tt>Z_OK</tt> to make sure that it was able to |
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112 | allocate memory for the internal state, and that the provided arguments were valid. |
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113 | <tt>deflateInit()</tt> will also check that the version of <em>zlib</em> that the <tt>zlib.h</tt> |
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114 | file came from matches the version of <em>zlib</em> actually linked with the program. This |
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115 | is especially important for environments in which <em>zlib</em> is a shared library. |
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116 | <p> |
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117 | Note that an application can initialize multiple, independent <em>zlib</em> streams, which can |
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118 | operate in parallel. The state information maintained in the structure allows the <em>zlib</em> |
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119 | routines to be reentrant. |
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120 | <pre><b> |
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121 | /* allocate deflate state */ |
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122 | strm.zalloc = Z_NULL; |
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123 | strm.zfree = Z_NULL; |
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124 | strm.opaque = Z_NULL; |
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125 | ret = deflateInit(&strm, level); |
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126 | if (ret != Z_OK) |
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127 | return ret; |
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128 | </b></pre><!-- --> |
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129 | With the pleasantries out of the way, now we can get down to business. The outer <tt>do</tt>-loop |
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130 | reads all of the input file and exits at the bottom of the loop once end-of-file is reached. |
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131 | This loop contains the only call of <tt>deflate()</tt>. So we must make sure that all of the |
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132 | input data has been processed and that all of the output data has been generated and consumed |
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133 | before we fall out of the loop at the bottom. |
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134 | <pre><b> |
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135 | /* compress until end of file */ |
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136 | do { |
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137 | </b></pre> |
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138 | We start off by reading data from the input file. The number of bytes read is put directly |
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139 | into <tt>avail_in</tt>, and a pointer to those bytes is put into <tt>next_in</tt>. We also |
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140 | check to see if end-of-file on the input has been reached. If we are at the end of file, then <tt>flush</tt> is set to the |
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141 | <em>zlib</em> constant <tt>Z_FINISH</tt>, which is later passed to <tt>deflate()</tt> to |
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142 | indicate that this is the last chunk of input data to compress. We need to use <tt>feof()</tt> |
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143 | to check for end-of-file as opposed to seeing if fewer than <tt>CHUNK</tt> bytes have been read. The |
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144 | reason is that if the input file length is an exact multiple of <tt>CHUNK</tt>, we will miss |
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145 | the fact that we got to the end-of-file, and not know to tell <tt>deflate()</tt> to finish |
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146 | up the compressed stream. If we are not yet at the end of the input, then the <em>zlib</em> |
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147 | constant <tt>Z_NO_FLUSH</tt> will be passed to <tt>deflate</tt> to indicate that we are still |
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148 | in the middle of the uncompressed data. |
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149 | <p> |
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150 | If there is an error in reading from the input file, the process is aborted with |
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151 | <tt>deflateEnd()</tt> being called to free the allocated <em>zlib</em> state before returning |
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152 | the error. We wouldn't want a memory leak, now would we? <tt>deflateEnd()</tt> can be called |
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153 | at any time after the state has been initialized. Once that's done, <tt>deflateInit()</tt> (or |
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154 | <tt>deflateInit2()</tt>) would have to be called to start a new compression process. There is |
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155 | no point here in checking the <tt>deflateEnd()</tt> return code. The deallocation can't fail. |
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156 | <pre><b> |
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157 | strm.avail_in = fread(in, 1, CHUNK, source); |
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158 | if (ferror(source)) { |
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159 | (void)deflateEnd(&strm); |
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160 | return Z_ERRNO; |
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161 | } |
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162 | flush = feof(source) ? Z_FINISH : Z_NO_FLUSH; |
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163 | strm.next_in = in; |
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164 | </b></pre><!-- --> |
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165 | The inner <tt>do</tt>-loop passes our chunk of input data to <tt>deflate()</tt>, and then |
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166 | keeps calling <tt>deflate()</tt> until it is done producing output. Once there is no more |
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167 | new output, <tt>deflate()</tt> is guaranteed to have consumed all of the input, i.e., |
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168 | <tt>avail_in</tt> will be zero. |
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169 | <pre><b> |
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170 | /* run deflate() on input until output buffer not full, finish |
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171 | compression if all of source has been read in */ |
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172 | do { |
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173 | </b></pre> |
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174 | Output space is provided to <tt>deflate()</tt> by setting <tt>avail_out</tt> to the number |
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175 | of available output bytes and <tt>next_out</tt> to a pointer to that space. |
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176 | <pre><b> |
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177 | strm.avail_out = CHUNK; |
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178 | strm.next_out = out; |
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179 | </b></pre> |
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180 | Now we call the compression engine itself, <tt>deflate()</tt>. It takes as many of the |
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181 | <tt>avail_in</tt> bytes at <tt>next_in</tt> as it can process, and writes as many as |
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182 | <tt>avail_out</tt> bytes to <tt>next_out</tt>. Those counters and pointers are then |
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183 | updated past the input data consumed and the output data written. It is the amount of |
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184 | output space available that may limit how much input is consumed. |
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185 | Hence the inner loop to make sure that |
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186 | all of the input is consumed by providing more output space each time. Since <tt>avail_in</tt> |
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187 | and <tt>next_in</tt> are updated by <tt>deflate()</tt>, we don't have to mess with those |
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188 | between <tt>deflate()</tt> calls until it's all used up. |
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189 | <p> |
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190 | The parameters to <tt>deflate()</tt> are a pointer to the <tt>strm</tt> structure containing |
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191 | the input and output information and the internal compression engine state, and a parameter |
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192 | indicating whether and how to flush data to the output. Normally <tt>deflate</tt> will consume |
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193 | several K bytes of input data before producing any output (except for the header), in order |
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194 | to accumulate statistics on the data for optimum compression. It will then put out a burst of |
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195 | compressed data, and proceed to consume more input before the next burst. Eventually, |
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196 | <tt>deflate()</tt> |
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197 | must be told to terminate the stream, complete the compression with provided input data, and |
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198 | write out the trailer check value. <tt>deflate()</tt> will continue to compress normally as long |
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199 | as the flush parameter is <tt>Z_NO_FLUSH</tt>. Once the <tt>Z_FINISH</tt> parameter is provided, |
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200 | <tt>deflate()</tt> will begin to complete the compressed output stream. However depending on how |
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201 | much output space is provided, <tt>deflate()</tt> may have to be called several times until it |
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202 | has provided the complete compressed stream, even after it has consumed all of the input. The flush |
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203 | parameter must continue to be <tt>Z_FINISH</tt> for those subsequent calls. |
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204 | <p> |
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205 | There are other values of the flush parameter that are used in more advanced applications. You can |
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206 | force <tt>deflate()</tt> to produce a burst of output that encodes all of the input data provided |
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207 | so far, even if it wouldn't have otherwise, for example to control data latency on a link with |
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208 | compressed data. You can also ask that <tt>deflate()</tt> do that as well as erase any history up to |
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209 | that point so that what follows can be decompressed independently, for example for random access |
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210 | applications. Both requests will degrade compression by an amount depending on how often such |
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211 | requests are made. |
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212 | <p> |
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213 | <tt>deflate()</tt> has a return value that can indicate errors, yet we do not check it here. Why |
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214 | not? Well, it turns out that <tt>deflate()</tt> can do no wrong here. Let's go through |
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215 | <tt>deflate()</tt>'s return values and dispense with them one by one. The possible values are |
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216 | <tt>Z_OK</tt>, <tt>Z_STREAM_END</tt>, <tt>Z_STREAM_ERROR</tt>, or <tt>Z_BUF_ERROR</tt>. <tt>Z_OK</tt> |
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217 | is, well, ok. <tt>Z_STREAM_END</tt> is also ok and will be returned for the last call of |
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218 | <tt>deflate()</tt>. This is already guaranteed by calling <tt>deflate()</tt> with <tt>Z_FINISH</tt> |
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219 | until it has no more output. <tt>Z_STREAM_ERROR</tt> is only possible if the stream is not |
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220 | initialized properly, but we did initialize it properly. There is no harm in checking for |
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221 | <tt>Z_STREAM_ERROR</tt> here, for example to check for the possibility that some |
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222 | other part of the application inadvertently clobbered the memory containing the <em>zlib</em> state. |
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223 | <tt>Z_BUF_ERROR</tt> will be explained further below, but |
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224 | suffice it to say that this is simply an indication that <tt>deflate()</tt> could not consume |
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225 | more input or produce more output. <tt>deflate()</tt> can be called again with more output space |
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226 | or more available input, which it will be in this code. |
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227 | <pre><b> |
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228 | ret = deflate(&strm, flush); /* no bad return value */ |
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229 | assert(ret != Z_STREAM_ERROR); /* state not clobbered */ |
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230 | </b></pre> |
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231 | Now we compute how much output <tt>deflate()</tt> provided on the last call, which is the |
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232 | difference between how much space was provided before the call, and how much output space |
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233 | is still available after the call. Then that data, if any, is written to the output file. |
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234 | We can then reuse the output buffer for the next call of <tt>deflate()</tt>. Again if there |
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235 | is a file i/o error, we call <tt>deflateEnd()</tt> before returning to avoid a memory leak. |
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236 | <pre><b> |
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237 | have = CHUNK - strm.avail_out; |
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238 | if (fwrite(out, 1, have, dest) != have || ferror(dest)) { |
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239 | (void)deflateEnd(&strm); |
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240 | return Z_ERRNO; |
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241 | } |
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242 | </b></pre> |
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243 | The inner <tt>do</tt>-loop is repeated until the last <tt>deflate()</tt> call fails to fill the |
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244 | provided output buffer. Then we know that <tt>deflate()</tt> has done as much as it can with |
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245 | the provided input, and that all of that input has been consumed. We can then fall out of this |
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246 | loop and reuse the input buffer. |
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247 | <p> |
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248 | The way we tell that <tt>deflate()</tt> has no more output is by seeing that it did not fill |
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249 | the output buffer, leaving <tt>avail_out</tt> greater than zero. However suppose that |
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250 | <tt>deflate()</tt> has no more output, but just so happened to exactly fill the output buffer! |
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251 | <tt>avail_out</tt> is zero, and we can't tell that <tt>deflate()</tt> has done all it can. |
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252 | As far as we know, <tt>deflate()</tt> |
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253 | has more output for us. So we call it again. But now <tt>deflate()</tt> produces no output |
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254 | at all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call |
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255 | wasn't able to do anything, either consume input or produce output, and so it returns |
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256 | <tt>Z_BUF_ERROR</tt>. (See, I told you I'd cover this later.) However this is not a problem at |
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257 | all. Now we finally have the desired indication that <tt>deflate()</tt> is really done, |
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258 | and so we drop out of the inner loop to provide more input to <tt>deflate()</tt>. |
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259 | <p> |
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260 | With <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will |
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261 | complete the output stream. Once that is done, subsequent calls of <tt>deflate()</tt> would return |
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262 | <tt>Z_STREAM_ERROR</tt> if the flush parameter is not <tt>Z_FINISH</tt>, and do no more processing |
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263 | until the state is reinitialized. |
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264 | <p> |
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265 | Some applications of <em>zlib</em> have two loops that call <tt>deflate()</tt> |
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266 | instead of the single inner loop we have here. The first loop would call |
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267 | without flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call |
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268 | <tt>deflate()</tt> with no more |
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269 | data and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this |
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270 | example, that can be avoided by simply keeping track of the current flush state. |
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271 | <pre><b> |
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272 | } while (strm.avail_out == 0); |
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273 | assert(strm.avail_in == 0); /* all input will be used */ |
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274 | </b></pre><!-- --> |
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275 | Now we check to see if we have already processed all of the input file. That information was |
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276 | saved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so, |
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277 | then we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt> |
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278 | from the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was |
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279 | consumed and all of the output was generated. |
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280 | <pre><b> |
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281 | /* done when last data in file processed */ |
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282 | } while (flush != Z_FINISH); |
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283 | assert(ret == Z_STREAM_END); /* stream will be complete */ |
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284 | </b></pre><!-- --> |
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285 | The process is complete, but we still need to deallocate the state to avoid a memory leak |
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286 | (or rather more like a memory hemorrhage if you didn't do this). Then |
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287 | finally we can return with a happy return value. |
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288 | <pre><b> |
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289 | /* clean up and return */ |
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290 | (void)deflateEnd(&strm); |
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291 | return Z_OK; |
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292 | } |
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293 | </b></pre><!-- --> |
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294 | Now we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt> |
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295 | decompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the |
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296 | uncompressed data to the output file. Much of the discussion above for <tt>def()</tt> |
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297 | applies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between |
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298 | the two. |
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299 | <pre><b> |
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300 | /* Decompress from file source to file dest until stream ends or EOF. |
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301 | inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be |
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302 | allocated for processing, Z_DATA_ERROR if the deflate data is |
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303 | invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and |
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304 | the version of the library linked do not match, or Z_ERRNO if there |
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305 | is an error reading or writing the files. */ |
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306 | int inf(FILE *source, FILE *dest) |
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307 | { |
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308 | </b></pre> |
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309 | The local variables have the same functionality as they do for <tt>def()</tt>. The |
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310 | only difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt> |
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311 | can tell from the <em>zlib</em> stream itself when the stream is complete. |
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312 | <pre><b> |
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313 | int ret; |
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314 | unsigned have; |
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315 | z_stream strm; |
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316 | char in[CHUNK]; |
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317 | char out[CHUNK]; |
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318 | </b></pre><!-- --> |
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319 | The initialization of the state is the same, except that there is no compression level, |
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320 | of course, and two more elements of the structure are initialized. <tt>avail_in</tt> |
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321 | and <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This |
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322 | is because the application has the option to provide the start of the zlib stream in |
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323 | order for <tt>inflateInit()</tt> to have access to information about the compression |
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324 | method to aid in memory allocation. In the current implementation of <em>zlib</em> |
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325 | (up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of |
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326 | <tt>inflate()</tt> anyway. However those fields must be initialized since later versions |
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327 | of <em>zlib</em> that provide more compression methods may take advantage of this interface. |
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328 | In any case, no decompression is performed by <tt>inflateInit()</tt>, so the |
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329 | <tt>avail_out</tt> and <tt>next_out</tt> fields do not need to be initialized before calling. |
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330 | <p> |
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331 | Here <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to |
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332 | indicate that no input data is being provided. |
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333 | <pre><b> |
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334 | /* allocate inflate state */ |
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335 | strm.zalloc = Z_NULL; |
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336 | strm.zfree = Z_NULL; |
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337 | strm.opaque = Z_NULL; |
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338 | strm.avail_in = 0; |
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339 | strm.next_in = Z_NULL; |
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340 | ret = inflateInit(&strm); |
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341 | if (ret != Z_OK) |
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342 | return ret; |
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343 | </b></pre><!-- --> |
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344 | The outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates |
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345 | that it has reached the end of the compressed data and has produced all of the uncompressed |
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346 | output. This is in contrast to <tt>def()</tt> which processes all of the input file. |
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347 | If end-of-file is reached before the compressed data self-terminates, then the compressed |
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348 | data is incomplete and an error is returned. |
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349 | <pre><b> |
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350 | /* decompress until deflate stream ends or end of file */ |
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351 | do { |
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352 | </b></pre> |
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353 | We read input data and set the <tt>strm</tt> structure accordingly. If we've reached the |
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354 | end of the input file, then we leave the outer loop and report an error, since the |
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355 | compressed data is incomplete. Note that we may read more data than is eventually consumed |
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356 | by <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream. |
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357 | For applications where <em>zlib</em> streams are embedded in other data, this routine would |
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358 | need to be modified to return the unused data, or at least indicate how much of the input |
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359 | data was not used, so the application would know where to pick up after the <em>zlib</em> stream. |
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360 | <pre><b> |
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361 | strm.avail_in = fread(in, 1, CHUNK, source); |
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362 | if (ferror(source)) { |
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363 | (void)inflateEnd(&strm); |
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364 | return Z_ERRNO; |
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365 | } |
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366 | if (strm.avail_in == 0) |
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367 | break; |
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368 | strm.next_in = in; |
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369 | </b></pre><!-- --> |
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370 | The inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to |
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371 | keep calling <tt>inflate()</tt> until has generated all of the output it can with the |
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372 | provided input. |
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373 | <pre><b> |
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374 | /* run inflate() on input until output buffer not full */ |
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375 | do { |
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376 | </b></pre> |
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377 | Just like in <tt>def()</tt>, the same output space is provided for each call of <tt>inflate()</tt>. |
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378 | <pre><b> |
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379 | strm.avail_out = CHUNK; |
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380 | strm.next_out = out; |
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381 | </b></pre> |
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382 | Now we run the decompression engine itself. There is no need to adjust the flush parameter, since |
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383 | the <em>zlib</em> format is self-terminating. The main difference here is that there are |
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384 | return values that we need to pay attention to. <tt>Z_DATA_ERROR</tt> |
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385 | indicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format, |
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386 | which means that either the data is not a <em>zlib</em> stream to begin with, or that the data was |
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387 | corrupted somewhere along the way since it was compressed. The other error to be processed is |
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388 | <tt>Z_MEM_ERROR</tt>, which can occur since memory allocation is deferred until <tt>inflate()</tt> |
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389 | needs it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>. |
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390 | <p> |
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391 | Advanced applications may use |
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392 | <tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the |
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393 | first 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt> |
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394 | requests that that dictionary be provided before it can start to decompress. Without the dictionary, |
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395 | correct decompression is not possible. For this routine, we have no idea what the dictionary is, |
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396 | so the <tt>Z_NEED_DICT</tt> indication is converted to a <tt>Z_DATA_ERROR</tt>. |
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397 | <p> |
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398 | <tt>inflate()</tt> can also return <tt>Z_STREAM_ERROR</tt>, which should not be possible here, |
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399 | but could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be |
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400 | checked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be |
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401 | checked for later. |
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402 | <pre><b> |
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403 | ret = inflate(&strm, Z_NO_FLUSH); |
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404 | assert(ret != Z_STREAM_ERROR); /* state not clobbered */ |
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405 | switch (ret) { |
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406 | case Z_NEED_DICT: |
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407 | ret = Z_DATA_ERROR; /* and fall through */ |
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408 | case Z_DATA_ERROR: |
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409 | case Z_MEM_ERROR: |
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410 | (void)inflateEnd(&strm); |
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411 | return ret; |
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412 | } |
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413 | </b></pre> |
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414 | The output of <tt>inflate()</tt> is handled identically to that of <tt>deflate()</tt>. |
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415 | <pre><b> |
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416 | have = CHUNK - strm.avail_out; |
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417 | if (fwrite(out, 1, have, dest) != have || ferror(dest)) { |
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418 | (void)inflateEnd(&strm); |
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419 | return Z_ERRNO; |
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420 | } |
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421 | </b></pre> |
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422 | The inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated |
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423 | by not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot |
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424 | assert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file |
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425 | does. |
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426 | <pre><b> |
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427 | } while (strm.avail_out == 0); |
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428 | </b></pre><!-- --> |
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429 | The outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the |
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430 | end of the input <em>zlib</em> stream, has completed the decompression and integrity |
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431 | check, and has provided all of the output. This is indicated by the <tt>inflate()</tt> |
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432 | return value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt> |
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433 | equal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end |
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434 | of the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the |
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435 | loop continues to read more input. |
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436 | <pre><b> |
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437 | /* done when inflate() says it's done */ |
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438 | } while (ret != Z_STREAM_END); |
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439 | </b></pre><!-- --> |
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440 | At this point, decompression successfully completed, or we broke out of the loop due to no |
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441 | more data being available from the input file. If the last <tt>inflate()</tt> return value |
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442 | is not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error |
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443 | is returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt> |
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444 | is called first to avoid a memory leak. |
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445 | <pre><b> |
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446 | /* clean up and return */ |
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447 | (void)inflateEnd(&strm); |
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448 | return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR; |
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449 | } |
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450 | </b></pre><!-- --> |
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451 | That ends the routines that directly use <em>zlib</em>. The following routines make this |
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452 | a command-line program by running data through the above routines from <tt>stdin</tt> to |
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453 | <tt>stdout</tt>, and handling any errors reported by <tt>def()</tt> or <tt>inf()</tt>. |
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454 | <p> |
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455 | <tt>zerr()</tt> is used to interpret the possible error codes from <tt>def()</tt> |
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456 | and <tt>inf()</tt>, as detailed in their comments above, and print out an error message. |
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457 | Note that these are only a subset of the possible return values from <tt>deflate()</tt> |
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458 | and <tt>inflate()</tt>. |
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459 | <pre><b> |
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460 | /* report a zlib or i/o error */ |
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461 | void zerr(int ret) |
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462 | { |
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463 | fputs("zpipe: ", stderr); |
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464 | switch (ret) { |
---|
465 | case Z_ERRNO: |
---|
466 | if (ferror(stdin)) |
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467 | fputs("error reading stdin\n", stderr); |
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468 | if (ferror(stdout)) |
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469 | fputs("error writing stdout\n", stderr); |
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470 | break; |
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471 | case Z_STREAM_ERROR: |
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472 | fputs("invalid compression level\n", stderr); |
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473 | break; |
---|
474 | case Z_DATA_ERROR: |
---|
475 | fputs("invalid or incomplete deflate data\n", stderr); |
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476 | break; |
---|
477 | case Z_MEM_ERROR: |
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478 | fputs("out of memory\n", stderr); |
---|
479 | break; |
---|
480 | case Z_VERSION_ERROR: |
---|
481 | fputs("zlib version mismatch!\n", stderr); |
---|
482 | } |
---|
483 | } |
---|
484 | </b></pre><!-- --> |
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485 | Here is the <tt>main()</tt> routine used to test <tt>def()</tt> and <tt>inf()</tt>. The |
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486 | <tt>zpipe</tt> command is simply a compression pipe from <tt>stdin</tt> to <tt>stdout</tt>, if |
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487 | no arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other |
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488 | arguments are provided, no compression or decompression is performed. Instead a usage |
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489 | message is displayed. Examples are <tt>zpipe < foo.txt > foo.txt.z</tt> to compress, and |
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490 | <tt>zpipe -d < foo.txt.z > foo.txt</tt> to decompress. |
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491 | <pre><b> |
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492 | /* compress or decompress from stdin to stdout */ |
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493 | int main(int argc, char **argv) |
---|
494 | { |
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495 | int ret; |
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496 | |
---|
497 | /* do compression if no arguments */ |
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498 | if (argc == 1) { |
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499 | ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION); |
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500 | if (ret != Z_OK) |
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501 | zerr(ret); |
---|
502 | return ret; |
---|
503 | } |
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504 | |
---|
505 | /* do decompression if -d specified */ |
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506 | else if (argc == 2 && strcmp(argv[1], "-d") == 0) { |
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507 | ret = inf(stdin, stdout); |
---|
508 | if (ret != Z_OK) |
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509 | zerr(ret); |
---|
510 | return ret; |
---|
511 | } |
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512 | |
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513 | /* otherwise, report usage */ |
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514 | else { |
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515 | fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr); |
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516 | return 1; |
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517 | } |
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518 | } |
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519 | </b></pre> |
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520 | <hr> |
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521 | <i>Copyright (c) 2004 by Mark Adler<br>Last modified 13 November 2004</i> |
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522 | </body> |
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523 | </html> |
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