| 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
|
|---|
| 2 | "http://www.w3.org/TR/REC-html40/loose.dtd">
|
|---|
| 3 | <html>
|
|---|
| 4 | <head>
|
|---|
| 5 | <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
|
|---|
| 6 | <title>zlib Usage Example</title>
|
|---|
| 7 | <!-- Copyright (c) 2004 Mark Adler. -->
|
|---|
| 8 | </head>
|
|---|
| 9 | <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#00A000">
|
|---|
| 10 | <h2 align="center"> zlib Usage Example </h2>
|
|---|
| 11 | We often get questions about how the <tt>deflate()</tt> and <tt>inflate()</tt> functions should be used.
|
|---|
| 12 | Users wonder when they should provide more input, when they should use more output,
|
|---|
| 13 | what to do with a <tt>Z_BUF_ERROR</tt>, how to make sure the process terminates properly, and
|
|---|
| 14 | so on. So for those who have read <tt>zlib.h</tt> (a few times), and
|
|---|
| 15 | would like further edification, below is an annotated example in C of simple routines to compress and decompress
|
|---|
| 16 | from an input file to an output file using <tt>deflate()</tt> and <tt>inflate()</tt> respectively. The
|
|---|
| 17 | annotations are interspersed between lines of the code. So please read between the lines.
|
|---|
| 18 | We hope this helps explain some of the intricacies of <em>zlib</em>.
|
|---|
| 19 | <p>
|
|---|
| 20 | Without further adieu, here is the program <a href="zpipe.c"><tt>zpipe.c</tt></a>:
|
|---|
| 21 | <pre><b>
|
|---|
| 22 | /* zpipe.c: example of proper use of zlib's inflate() and deflate()
|
|---|
| 23 | Not copyrighted -- provided to the public domain
|
|---|
| 24 | Version 1.2 9 November 2004 Mark Adler */
|
|---|
| 25 |
|
|---|
| 26 | /* Version history:
|
|---|
| 27 | 1.0 30 Oct 2004 First version
|
|---|
| 28 | 1.1 8 Nov 2004 Add void casting for unused return values
|
|---|
| 29 | Use switch statement for inflate() return values
|
|---|
| 30 | 1.2 9 Nov 2004 Add assertions to document zlib guarantees
|
|---|
| 31 | */
|
|---|
| 32 | </b></pre><!-- -->
|
|---|
| 33 | We now include the header files for the required definitions. From
|
|---|
| 34 | <tt>stdio.h</tt> we use <tt>fopen()</tt>, <tt>fread()</tt>, <tt>fwrite()</tt>,
|
|---|
| 35 | <tt>feof()</tt>, <tt>ferror()</tt>, and <tt>fclose()</tt> for file i/o, and
|
|---|
| 36 | <tt>fputs()</tt> for error messages. From <tt>string.h</tt> we use
|
|---|
| 37 | <tt>strcmp()</tt> for command line argument processing.
|
|---|
| 38 | From <tt>assert.h</tt> we use the <tt>assert()</tt> macro.
|
|---|
| 39 | From <tt>zlib.h</tt>
|
|---|
| 40 | we use the basic compression functions <tt>deflateInit()</tt>,
|
|---|
| 41 | <tt>deflate()</tt>, and <tt>deflateEnd()</tt>, and the basic decompression
|
|---|
| 42 | functions <tt>inflateInit()</tt>, <tt>inflate()</tt>, and
|
|---|
| 43 | <tt>inflateEnd()</tt>.
|
|---|
| 44 | <pre><b>
|
|---|
| 45 | #include <stdio.h>
|
|---|
| 46 | #include <string.h>
|
|---|
| 47 | #include <assert.h>
|
|---|
| 48 | #include "zlib.h"
|
|---|
| 49 | </b></pre><!-- -->
|
|---|
| 50 | <tt>CHUNK</tt> is simply the buffer size for feeding data to and pulling data
|
|---|
| 51 | from the <em>zlib</em> routines. Larger buffer sizes would be more efficient,
|
|---|
| 52 | especially for <tt>inflate()</tt>. If the memory is available, buffers sizes
|
|---|
| 53 | on the order of 128K or 256K bytes should be used.
|
|---|
| 54 | <pre><b>
|
|---|
| 55 | #define CHUNK 16384
|
|---|
| 56 | </b></pre><!-- -->
|
|---|
| 57 | The <tt>def()</tt> routine compresses data from an input file to an output file. The output data
|
|---|
| 58 | will be in the <em>zlib</em> format, which is different from the <em>gzip</em> or <em>zip</em>
|
|---|
| 59 | formats. The <em>zlib</em> format has a very small header of only two bytes to identify it as
|
|---|
| 60 | a <em>zlib</em> stream and to provide decoding information, and a four-byte trailer with a fast
|
|---|
| 61 | check value to verify the integrity of the uncompressed data after decoding.
|
|---|
| 62 | <pre><b>
|
|---|
| 63 | /* Compress from file source to file dest until EOF on source.
|
|---|
| 64 | def() returns Z_OK on success, Z_MEM_ERROR if memory could not be
|
|---|
| 65 | allocated for processing, Z_STREAM_ERROR if an invalid compression
|
|---|
| 66 | level is supplied, Z_VERSION_ERROR if the version of zlib.h and the
|
|---|
| 67 | version of the library linked do not match, or Z_ERRNO if there is
|
|---|
| 68 | an error reading or writing the files. */
|
|---|
| 69 | int def(FILE *source, FILE *dest, int level)
|
|---|
| 70 | {
|
|---|
| 71 | </b></pre>
|
|---|
| 72 | Here are the local variables for <tt>def()</tt>. <tt>ret</tt> will be used for <em>zlib</em>
|
|---|
| 73 | return codes. <tt>flush</tt> will keep track of the current flushing state for <tt>deflate()</tt>,
|
|---|
| 74 | which is either no flushing, or flush to completion after the end of the input file is reached.
|
|---|
| 75 | <tt>have</tt> is the amount of data returned from <tt>deflate()</tt>. The <tt>strm</tt> structure
|
|---|
| 76 | is used to pass information to and from the <em>zlib</em> routines, and to maintain the
|
|---|
| 77 | <tt>deflate()</tt> state. <tt>in</tt> and <tt>out</tt> are the input and output buffers for
|
|---|
| 78 | <tt>deflate()</tt>.
|
|---|
| 79 | <pre><b>
|
|---|
| 80 | int ret, flush;
|
|---|
| 81 | unsigned have;
|
|---|
| 82 | z_stream strm;
|
|---|
| 83 | char in[CHUNK];
|
|---|
| 84 | char out[CHUNK];
|
|---|
| 85 | </b></pre><!-- -->
|
|---|
| 86 | The first thing we do is to initialize the <em>zlib</em> state for compression using
|
|---|
| 87 | <tt>deflateInit()</tt>. This must be done before the first use of <tt>deflate()</tt>.
|
|---|
| 88 | The <tt>zalloc</tt>, <tt>zfree</tt>, and <tt>opaque</tt> fields in the <tt>strm</tt>
|
|---|
| 89 | structure must be initialized before calling <tt>deflateInit()</tt>. Here they are
|
|---|
| 90 | set to the <em>zlib</em> constant <tt>Z_NULL</tt> to request that <em>zlib</em> use
|
|---|
| 91 | the default memory allocation routines. An application may also choose to provide
|
|---|
| 92 | custom memory allocation routines here. <tt>deflateInit()</tt> will allocate on the
|
|---|
| 93 | order of 256K bytes for the internal state.
|
|---|
| 94 | (See <a href="zlib_tech.html"><em>zlib Technical Details</em></a>.)
|
|---|
| 95 | <p>
|
|---|
| 96 | <tt>deflateInit()</tt> is called with a pointer to the structure to be initialized and
|
|---|
| 97 | the compression level, which is an integer in the range of -1 to 9. Lower compression
|
|---|
| 98 | levels result in faster execution, but less compression. Higher levels result in
|
|---|
| 99 | greater compression, but slower execution. The <em>zlib</em> constant Z_DEFAULT_COMPRESSION,
|
|---|
| 100 | equal to -1,
|
|---|
| 101 | provides a good compromise between compression and speed and is equivalent to level 6.
|
|---|
| 102 | Level 0 actually does no compression at all, and in fact expands the data slightly to produce
|
|---|
| 103 | the <em>zlib</em> format (it is not a byte-for-byte copy of the input).
|
|---|
| 104 | More advanced applications of <em>zlib</em>
|
|---|
| 105 | may use <tt>deflateInit2()</tt> here instead. Such an application may want to reduce how
|
|---|
| 106 | much memory will be used, at some price in compression. Or it may need to request a
|
|---|
| 107 | <em>gzip</em> header and trailer instead of a <em>zlib</em> header and trailer, or raw
|
|---|
| 108 | encoding with no header or trailer at all.
|
|---|
| 109 | <p>
|
|---|
| 110 | We must check the return value of <tt>deflateInit()</tt> against the <em>zlib</em> constant
|
|---|
| 111 | <tt>Z_OK</tt> to make sure that it was able to
|
|---|
| 112 | allocate memory for the internal state, and that the provided arguments were valid.
|
|---|
| 113 | <tt>deflateInit()</tt> will also check that the version of <em>zlib</em> that the <tt>zlib.h</tt>
|
|---|
| 114 | file came from matches the version of <em>zlib</em> actually linked with the program. This
|
|---|
| 115 | is especially important for environments in which <em>zlib</em> is a shared library.
|
|---|
| 116 | <p>
|
|---|
| 117 | Note that an application can initialize multiple, independent <em>zlib</em> streams, which can
|
|---|
| 118 | operate in parallel. The state information maintained in the structure allows the <em>zlib</em>
|
|---|
| 119 | routines to be reentrant.
|
|---|
| 120 | <pre><b>
|
|---|
| 121 | /* allocate deflate state */
|
|---|
| 122 | strm.zalloc = Z_NULL;
|
|---|
| 123 | strm.zfree = Z_NULL;
|
|---|
| 124 | strm.opaque = Z_NULL;
|
|---|
| 125 | ret = deflateInit(&strm, level);
|
|---|
| 126 | if (ret != Z_OK)
|
|---|
| 127 | return ret;
|
|---|
| 128 | </b></pre><!-- -->
|
|---|
| 129 | With the pleasantries out of the way, now we can get down to business. The outer <tt>do</tt>-loop
|
|---|
| 130 | reads all of the input file and exits at the bottom of the loop once end-of-file is reached.
|
|---|
| 131 | This loop contains the only call of <tt>deflate()</tt>. So we must make sure that all of the
|
|---|
| 132 | input data has been processed and that all of the output data has been generated and consumed
|
|---|
| 133 | before we fall out of the loop at the bottom.
|
|---|
| 134 | <pre><b>
|
|---|
| 135 | /* compress until end of file */
|
|---|
| 136 | do {
|
|---|
| 137 | </b></pre>
|
|---|
| 138 | We start off by reading data from the input file. The number of bytes read is put directly
|
|---|
| 139 | into <tt>avail_in</tt>, and a pointer to those bytes is put into <tt>next_in</tt>. We also
|
|---|
| 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
|
|---|
| 141 | <em>zlib</em> constant <tt>Z_FINISH</tt>, which is later passed to <tt>deflate()</tt> to
|
|---|
| 142 | indicate that this is the last chunk of input data to compress. We need to use <tt>feof()</tt>
|
|---|
| 143 | to check for end-of-file as opposed to seeing if fewer than <tt>CHUNK</tt> bytes have been read. The
|
|---|
| 144 | reason is that if the input file length is an exact multiple of <tt>CHUNK</tt>, we will miss
|
|---|
| 145 | the fact that we got to the end-of-file, and not know to tell <tt>deflate()</tt> to finish
|
|---|
| 146 | up the compressed stream. If we are not yet at the end of the input, then the <em>zlib</em>
|
|---|
| 147 | constant <tt>Z_NO_FLUSH</tt> will be passed to <tt>deflate</tt> to indicate that we are still
|
|---|
| 148 | in the middle of the uncompressed data.
|
|---|
| 149 | <p>
|
|---|
| 150 | If there is an error in reading from the input file, the process is aborted with
|
|---|
| 151 | <tt>deflateEnd()</tt> being called to free the allocated <em>zlib</em> state before returning
|
|---|
| 152 | the error. We wouldn't want a memory leak, now would we? <tt>deflateEnd()</tt> can be called
|
|---|
| 153 | at any time after the state has been initialized. Once that's done, <tt>deflateInit()</tt> (or
|
|---|
| 154 | <tt>deflateInit2()</tt>) would have to be called to start a new compression process. There is
|
|---|
| 155 | no point here in checking the <tt>deflateEnd()</tt> return code. The deallocation can't fail.
|
|---|
| 156 | <pre><b>
|
|---|
| 157 | strm.avail_in = fread(in, 1, CHUNK, source);
|
|---|
| 158 | if (ferror(source)) {
|
|---|
| 159 | (void)deflateEnd(&strm);
|
|---|
| 160 | return Z_ERRNO;
|
|---|
| 161 | }
|
|---|
| 162 | flush = feof(source) ? Z_FINISH : Z_NO_FLUSH;
|
|---|
| 163 | strm.next_in = in;
|
|---|
| 164 | </b></pre><!-- -->
|
|---|
| 165 | The inner <tt>do</tt>-loop passes our chunk of input data to <tt>deflate()</tt>, and then
|
|---|
| 166 | keeps calling <tt>deflate()</tt> until it is done producing output. Once there is no more
|
|---|
| 167 | new output, <tt>deflate()</tt> is guaranteed to have consumed all of the input, i.e.,
|
|---|
| 168 | <tt>avail_in</tt> will be zero.
|
|---|
| 169 | <pre><b>
|
|---|
| 170 | /* run deflate() on input until output buffer not full, finish
|
|---|
| 171 | compression if all of source has been read in */
|
|---|
| 172 | do {
|
|---|
| 173 | </b></pre>
|
|---|
| 174 | Output space is provided to <tt>deflate()</tt> by setting <tt>avail_out</tt> to the number
|
|---|
| 175 | of available output bytes and <tt>next_out</tt> to a pointer to that space.
|
|---|
| 176 | <pre><b>
|
|---|
| 177 | strm.avail_out = CHUNK;
|
|---|
| 178 | strm.next_out = out;
|
|---|
| 179 | </b></pre>
|
|---|
| 180 | Now we call the compression engine itself, <tt>deflate()</tt>. It takes as many of the
|
|---|
| 181 | <tt>avail_in</tt> bytes at <tt>next_in</tt> as it can process, and writes as many as
|
|---|
| 182 | <tt>avail_out</tt> bytes to <tt>next_out</tt>. Those counters and pointers are then
|
|---|
| 183 | updated past the input data consumed and the output data written. It is the amount of
|
|---|
| 184 | output space available that may limit how much input is consumed.
|
|---|
| 185 | Hence the inner loop to make sure that
|
|---|
| 186 | all of the input is consumed by providing more output space each time. Since <tt>avail_in</tt>
|
|---|
| 187 | and <tt>next_in</tt> are updated by <tt>deflate()</tt>, we don't have to mess with those
|
|---|
| 188 | between <tt>deflate()</tt> calls until it's all used up.
|
|---|
| 189 | <p>
|
|---|
| 190 | The parameters to <tt>deflate()</tt> are a pointer to the <tt>strm</tt> structure containing
|
|---|
| 191 | the input and output information and the internal compression engine state, and a parameter
|
|---|
| 192 | indicating whether and how to flush data to the output. Normally <tt>deflate</tt> will consume
|
|---|
| 193 | several K bytes of input data before producing any output (except for the header), in order
|
|---|
| 194 | to accumulate statistics on the data for optimum compression. It will then put out a burst of
|
|---|
| 195 | compressed data, and proceed to consume more input before the next burst. Eventually,
|
|---|
| 196 | <tt>deflate()</tt>
|
|---|
| 197 | must be told to terminate the stream, complete the compression with provided input data, and
|
|---|
| 198 | write out the trailer check value. <tt>deflate()</tt> will continue to compress normally as long
|
|---|
| 199 | as the flush parameter is <tt>Z_NO_FLUSH</tt>. Once the <tt>Z_FINISH</tt> parameter is provided,
|
|---|
| 200 | <tt>deflate()</tt> will begin to complete the compressed output stream. However depending on how
|
|---|
| 201 | much output space is provided, <tt>deflate()</tt> may have to be called several times until it
|
|---|
| 202 | has provided the complete compressed stream, even after it has consumed all of the input. The flush
|
|---|
| 203 | parameter must continue to be <tt>Z_FINISH</tt> for those subsequent calls.
|
|---|
| 204 | <p>
|
|---|
| 205 | There are other values of the flush parameter that are used in more advanced applications. You can
|
|---|
| 206 | force <tt>deflate()</tt> to produce a burst of output that encodes all of the input data provided
|
|---|
| 207 | so far, even if it wouldn't have otherwise, for example to control data latency on a link with
|
|---|
| 208 | compressed data. You can also ask that <tt>deflate()</tt> do that as well as erase any history up to
|
|---|
| 209 | that point so that what follows can be decompressed independently, for example for random access
|
|---|
| 210 | applications. Both requests will degrade compression by an amount depending on how often such
|
|---|
| 211 | requests are made.
|
|---|
| 212 | <p>
|
|---|
| 213 | <tt>deflate()</tt> has a return value that can indicate errors, yet we do not check it here. Why
|
|---|
| 214 | not? Well, it turns out that <tt>deflate()</tt> can do no wrong here. Let's go through
|
|---|
| 215 | <tt>deflate()</tt>'s return values and dispense with them one by one. The possible values are
|
|---|
| 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>
|
|---|
| 217 | is, well, ok. <tt>Z_STREAM_END</tt> is also ok and will be returned for the last call of
|
|---|
| 218 | <tt>deflate()</tt>. This is already guaranteed by calling <tt>deflate()</tt> with <tt>Z_FINISH</tt>
|
|---|
| 219 | until it has no more output. <tt>Z_STREAM_ERROR</tt> is only possible if the stream is not
|
|---|
| 220 | initialized properly, but we did initialize it properly. There is no harm in checking for
|
|---|
| 221 | <tt>Z_STREAM_ERROR</tt> here, for example to check for the possibility that some
|
|---|
| 222 | other part of the application inadvertently clobbered the memory containing the <em>zlib</em> state.
|
|---|
| 223 | <tt>Z_BUF_ERROR</tt> will be explained further below, but
|
|---|
| 224 | suffice it to say that this is simply an indication that <tt>deflate()</tt> could not consume
|
|---|
| 225 | more input or produce more output. <tt>deflate()</tt> can be called again with more output space
|
|---|
| 226 | or more available input, which it will be in this code.
|
|---|
| 227 | <pre><b>
|
|---|
| 228 | ret = deflate(&strm, flush); /* no bad return value */
|
|---|
| 229 | assert(ret != Z_STREAM_ERROR); /* state not clobbered */
|
|---|
| 230 | </b></pre>
|
|---|
| 231 | Now we compute how much output <tt>deflate()</tt> provided on the last call, which is the
|
|---|
| 232 | difference between how much space was provided before the call, and how much output space
|
|---|
| 233 | is still available after the call. Then that data, if any, is written to the output file.
|
|---|
| 234 | We can then reuse the output buffer for the next call of <tt>deflate()</tt>. Again if there
|
|---|
| 235 | is a file i/o error, we call <tt>deflateEnd()</tt> before returning to avoid a memory leak.
|
|---|
| 236 | <pre><b>
|
|---|
| 237 | have = CHUNK - strm.avail_out;
|
|---|
| 238 | if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
|
|---|
| 239 | (void)deflateEnd(&strm);
|
|---|
| 240 | return Z_ERRNO;
|
|---|
| 241 | }
|
|---|
| 242 | </b></pre>
|
|---|
| 243 | The inner <tt>do</tt>-loop is repeated until the last <tt>deflate()</tt> call fails to fill the
|
|---|
| 244 | provided output buffer. Then we know that <tt>deflate()</tt> has done as much as it can with
|
|---|
| 245 | the provided input, and that all of that input has been consumed. We can then fall out of this
|
|---|
| 246 | loop and reuse the input buffer.
|
|---|
| 247 | <p>
|
|---|
| 248 | The way we tell that <tt>deflate()</tt> has no more output is by seeing that it did not fill
|
|---|
| 249 | the output buffer, leaving <tt>avail_out</tt> greater than zero. However suppose that
|
|---|
| 250 | <tt>deflate()</tt> has no more output, but just so happened to exactly fill the output buffer!
|
|---|
| 251 | <tt>avail_out</tt> is zero, and we can't tell that <tt>deflate()</tt> has done all it can.
|
|---|
| 252 | As far as we know, <tt>deflate()</tt>
|
|---|
| 253 | has more output for us. So we call it again. But now <tt>deflate()</tt> produces no output
|
|---|
| 254 | at all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call
|
|---|
| 255 | wasn't able to do anything, either consume input or produce output, and so it returns
|
|---|
| 256 | <tt>Z_BUF_ERROR</tt>. (See, I told you I'd cover this later.) However this is not a problem at
|
|---|
| 257 | all. Now we finally have the desired indication that <tt>deflate()</tt> is really done,
|
|---|
| 258 | and so we drop out of the inner loop to provide more input to <tt>deflate()</tt>.
|
|---|
| 259 | <p>
|
|---|
| 260 | With <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will
|
|---|
| 261 | complete the output stream. Once that is done, subsequent calls of <tt>deflate()</tt> would return
|
|---|
| 262 | <tt>Z_STREAM_ERROR</tt> if the flush parameter is not <tt>Z_FINISH</tt>, and do no more processing
|
|---|
| 263 | until the state is reinitialized.
|
|---|
| 264 | <p>
|
|---|
| 265 | Some applications of <em>zlib</em> have two loops that call <tt>deflate()</tt>
|
|---|
| 266 | instead of the single inner loop we have here. The first loop would call
|
|---|
| 267 | without flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call
|
|---|
| 268 | <tt>deflate()</tt> with no more
|
|---|
| 269 | data and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this
|
|---|
| 270 | example, that can be avoided by simply keeping track of the current flush state.
|
|---|
| 271 | <pre><b>
|
|---|
| 272 | } while (strm.avail_out == 0);
|
|---|
| 273 | assert(strm.avail_in == 0); /* all input will be used */
|
|---|
| 274 | </b></pre><!-- -->
|
|---|
| 275 | Now we check to see if we have already processed all of the input file. That information was
|
|---|
| 276 | saved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so,
|
|---|
| 277 | then we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt>
|
|---|
| 278 | from the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was
|
|---|
| 279 | consumed and all of the output was generated.
|
|---|
| 280 | <pre><b>
|
|---|
| 281 | /* done when last data in file processed */
|
|---|
| 282 | } while (flush != Z_FINISH);
|
|---|
| 283 | assert(ret == Z_STREAM_END); /* stream will be complete */
|
|---|
| 284 | </b></pre><!-- -->
|
|---|
| 285 | The process is complete, but we still need to deallocate the state to avoid a memory leak
|
|---|
| 286 | (or rather more like a memory hemorrhage if you didn't do this). Then
|
|---|
| 287 | finally we can return with a happy return value.
|
|---|
| 288 | <pre><b>
|
|---|
| 289 | /* clean up and return */
|
|---|
| 290 | (void)deflateEnd(&strm);
|
|---|
| 291 | return Z_OK;
|
|---|
| 292 | }
|
|---|
| 293 | </b></pre><!-- -->
|
|---|
| 294 | Now we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt>
|
|---|
| 295 | decompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the
|
|---|
| 296 | uncompressed data to the output file. Much of the discussion above for <tt>def()</tt>
|
|---|
| 297 | applies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between
|
|---|
| 298 | the two.
|
|---|
| 299 | <pre><b>
|
|---|
| 300 | /* Decompress from file source to file dest until stream ends or EOF.
|
|---|
| 301 | inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be
|
|---|
| 302 | allocated for processing, Z_DATA_ERROR if the deflate data is
|
|---|
| 303 | invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and
|
|---|
| 304 | the version of the library linked do not match, or Z_ERRNO if there
|
|---|
| 305 | is an error reading or writing the files. */
|
|---|
| 306 | int inf(FILE *source, FILE *dest)
|
|---|
| 307 | {
|
|---|
| 308 | </b></pre>
|
|---|
| 309 | The local variables have the same functionality as they do for <tt>def()</tt>. The
|
|---|
| 310 | only difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt>
|
|---|
| 311 | can tell from the <em>zlib</em> stream itself when the stream is complete.
|
|---|
| 312 | <pre><b>
|
|---|
| 313 | int ret;
|
|---|
| 314 | unsigned have;
|
|---|
| 315 | z_stream strm;
|
|---|
| 316 | char in[CHUNK];
|
|---|
| 317 | char out[CHUNK];
|
|---|
| 318 | </b></pre><!-- -->
|
|---|
| 319 | The initialization of the state is the same, except that there is no compression level,
|
|---|
| 320 | of course, and two more elements of the structure are initialized. <tt>avail_in</tt>
|
|---|
| 321 | and <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This
|
|---|
| 322 | is because the application has the option to provide the start of the zlib stream in
|
|---|
| 323 | order for <tt>inflateInit()</tt> to have access to information about the compression
|
|---|
| 324 | method to aid in memory allocation. In the current implementation of <em>zlib</em>
|
|---|
| 325 | (up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of
|
|---|
| 326 | <tt>inflate()</tt> anyway. However those fields must be initialized since later versions
|
|---|
| 327 | of <em>zlib</em> that provide more compression methods may take advantage of this interface.
|
|---|
| 328 | In any case, no decompression is performed by <tt>inflateInit()</tt>, so the
|
|---|
| 329 | <tt>avail_out</tt> and <tt>next_out</tt> fields do not need to be initialized before calling.
|
|---|
| 330 | <p>
|
|---|
| 331 | Here <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to
|
|---|
| 332 | indicate that no input data is being provided.
|
|---|
| 333 | <pre><b>
|
|---|
| 334 | /* allocate inflate state */
|
|---|
| 335 | strm.zalloc = Z_NULL;
|
|---|
| 336 | strm.zfree = Z_NULL;
|
|---|
| 337 | strm.opaque = Z_NULL;
|
|---|
| 338 | strm.avail_in = 0;
|
|---|
| 339 | strm.next_in = Z_NULL;
|
|---|
| 340 | ret = inflateInit(&strm);
|
|---|
| 341 | if (ret != Z_OK)
|
|---|
| 342 | return ret;
|
|---|
| 343 | </b></pre><!-- -->
|
|---|
| 344 | The outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates
|
|---|
| 345 | that it has reached the end of the compressed data and has produced all of the uncompressed
|
|---|
| 346 | output. This is in contrast to <tt>def()</tt> which processes all of the input file.
|
|---|
| 347 | If end-of-file is reached before the compressed data self-terminates, then the compressed
|
|---|
| 348 | data is incomplete and an error is returned.
|
|---|
| 349 | <pre><b>
|
|---|
| 350 | /* decompress until deflate stream ends or end of file */
|
|---|
| 351 | do {
|
|---|
| 352 | </b></pre>
|
|---|
| 353 | We read input data and set the <tt>strm</tt> structure accordingly. If we've reached the
|
|---|
| 354 | end of the input file, then we leave the outer loop and report an error, since the
|
|---|
| 355 | compressed data is incomplete. Note that we may read more data than is eventually consumed
|
|---|
| 356 | by <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream.
|
|---|
| 357 | For applications where <em>zlib</em> streams are embedded in other data, this routine would
|
|---|
| 358 | need to be modified to return the unused data, or at least indicate how much of the input
|
|---|
| 359 | data was not used, so the application would know where to pick up after the <em>zlib</em> stream.
|
|---|
| 360 | <pre><b>
|
|---|
| 361 | strm.avail_in = fread(in, 1, CHUNK, source);
|
|---|
| 362 | if (ferror(source)) {
|
|---|
| 363 | (void)inflateEnd(&strm);
|
|---|
| 364 | return Z_ERRNO;
|
|---|
| 365 | }
|
|---|
| 366 | if (strm.avail_in == 0)
|
|---|
| 367 | break;
|
|---|
| 368 | strm.next_in = in;
|
|---|
| 369 | </b></pre><!-- -->
|
|---|
| 370 | The inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to
|
|---|
| 371 | keep calling <tt>inflate()</tt> until has generated all of the output it can with the
|
|---|
| 372 | provided input.
|
|---|
| 373 | <pre><b>
|
|---|
| 374 | /* run inflate() on input until output buffer not full */
|
|---|
| 375 | do {
|
|---|
| 376 | </b></pre>
|
|---|
| 377 | Just like in <tt>def()</tt>, the same output space is provided for each call of <tt>inflate()</tt>.
|
|---|
| 378 | <pre><b>
|
|---|
| 379 | strm.avail_out = CHUNK;
|
|---|
| 380 | strm.next_out = out;
|
|---|
| 381 | </b></pre>
|
|---|
| 382 | Now we run the decompression engine itself. There is no need to adjust the flush parameter, since
|
|---|
| 383 | the <em>zlib</em> format is self-terminating. The main difference here is that there are
|
|---|
| 384 | return values that we need to pay attention to. <tt>Z_DATA_ERROR</tt>
|
|---|
| 385 | indicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format,
|
|---|
| 386 | which means that either the data is not a <em>zlib</em> stream to begin with, or that the data was
|
|---|
| 387 | corrupted somewhere along the way since it was compressed. The other error to be processed is
|
|---|
| 388 | <tt>Z_MEM_ERROR</tt>, which can occur since memory allocation is deferred until <tt>inflate()</tt>
|
|---|
| 389 | needs it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>.
|
|---|
| 390 | <p>
|
|---|
| 391 | Advanced applications may use
|
|---|
| 392 | <tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the
|
|---|
| 393 | first 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt>
|
|---|
| 394 | requests that that dictionary be provided before it can start to decompress. Without the dictionary,
|
|---|
| 395 | correct decompression is not possible. For this routine, we have no idea what the dictionary is,
|
|---|
| 396 | so the <tt>Z_NEED_DICT</tt> indication is converted to a <tt>Z_DATA_ERROR</tt>.
|
|---|
| 397 | <p>
|
|---|
| 398 | <tt>inflate()</tt> can also return <tt>Z_STREAM_ERROR</tt>, which should not be possible here,
|
|---|
| 399 | but could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be
|
|---|
| 400 | checked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be
|
|---|
| 401 | checked for later.
|
|---|
| 402 | <pre><b>
|
|---|
| 403 | ret = inflate(&strm, Z_NO_FLUSH);
|
|---|
| 404 | assert(ret != Z_STREAM_ERROR); /* state not clobbered */
|
|---|
| 405 | switch (ret) {
|
|---|
| 406 | case Z_NEED_DICT:
|
|---|
| 407 | ret = Z_DATA_ERROR; /* and fall through */
|
|---|
| 408 | case Z_DATA_ERROR:
|
|---|
| 409 | case Z_MEM_ERROR:
|
|---|
| 410 | (void)inflateEnd(&strm);
|
|---|
| 411 | return ret;
|
|---|
| 412 | }
|
|---|
| 413 | </b></pre>
|
|---|
| 414 | The output of <tt>inflate()</tt> is handled identically to that of <tt>deflate()</tt>.
|
|---|
| 415 | <pre><b>
|
|---|
| 416 | have = CHUNK - strm.avail_out;
|
|---|
| 417 | if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
|
|---|
| 418 | (void)inflateEnd(&strm);
|
|---|
| 419 | return Z_ERRNO;
|
|---|
| 420 | }
|
|---|
| 421 | </b></pre>
|
|---|
| 422 | The inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated
|
|---|
| 423 | by not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot
|
|---|
| 424 | assert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file
|
|---|
| 425 | does.
|
|---|
| 426 | <pre><b>
|
|---|
| 427 | } while (strm.avail_out == 0);
|
|---|
| 428 | </b></pre><!-- -->
|
|---|
| 429 | The outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the
|
|---|
| 430 | end of the input <em>zlib</em> stream, has completed the decompression and integrity
|
|---|
| 431 | check, and has provided all of the output. This is indicated by the <tt>inflate()</tt>
|
|---|
| 432 | return value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt>
|
|---|
| 433 | equal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end
|
|---|
| 434 | of the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the
|
|---|
| 435 | loop continues to read more input.
|
|---|
| 436 | <pre><b>
|
|---|
| 437 | /* done when inflate() says it's done */
|
|---|
| 438 | } while (ret != Z_STREAM_END);
|
|---|
| 439 | </b></pre><!-- -->
|
|---|
| 440 | At this point, decompression successfully completed, or we broke out of the loop due to no
|
|---|
| 441 | more data being available from the input file. If the last <tt>inflate()</tt> return value
|
|---|
| 442 | is not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error
|
|---|
| 443 | is returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt>
|
|---|
| 444 | is called first to avoid a memory leak.
|
|---|
| 445 | <pre><b>
|
|---|
| 446 | /* clean up and return */
|
|---|
| 447 | (void)inflateEnd(&strm);
|
|---|
| 448 | return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR;
|
|---|
| 449 | }
|
|---|
| 450 | </b></pre><!-- -->
|
|---|
| 451 | That ends the routines that directly use <em>zlib</em>. The following routines make this
|
|---|
| 452 | a command-line program by running data through the above routines from <tt>stdin</tt> to
|
|---|
| 453 | <tt>stdout</tt>, and handling any errors reported by <tt>def()</tt> or <tt>inf()</tt>.
|
|---|
| 454 | <p>
|
|---|
| 455 | <tt>zerr()</tt> is used to interpret the possible error codes from <tt>def()</tt>
|
|---|
| 456 | and <tt>inf()</tt>, as detailed in their comments above, and print out an error message.
|
|---|
| 457 | Note that these are only a subset of the possible return values from <tt>deflate()</tt>
|
|---|
| 458 | and <tt>inflate()</tt>.
|
|---|
| 459 | <pre><b>
|
|---|
| 460 | /* report a zlib or i/o error */
|
|---|
| 461 | void zerr(int ret)
|
|---|
| 462 | {
|
|---|
| 463 | fputs("zpipe: ", stderr);
|
|---|
| 464 | switch (ret) {
|
|---|
| 465 | case Z_ERRNO:
|
|---|
| 466 | if (ferror(stdin))
|
|---|
| 467 | fputs("error reading stdin\n", stderr);
|
|---|
| 468 | if (ferror(stdout))
|
|---|
| 469 | fputs("error writing stdout\n", stderr);
|
|---|
| 470 | break;
|
|---|
| 471 | case Z_STREAM_ERROR:
|
|---|
| 472 | fputs("invalid compression level\n", stderr);
|
|---|
| 473 | break;
|
|---|
| 474 | case Z_DATA_ERROR:
|
|---|
| 475 | fputs("invalid or incomplete deflate data\n", stderr);
|
|---|
| 476 | break;
|
|---|
| 477 | case Z_MEM_ERROR:
|
|---|
| 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><!-- -->
|
|---|
| 485 | Here is the <tt>main()</tt> routine used to test <tt>def()</tt> and <tt>inf()</tt>. The
|
|---|
| 486 | <tt>zpipe</tt> command is simply a compression pipe from <tt>stdin</tt> to <tt>stdout</tt>, if
|
|---|
| 487 | no arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other
|
|---|
| 488 | arguments are provided, no compression or decompression is performed. Instead a usage
|
|---|
| 489 | message is displayed. Examples are <tt>zpipe < foo.txt > foo.txt.z</tt> to compress, and
|
|---|
| 490 | <tt>zpipe -d < foo.txt.z > foo.txt</tt> to decompress.
|
|---|
| 491 | <pre><b>
|
|---|
| 492 | /* compress or decompress from stdin to stdout */
|
|---|
| 493 | int main(int argc, char **argv)
|
|---|
| 494 | {
|
|---|
| 495 | int ret;
|
|---|
| 496 |
|
|---|
| 497 | /* do compression if no arguments */
|
|---|
| 498 | if (argc == 1) {
|
|---|
| 499 | ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION);
|
|---|
| 500 | if (ret != Z_OK)
|
|---|
| 501 | zerr(ret);
|
|---|
| 502 | return ret;
|
|---|
| 503 | }
|
|---|
| 504 |
|
|---|
| 505 | /* do decompression if -d specified */
|
|---|
| 506 | else if (argc == 2 && strcmp(argv[1], "-d") == 0) {
|
|---|
| 507 | ret = inf(stdin, stdout);
|
|---|
| 508 | if (ret != Z_OK)
|
|---|
| 509 | zerr(ret);
|
|---|
| 510 | return ret;
|
|---|
| 511 | }
|
|---|
| 512 |
|
|---|
| 513 | /* otherwise, report usage */
|
|---|
| 514 | else {
|
|---|
| 515 | fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr);
|
|---|
| 516 | return 1;
|
|---|
| 517 | }
|
|---|
| 518 | }
|
|---|
| 519 | </b></pre>
|
|---|
| 520 | <hr>
|
|---|
| 521 | <i>Copyright (c) 2004 by Mark Adler<br>Last modified 13 November 2004</i>
|
|---|
| 522 | </body>
|
|---|
| 523 | </html>
|
|---|
