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546 lines
29 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
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"http://www.w3.org/TR/REC-html40/loose.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>zlib Usage Example</title>
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<!-- Copyright (c) 2004, 2005 Mark Adler. -->
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</head>
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#00A000">
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<h2 align="center"> zlib Usage Example </h2>
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We often get questions about how the <tt>deflate()</tt> and <tt>inflate()</tt> functions should be used.
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Users wonder when they should provide more input, when they should use more output,
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what to do with a <tt>Z_BUF_ERROR</tt>, how to make sure the process terminates properly, and
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so on. So for those who have read <tt>zlib.h</tt> (a few times), and
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would like further edification, below is an annotated example in C of simple routines to compress and decompress
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from an input file to an output file using <tt>deflate()</tt> and <tt>inflate()</tt> respectively. The
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annotations are interspersed between lines of the code. So please read between the lines.
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We hope this helps explain some of the intricacies of <em>zlib</em>.
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<p>
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Without further adieu, here is the program <a href="zpipe.c"><tt>zpipe.c</tt></a>:
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<pre><b>
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/* zpipe.c: example of proper use of zlib's inflate() and deflate()
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Not copyrighted -- provided to the public domain
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Version 1.4 11 December 2005 Mark Adler */
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/* Version history:
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1.0 30 Oct 2004 First version
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1.1 8 Nov 2004 Add void casting for unused return values
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Use switch statement for inflate() return values
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1.2 9 Nov 2004 Add assertions to document zlib guarantees
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1.3 6 Apr 2005 Remove incorrect assertion in inf()
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1.4 11 Dec 2005 Add hack to avoid MSDOS end-of-line conversions
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Avoid some compiler warnings for input and output buffers
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*/
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</b></pre><!-- -->
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We now include the header files for the required definitions. From
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<tt>stdio.h</tt> we use <tt>fopen()</tt>, <tt>fread()</tt>, <tt>fwrite()</tt>,
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<tt>feof()</tt>, <tt>ferror()</tt>, and <tt>fclose()</tt> for file i/o, and
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<tt>fputs()</tt> for error messages. From <tt>string.h</tt> we use
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<tt>strcmp()</tt> for command line argument processing.
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From <tt>assert.h</tt> we use the <tt>assert()</tt> macro.
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From <tt>zlib.h</tt>
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we use the basic compression functions <tt>deflateInit()</tt>,
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<tt>deflate()</tt>, and <tt>deflateEnd()</tt>, and the basic decompression
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functions <tt>inflateInit()</tt>, <tt>inflate()</tt>, and
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<tt>inflateEnd()</tt>.
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<pre><b>
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#include <stdio.h>
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#include <string.h>
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#include <assert.h>
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#include "zlib.h"
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</b></pre><!-- -->
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This is an ugly hack required to avoid corruption of the input and output data on
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Windows/MS-DOS systems. Without this, those systems would assume that the input and output
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files are text, and try to convert the end-of-line characters from one standard to
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another. That would corrupt binary data, and in particular would render the compressed data unusable.
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This sets the input and output to binary which suppresses the end-of-line conversions.
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<tt>SET_BINARY_MODE()</tt> will be used later on <tt>stdin</tt> and <tt>stdout</tt>, at the beginning of <tt>main()</tt>.
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<pre><b>
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#if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(__CYGWIN__)
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# include <fcntl.h>
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# include <io.h>
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# define SET_BINARY_MODE(file) setmode(fileno(file), O_BINARY)
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#else
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# define SET_BINARY_MODE(file)
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#endif
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</b></pre><!-- -->
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<tt>CHUNK</tt> is simply the buffer size for feeding data to and pulling data
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from the <em>zlib</em> routines. Larger buffer sizes would be more efficient,
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especially for <tt>inflate()</tt>. If the memory is available, buffers sizes
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on the order of 128K or 256K bytes should be used.
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<pre><b>
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#define CHUNK 16384
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</b></pre><!-- -->
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The <tt>def()</tt> routine compresses data from an input file to an output file. The output data
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will be in the <em>zlib</em> format, which is different from the <em>gzip</em> or <em>zip</em>
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formats. The <em>zlib</em> format has a very small header of only two bytes to identify it as
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a <em>zlib</em> stream and to provide decoding information, and a four-byte trailer with a fast
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check value to verify the integrity of the uncompressed data after decoding.
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<pre><b>
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/* Compress from file source to file dest until EOF on source.
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def() returns Z_OK on success, Z_MEM_ERROR if memory could not be
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allocated for processing, Z_STREAM_ERROR if an invalid compression
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level is supplied, Z_VERSION_ERROR if the version of zlib.h and the
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version of the library linked do not match, or Z_ERRNO if there is
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an error reading or writing the files. */
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int def(FILE *source, FILE *dest, int level)
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{
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</b></pre>
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Here are the local variables for <tt>def()</tt>. <tt>ret</tt> will be used for <em>zlib</em>
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return codes. <tt>flush</tt> will keep track of the current flushing state for <tt>deflate()</tt>,
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which is either no flushing, or flush to completion after the end of the input file is reached.
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<tt>have</tt> is the amount of data returned from <tt>deflate()</tt>. The <tt>strm</tt> structure
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is used to pass information to and from the <em>zlib</em> routines, and to maintain the
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<tt>deflate()</tt> state. <tt>in</tt> and <tt>out</tt> are the input and output buffers for
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<tt>deflate()</tt>.
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<pre><b>
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int ret, flush;
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unsigned have;
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z_stream strm;
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unsigned char in[CHUNK];
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unsigned char out[CHUNK];
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</b></pre><!-- -->
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The first thing we do is to initialize the <em>zlib</em> state for compression using
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<tt>deflateInit()</tt>. This must be done before the first use of <tt>deflate()</tt>.
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The <tt>zalloc</tt>, <tt>zfree</tt>, and <tt>opaque</tt> fields in the <tt>strm</tt>
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structure must be initialized before calling <tt>deflateInit()</tt>. Here they are
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set to the <em>zlib</em> constant <tt>Z_NULL</tt> to request that <em>zlib</em> use
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the default memory allocation routines. An application may also choose to provide
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custom memory allocation routines here. <tt>deflateInit()</tt> will allocate on the
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order of 256K bytes for the internal state.
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(See <a href="zlib_tech.html"><em>zlib Technical Details</em></a>.)
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<p>
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<tt>deflateInit()</tt> is called with a pointer to the structure to be initialized and
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the compression level, which is an integer in the range of -1 to 9. Lower compression
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levels result in faster execution, but less compression. Higher levels result in
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greater compression, but slower execution. The <em>zlib</em> constant Z_DEFAULT_COMPRESSION,
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equal to -1,
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provides a good compromise between compression and speed and is equivalent to level 6.
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Level 0 actually does no compression at all, and in fact expands the data slightly to produce
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the <em>zlib</em> format (it is not a byte-for-byte copy of the input).
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More advanced applications of <em>zlib</em>
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may use <tt>deflateInit2()</tt> here instead. Such an application may want to reduce how
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much memory will be used, at some price in compression. Or it may need to request a
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<em>gzip</em> header and trailer instead of a <em>zlib</em> header and trailer, or raw
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encoding with no header or trailer at all.
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<p>
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We must check the return value of <tt>deflateInit()</tt> against the <em>zlib</em> constant
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<tt>Z_OK</tt> to make sure that it was able to
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allocate memory for the internal state, and that the provided arguments were valid.
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<tt>deflateInit()</tt> will also check that the version of <em>zlib</em> that the <tt>zlib.h</tt>
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file came from matches the version of <em>zlib</em> actually linked with the program. This
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is especially important for environments in which <em>zlib</em> is a shared library.
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<p>
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Note that an application can initialize multiple, independent <em>zlib</em> streams, which can
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operate in parallel. The state information maintained in the structure allows the <em>zlib</em>
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routines to be reentrant.
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<pre><b>
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/* allocate deflate state */
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strm.zalloc = Z_NULL;
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strm.zfree = Z_NULL;
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strm.opaque = Z_NULL;
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ret = deflateInit(&strm, level);
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if (ret != Z_OK)
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return ret;
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</b></pre><!-- -->
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With the pleasantries out of the way, now we can get down to business. The outer <tt>do</tt>-loop
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reads all of the input file and exits at the bottom of the loop once end-of-file is reached.
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This loop contains the only call of <tt>deflate()</tt>. So we must make sure that all of the
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input data has been processed and that all of the output data has been generated and consumed
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before we fall out of the loop at the bottom.
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<pre><b>
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/* compress until end of file */
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do {
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</b></pre>
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We start off by reading data from the input file. The number of bytes read is put directly
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into <tt>avail_in</tt>, and a pointer to those bytes is put into <tt>next_in</tt>. We also
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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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<em>zlib</em> constant <tt>Z_FINISH</tt>, which is later passed to <tt>deflate()</tt> to
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indicate that this is the last chunk of input data to compress. We need to use <tt>feof()</tt>
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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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reason is that if the input file length is an exact multiple of <tt>CHUNK</tt>, we will miss
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the fact that we got to the end-of-file, and not know to tell <tt>deflate()</tt> to finish
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up the compressed stream. If we are not yet at the end of the input, then the <em>zlib</em>
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constant <tt>Z_NO_FLUSH</tt> will be passed to <tt>deflate</tt> to indicate that we are still
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in the middle of the uncompressed data.
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<p>
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If there is an error in reading from the input file, the process is aborted with
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<tt>deflateEnd()</tt> being called to free the allocated <em>zlib</em> state before returning
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the error. We wouldn't want a memory leak, now would we? <tt>deflateEnd()</tt> can be called
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at any time after the state has been initialized. Once that's done, <tt>deflateInit()</tt> (or
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<tt>deflateInit2()</tt>) would have to be called to start a new compression process. There is
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no point here in checking the <tt>deflateEnd()</tt> return code. The deallocation can't fail.
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<pre><b>
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strm.avail_in = fread(in, 1, CHUNK, source);
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if (ferror(source)) {
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(void)deflateEnd(&strm);
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return Z_ERRNO;
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}
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flush = feof(source) ? Z_FINISH : Z_NO_FLUSH;
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strm.next_in = in;
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</b></pre><!-- -->
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The inner <tt>do</tt>-loop passes our chunk of input data to <tt>deflate()</tt>, and then
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keeps calling <tt>deflate()</tt> until it is done producing output. Once there is no more
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new output, <tt>deflate()</tt> is guaranteed to have consumed all of the input, i.e.,
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<tt>avail_in</tt> will be zero.
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<pre><b>
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/* run deflate() on input until output buffer not full, finish
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compression if all of source has been read in */
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do {
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</b></pre>
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Output space is provided to <tt>deflate()</tt> by setting <tt>avail_out</tt> to the number
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of available output bytes and <tt>next_out</tt> to a pointer to that space.
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<pre><b>
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strm.avail_out = CHUNK;
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strm.next_out = out;
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</b></pre>
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Now we call the compression engine itself, <tt>deflate()</tt>. It takes as many of the
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<tt>avail_in</tt> bytes at <tt>next_in</tt> as it can process, and writes as many as
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<tt>avail_out</tt> bytes to <tt>next_out</tt>. Those counters and pointers are then
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updated past the input data consumed and the output data written. It is the amount of
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output space available that may limit how much input is consumed.
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Hence the inner loop to make sure that
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all of the input is consumed by providing more output space each time. Since <tt>avail_in</tt>
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and <tt>next_in</tt> are updated by <tt>deflate()</tt>, we don't have to mess with those
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between <tt>deflate()</tt> calls until it's all used up.
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<p>
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The parameters to <tt>deflate()</tt> are a pointer to the <tt>strm</tt> structure containing
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the input and output information and the internal compression engine state, and a parameter
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indicating whether and how to flush data to the output. Normally <tt>deflate</tt> will consume
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several K bytes of input data before producing any output (except for the header), in order
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to accumulate statistics on the data for optimum compression. It will then put out a burst of
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compressed data, and proceed to consume more input before the next burst. Eventually,
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<tt>deflate()</tt>
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must be told to terminate the stream, complete the compression with provided input data, and
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write out the trailer check value. <tt>deflate()</tt> will continue to compress normally as long
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as the flush parameter is <tt>Z_NO_FLUSH</tt>. Once the <tt>Z_FINISH</tt> parameter is provided,
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<tt>deflate()</tt> will begin to complete the compressed output stream. However depending on how
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much output space is provided, <tt>deflate()</tt> may have to be called several times until it
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has provided the complete compressed stream, even after it has consumed all of the input. The flush
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parameter must continue to be <tt>Z_FINISH</tt> for those subsequent calls.
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<p>
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There are other values of the flush parameter that are used in more advanced applications. You can
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force <tt>deflate()</tt> to produce a burst of output that encodes all of the input data provided
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so far, even if it wouldn't have otherwise, for example to control data latency on a link with
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compressed data. You can also ask that <tt>deflate()</tt> do that as well as erase any history up to
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that point so that what follows can be decompressed independently, for example for random access
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applications. Both requests will degrade compression by an amount depending on how often such
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requests are made.
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<p>
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<tt>deflate()</tt> has a return value that can indicate errors, yet we do not check it here. Why
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not? Well, it turns out that <tt>deflate()</tt> can do no wrong here. Let's go through
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<tt>deflate()</tt>'s return values and dispense with them one by one. The possible values are
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<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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is, well, ok. <tt>Z_STREAM_END</tt> is also ok and will be returned for the last call of
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<tt>deflate()</tt>. This is already guaranteed by calling <tt>deflate()</tt> with <tt>Z_FINISH</tt>
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until it has no more output. <tt>Z_STREAM_ERROR</tt> is only possible if the stream is not
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initialized properly, but we did initialize it properly. There is no harm in checking for
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<tt>Z_STREAM_ERROR</tt> here, for example to check for the possibility that some
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other part of the application inadvertently clobbered the memory containing the <em>zlib</em> state.
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<tt>Z_BUF_ERROR</tt> will be explained further below, but
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suffice it to say that this is simply an indication that <tt>deflate()</tt> could not consume
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more input or produce more output. <tt>deflate()</tt> can be called again with more output space
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or more available input, which it will be in this code.
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<pre><b>
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ret = deflate(&strm, flush); /* no bad return value */
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assert(ret != Z_STREAM_ERROR); /* state not clobbered */
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</b></pre>
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Now we compute how much output <tt>deflate()</tt> provided on the last call, which is the
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difference between how much space was provided before the call, and how much output space
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is still available after the call. Then that data, if any, is written to the output file.
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We can then reuse the output buffer for the next call of <tt>deflate()</tt>. Again if there
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is a file i/o error, we call <tt>deflateEnd()</tt> before returning to avoid a memory leak.
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<pre><b>
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have = CHUNK - strm.avail_out;
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if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
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(void)deflateEnd(&strm);
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return Z_ERRNO;
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}
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</b></pre>
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The inner <tt>do</tt>-loop is repeated until the last <tt>deflate()</tt> call fails to fill the
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provided output buffer. Then we know that <tt>deflate()</tt> has done as much as it can with
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the provided input, and that all of that input has been consumed. We can then fall out of this
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loop and reuse the input buffer.
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<p>
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|
|
The way we tell that <tt>deflate()</tt> has no more output is by seeing that it did not fill
|
||
|
|
the output buffer, leaving <tt>avail_out</tt> greater than zero. However suppose that
|
||
|
|
<tt>deflate()</tt> has no more output, but just so happened to exactly fill the output buffer!
|
||
|
|
<tt>avail_out</tt> is zero, and we can't tell that <tt>deflate()</tt> has done all it can.
|
||
|
|
As far as we know, <tt>deflate()</tt>
|
||
|
|
has more output for us. So we call it again. But now <tt>deflate()</tt> produces no output
|
||
|
|
at all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call
|
||
|
|
wasn't able to do anything, either consume input or produce output, and so it returns
|
||
|
|
<tt>Z_BUF_ERROR</tt>. (See, I told you I'd cover this later.) However this is not a problem at
|
||
|
|
all. Now we finally have the desired indication that <tt>deflate()</tt> is really done,
|
||
|
|
and so we drop out of the inner loop to provide more input to <tt>deflate()</tt>.
|
||
|
|
<p>
|
||
|
|
With <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will
|
||
|
|
complete the output stream. Once that is done, subsequent calls of <tt>deflate()</tt> would return
|
||
|
|
<tt>Z_STREAM_ERROR</tt> if the flush parameter is not <tt>Z_FINISH</tt>, and do no more processing
|
||
|
|
until the state is reinitialized.
|
||
|
|
<p>
|
||
|
|
Some applications of <em>zlib</em> have two loops that call <tt>deflate()</tt>
|
||
|
|
instead of the single inner loop we have here. The first loop would call
|
||
|
|
without flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call
|
||
|
|
<tt>deflate()</tt> with no more
|
||
|
|
data and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this
|
||
|
|
example, that can be avoided by simply keeping track of the current flush state.
|
||
|
|
<pre><b>
|
||
|
|
} while (strm.avail_out == 0);
|
||
|
|
assert(strm.avail_in == 0); /* all input will be used */
|
||
|
|
</b></pre><!-- -->
|
||
|
|
Now we check to see if we have already processed all of the input file. That information was
|
||
|
|
saved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so,
|
||
|
|
then we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt>
|
||
|
|
from the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was
|
||
|
|
consumed and all of the output was generated.
|
||
|
|
<pre><b>
|
||
|
|
/* done when last data in file processed */
|
||
|
|
} while (flush != Z_FINISH);
|
||
|
|
assert(ret == Z_STREAM_END); /* stream will be complete */
|
||
|
|
</b></pre><!-- -->
|
||
|
|
The process is complete, but we still need to deallocate the state to avoid a memory leak
|
||
|
|
(or rather more like a memory hemorrhage if you didn't do this). Then
|
||
|
|
finally we can return with a happy return value.
|
||
|
|
<pre><b>
|
||
|
|
/* clean up and return */
|
||
|
|
(void)deflateEnd(&strm);
|
||
|
|
return Z_OK;
|
||
|
|
}
|
||
|
|
</b></pre><!-- -->
|
||
|
|
Now we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt>
|
||
|
|
decompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the
|
||
|
|
uncompressed data to the output file. Much of the discussion above for <tt>def()</tt>
|
||
|
|
applies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between
|
||
|
|
the two.
|
||
|
|
<pre><b>
|
||
|
|
/* Decompress from file source to file dest until stream ends or EOF.
|
||
|
|
inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be
|
||
|
|
allocated for processing, Z_DATA_ERROR if the deflate data is
|
||
|
|
invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and
|
||
|
|
the version of the library linked do not match, or Z_ERRNO if there
|
||
|
|
is an error reading or writing the files. */
|
||
|
|
int inf(FILE *source, FILE *dest)
|
||
|
|
{
|
||
|
|
</b></pre>
|
||
|
|
The local variables have the same functionality as they do for <tt>def()</tt>. The
|
||
|
|
only difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt>
|
||
|
|
can tell from the <em>zlib</em> stream itself when the stream is complete.
|
||
|
|
<pre><b>
|
||
|
|
int ret;
|
||
|
|
unsigned have;
|
||
|
|
z_stream strm;
|
||
|
|
unsigned char in[CHUNK];
|
||
|
|
unsigned char out[CHUNK];
|
||
|
|
</b></pre><!-- -->
|
||
|
|
The initialization of the state is the same, except that there is no compression level,
|
||
|
|
of course, and two more elements of the structure are initialized. <tt>avail_in</tt>
|
||
|
|
and <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This
|
||
|
|
is because the application has the option to provide the start of the zlib stream in
|
||
|
|
order for <tt>inflateInit()</tt> to have access to information about the compression
|
||
|
|
method to aid in memory allocation. In the current implementation of <em>zlib</em>
|
||
|
|
(up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of
|
||
|
|
<tt>inflate()</tt> anyway. However those fields must be initialized since later versions
|
||
|
|
of <em>zlib</em> that provide more compression methods may take advantage of this interface.
|
||
|
|
In any case, no decompression is performed by <tt>inflateInit()</tt>, so the
|
||
|
|
<tt>avail_out</tt> and <tt>next_out</tt> fields do not need to be initialized before calling.
|
||
|
|
<p>
|
||
|
|
Here <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to
|
||
|
|
indicate that no input data is being provided.
|
||
|
|
<pre><b>
|
||
|
|
/* allocate inflate state */
|
||
|
|
strm.zalloc = Z_NULL;
|
||
|
|
strm.zfree = Z_NULL;
|
||
|
|
strm.opaque = Z_NULL;
|
||
|
|
strm.avail_in = 0;
|
||
|
|
strm.next_in = Z_NULL;
|
||
|
|
ret = inflateInit(&strm);
|
||
|
|
if (ret != Z_OK)
|
||
|
|
return ret;
|
||
|
|
</b></pre><!-- -->
|
||
|
|
The outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates
|
||
|
|
that it has reached the end of the compressed data and has produced all of the uncompressed
|
||
|
|
output. This is in contrast to <tt>def()</tt> which processes all of the input file.
|
||
|
|
If end-of-file is reached before the compressed data self-terminates, then the compressed
|
||
|
|
data is incomplete and an error is returned.
|
||
|
|
<pre><b>
|
||
|
|
/* decompress until deflate stream ends or end of file */
|
||
|
|
do {
|
||
|
|
</b></pre>
|
||
|
|
We read input data and set the <tt>strm</tt> structure accordingly. If we've reached the
|
||
|
|
end of the input file, then we leave the outer loop and report an error, since the
|
||
|
|
compressed data is incomplete. Note that we may read more data than is eventually consumed
|
||
|
|
by <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream.
|
||
|
|
For applications where <em>zlib</em> streams are embedded in other data, this routine would
|
||
|
|
need to be modified to return the unused data, or at least indicate how much of the input
|
||
|
|
data was not used, so the application would know where to pick up after the <em>zlib</em> stream.
|
||
|
|
<pre><b>
|
||
|
|
strm.avail_in = fread(in, 1, CHUNK, source);
|
||
|
|
if (ferror(source)) {
|
||
|
|
(void)inflateEnd(&strm);
|
||
|
|
return Z_ERRNO;
|
||
|
|
}
|
||
|
|
if (strm.avail_in == 0)
|
||
|
|
break;
|
||
|
|
strm.next_in = in;
|
||
|
|
</b></pre><!-- -->
|
||
|
|
The inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to
|
||
|
|
keep calling <tt>inflate()</tt> until has generated all of the output it can with the
|
||
|
|
provided input.
|
||
|
|
<pre><b>
|
||
|
|
/* run inflate() on input until output buffer not full */
|
||
|
|
do {
|
||
|
|
</b></pre>
|
||
|
|
Just like in <tt>def()</tt>, the same output space is provided for each call of <tt>inflate()</tt>.
|
||
|
|
<pre><b>
|
||
|
|
strm.avail_out = CHUNK;
|
||
|
|
strm.next_out = out;
|
||
|
|
</b></pre>
|
||
|
|
Now we run the decompression engine itself. There is no need to adjust the flush parameter, since
|
||
|
|
the <em>zlib</em> format is self-terminating. The main difference here is that there are
|
||
|
|
return values that we need to pay attention to. <tt>Z_DATA_ERROR</tt>
|
||
|
|
indicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format,
|
||
|
|
which means that either the data is not a <em>zlib</em> stream to begin with, or that the data was
|
||
|
|
corrupted somewhere along the way since it was compressed. The other error to be processed is
|
||
|
|
<tt>Z_MEM_ERROR</tt>, which can occur since memory allocation is deferred until <tt>inflate()</tt>
|
||
|
|
needs it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>.
|
||
|
|
<p>
|
||
|
|
Advanced applications may use
|
||
|
|
<tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the
|
||
|
|
first 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt>
|
||
|
|
requests that that dictionary be provided before it can start to decompress. Without the dictionary,
|
||
|
|
correct decompression is not possible. For this routine, we have no idea what the dictionary is,
|
||
|
|
so the <tt>Z_NEED_DICT</tt> indication is converted to a <tt>Z_DATA_ERROR</tt>.
|
||
|
|
<p>
|
||
|
|
<tt>inflate()</tt> can also return <tt>Z_STREAM_ERROR</tt>, which should not be possible here,
|
||
|
|
but could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be
|
||
|
|
checked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be
|
||
|
|
checked for later.
|
||
|
|
<pre><b>
|
||
|
|
ret = inflate(&strm, Z_NO_FLUSH);
|
||
|
|
assert(ret != Z_STREAM_ERROR); /* state not clobbered */
|
||
|
|
switch (ret) {
|
||
|
|
case Z_NEED_DICT:
|
||
|
|
ret = Z_DATA_ERROR; /* and fall through */
|
||
|
|
case Z_DATA_ERROR:
|
||
|
|
case Z_MEM_ERROR:
|
||
|
|
(void)inflateEnd(&strm);
|
||
|
|
return ret;
|
||
|
|
}
|
||
|
|
</b></pre>
|
||
|
|
The output of <tt>inflate()</tt> is handled identically to that of <tt>deflate()</tt>.
|
||
|
|
<pre><b>
|
||
|
|
have = CHUNK - strm.avail_out;
|
||
|
|
if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
|
||
|
|
(void)inflateEnd(&strm);
|
||
|
|
return Z_ERRNO;
|
||
|
|
}
|
||
|
|
</b></pre>
|
||
|
|
The inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated
|
||
|
|
by not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot
|
||
|
|
assert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file
|
||
|
|
does.
|
||
|
|
<pre><b>
|
||
|
|
} while (strm.avail_out == 0);
|
||
|
|
</b></pre><!-- -->
|
||
|
|
The outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the
|
||
|
|
end of the input <em>zlib</em> stream, has completed the decompression and integrity
|
||
|
|
check, and has provided all of the output. This is indicated by the <tt>inflate()</tt>
|
||
|
|
return value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt>
|
||
|
|
equal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end
|
||
|
|
of the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the
|
||
|
|
loop continues to read more input.
|
||
|
|
<pre><b>
|
||
|
|
/* done when inflate() says it's done */
|
||
|
|
} while (ret != Z_STREAM_END);
|
||
|
|
</b></pre><!-- -->
|
||
|
|
At this point, decompression successfully completed, or we broke out of the loop due to no
|
||
|
|
more data being available from the input file. If the last <tt>inflate()</tt> return value
|
||
|
|
is not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error
|
||
|
|
is returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt>
|
||
|
|
is called first to avoid a memory leak.
|
||
|
|
<pre><b>
|
||
|
|
/* clean up and return */
|
||
|
|
(void)inflateEnd(&strm);
|
||
|
|
return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR;
|
||
|
|
}
|
||
|
|
</b></pre><!-- -->
|
||
|
|
That ends the routines that directly use <em>zlib</em>. The following routines make this
|
||
|
|
a command-line program by running data through the above routines from <tt>stdin</tt> to
|
||
|
|
<tt>stdout</tt>, and handling any errors reported by <tt>def()</tt> or <tt>inf()</tt>.
|
||
|
|
<p>
|
||
|
|
<tt>zerr()</tt> is used to interpret the possible error codes from <tt>def()</tt>
|
||
|
|
and <tt>inf()</tt>, as detailed in their comments above, and print out an error message.
|
||
|
|
Note that these are only a subset of the possible return values from <tt>deflate()</tt>
|
||
|
|
and <tt>inflate()</tt>.
|
||
|
|
<pre><b>
|
||
|
|
/* report a zlib or i/o error */
|
||
|
|
void zerr(int ret)
|
||
|
|
{
|
||
|
|
fputs("zpipe: ", stderr);
|
||
|
|
switch (ret) {
|
||
|
|
case Z_ERRNO:
|
||
|
|
if (ferror(stdin))
|
||
|
|
fputs("error reading stdin\n", stderr);
|
||
|
|
if (ferror(stdout))
|
||
|
|
fputs("error writing stdout\n", stderr);
|
||
|
|
break;
|
||
|
|
case Z_STREAM_ERROR:
|
||
|
|
fputs("invalid compression level\n", stderr);
|
||
|
|
break;
|
||
|
|
case Z_DATA_ERROR:
|
||
|
|
fputs("invalid or incomplete deflate data\n", stderr);
|
||
|
|
break;
|
||
|
|
case Z_MEM_ERROR:
|
||
|
|
fputs("out of memory\n", stderr);
|
||
|
|
break;
|
||
|
|
case Z_VERSION_ERROR:
|
||
|
|
fputs("zlib version mismatch!\n", stderr);
|
||
|
|
}
|
||
|
|
}
|
||
|
|
</b></pre><!-- -->
|
||
|
|
Here is the <tt>main()</tt> routine used to test <tt>def()</tt> and <tt>inf()</tt>. The
|
||
|
|
<tt>zpipe</tt> command is simply a compression pipe from <tt>stdin</tt> to <tt>stdout</tt>, if
|
||
|
|
no arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other
|
||
|
|
arguments are provided, no compression or decompression is performed. Instead a usage
|
||
|
|
message is displayed. Examples are <tt>zpipe < foo.txt > foo.txt.z</tt> to compress, and
|
||
|
|
<tt>zpipe -d < foo.txt.z > foo.txt</tt> to decompress.
|
||
|
|
<pre><b>
|
||
|
|
/* compress or decompress from stdin to stdout */
|
||
|
|
int main(int argc, char **argv)
|
||
|
|
{
|
||
|
|
int ret;
|
||
|
|
|
||
|
|
/* avoid end-of-line conversions */
|
||
|
|
SET_BINARY_MODE(stdin);
|
||
|
|
SET_BINARY_MODE(stdout);
|
||
|
|
|
||
|
|
/* do compression if no arguments */
|
||
|
|
if (argc == 1) {
|
||
|
|
ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION);
|
||
|
|
if (ret != Z_OK)
|
||
|
|
zerr(ret);
|
||
|
|
return ret;
|
||
|
|
}
|
||
|
|
|
||
|
|
/* do decompression if -d specified */
|
||
|
|
else if (argc == 2 && strcmp(argv[1], "-d") == 0) {
|
||
|
|
ret = inf(stdin, stdout);
|
||
|
|
if (ret != Z_OK)
|
||
|
|
zerr(ret);
|
||
|
|
return ret;
|
||
|
|
}
|
||
|
|
|
||
|
|
/* otherwise, report usage */
|
||
|
|
else {
|
||
|
|
fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr);
|
||
|
|
return 1;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
</b></pre>
|
||
|
|
<hr>
|
||
|
|
<i>Copyright (c) 2004, 2005 by Mark Adler<br>Last modified 11 December 2005</i>
|
||
|
|
</body>
|
||
|
|
</html>
|