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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" |
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"http://www.w3.org/TR/html4/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) 2004-2026 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 ado, 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 1.5 11 February 2026 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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1.5 11 Feb 2026 Use underscores for Windows POSIX names |
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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 using feof(). |
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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. |
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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 |
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the output buffer, leaving <tt>avail_out</tt> greater than zero. However suppose that |
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<tt>deflate()</tt> has no more output, but just so happened to exactly fill the output buffer! |
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<tt>avail_out</tt> is zero, and we can't tell that <tt>deflate()</tt> has done all it can. |
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As far as we know, <tt>deflate()</tt> |
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has more output for us. So we call it again. But now <tt>deflate()</tt> produces no output |
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at all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call |
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wasn't able to do anything, either consume input or produce output, and so it returns |
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<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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all. Now we finally have the desired indication that <tt>deflate()</tt> is really done, |
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and so we drop out of the inner loop to provide more input to <tt>deflate()</tt>. |
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<p> |
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With <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will |
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complete the output stream. Once that is done, subsequent calls of <tt>deflate()</tt> would return |
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<tt>Z_STREAM_ERROR</tt> if the flush parameter is not <tt>Z_FINISH</tt>, and do no more processing |
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until the state is reinitialized. |
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<p> |
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Some applications of <em>zlib</em> have two loops that call <tt>deflate()</tt> |
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instead of the single inner loop we have here. The first loop would call |
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without flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call |
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<tt>deflate()</tt> with no more |
|
286
|
data and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this |
|
287
|
example, that can be avoided by simply keeping track of the current flush state. |
|
288
|
<pre><b> |
|
289
|
} while (strm.avail_out == 0); |
|
290
|
assert(strm.avail_in == 0); /* all input will be used */ |
|
291
|
</b></pre><!-- --> |
|
292
|
Now we check to see if we have already processed all of the input file. That information was |
|
293
|
saved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so, |
|
294
|
then we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt> |
|
295
|
from the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was |
|
296
|
consumed and all of the output was generated. |
|
297
|
<pre><b> |
|
298
|
/* done when last data in file processed */ |
|
299
|
} while (flush != Z_FINISH); |
|
300
|
assert(ret == Z_STREAM_END); /* stream will be complete */ |
|
301
|
</b></pre><!-- --> |
|
302
|
The process is complete, but we still need to deallocate the state to avoid a memory leak |
|
303
|
(or rather more like a memory hemorrhage if you didn't do this). Then |
|
304
|
finally we can return with a happy return value. |
|
305
|
<pre><b> |
|
306
|
/* clean up and return */ |
|
307
|
(void)deflateEnd(&strm); |
|
308
|
return Z_OK; |
|
309
|
} |
|
310
|
</b></pre><!-- --> |
|
311
|
Now we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt> |
|
312
|
decompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the |
|
313
|
uncompressed data to the output file. Much of the discussion above for <tt>def()</tt> |
|
314
|
applies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between |
|
315
|
the two. |
|
316
|
<pre><b> |
|
317
|
/* Decompress from file source to file dest until stream ends or EOF. |
|
318
|
inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be |
|
319
|
allocated for processing, Z_DATA_ERROR if the deflate data is |
|
320
|
invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and |
|
321
|
the version of the library linked do not match, or Z_ERRNO if there |
|
322
|
is an error reading or writing the files. */ |
|
323
|
int inf(FILE *source, FILE *dest) |
|
324
|
{ |
|
325
|
</b></pre> |
|
326
|
The local variables have the same functionality as they do for <tt>def()</tt>. The |
|
327
|
only difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt> |
|
328
|
can tell from the <em>zlib</em> stream itself when the stream is complete. |
|
329
|
<pre><b> |
|
330
|
int ret; |
|
331
|
unsigned have; |
|
332
|
z_stream strm; |
|
333
|
unsigned char in[CHUNK]; |
|
334
|
unsigned char out[CHUNK]; |
|
335
|
</b></pre><!-- --> |
|
336
|
The initialization of the state is the same, except that there is no compression level, |
|
337
|
of course, and two more elements of the structure are initialized. <tt>avail_in</tt> |
|
338
|
and <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This |
|
339
|
is because the application has the option to provide the start of the zlib stream in |
|
340
|
order for <tt>inflateInit()</tt> to have access to information about the compression |
|
341
|
method to aid in memory allocation. In the current implementation of <em>zlib</em> |
|
342
|
(up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of |
|
343
|
<tt>inflate()</tt> anyway. However those fields must be initialized since later versions |
|
344
|
of <em>zlib</em> that provide more compression methods may take advantage of this interface. |
|
345
|
In any case, no decompression is performed by <tt>inflateInit()</tt>, so the |
|
346
|
<tt>avail_out</tt> and <tt>next_out</tt> fields do not need to be initialized before calling. |
|
347
|
<p> |
|
348
|
Here <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to |
|
349
|
indicate that no input data is being provided. |
|
350
|
<pre><b> |
|
351
|
/* allocate inflate state */ |
|
352
|
strm.zalloc = Z_NULL; |
|
353
|
strm.zfree = Z_NULL; |
|
354
|
strm.opaque = Z_NULL; |
|
355
|
strm.avail_in = 0; |
|
356
|
strm.next_in = Z_NULL; |
|
357
|
ret = inflateInit(&strm); |
|
358
|
if (ret != Z_OK) |
|
359
|
return ret; |
|
360
|
</b></pre><!-- --> |
|
361
|
The outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates |
|
362
|
that it has reached the end of the compressed data and has produced all of the uncompressed |
|
363
|
output. This is in contrast to <tt>def()</tt> which processes all of the input file. |
|
364
|
If end-of-file is reached before the compressed data self-terminates, then the compressed |
|
365
|
data is incomplete and an error is returned. |
|
366
|
<pre><b> |
|
367
|
/* decompress until deflate stream ends or end of file */ |
|
368
|
do { |
|
369
|
</b></pre> |
|
370
|
We read input data and set the <tt>strm</tt> structure accordingly. If we've reached the |
|
371
|
end of the input file, then we leave the outer loop and report an error, since the |
|
372
|
compressed data is incomplete. Note that we may read more data than is eventually consumed |
|
373
|
by <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream. |
|
374
|
For applications where <em>zlib</em> streams are embedded in other data, this routine would |
|
375
|
need to be modified to return the unused data, or at least indicate how much of the input |
|
376
|
data was not used, so the application would know where to pick up after the <em>zlib</em> stream. |
|
377
|
<pre><b> |
|
378
|
strm.avail_in = fread(in, 1, CHUNK, source); |
|
379
|
if (ferror(source)) { |
|
380
|
(void)inflateEnd(&strm); |
|
381
|
return Z_ERRNO; |
|
382
|
} |
|
383
|
if (strm.avail_in == 0) |
|
384
|
break; |
|
385
|
strm.next_in = in; |
|
386
|
</b></pre><!-- --> |
|
387
|
The inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to |
|
388
|
keep calling <tt>inflate()</tt> until has generated all of the output it can with the |
|
389
|
provided input. |
|
390
|
<pre><b> |
|
391
|
/* run inflate() on input until output buffer not full */ |
|
392
|
do { |
|
393
|
</b></pre> |
|
394
|
Just like in <tt>def()</tt>, the same output space is provided for each call of <tt>inflate()</tt>. |
|
395
|
<pre><b> |
|
396
|
strm.avail_out = CHUNK; |
|
397
|
strm.next_out = out; |
|
398
|
</b></pre> |
|
399
|
Now we run the decompression engine itself. There is no need to adjust the flush parameter, since |
|
400
|
the <em>zlib</em> format is self-terminating. The main difference here is that there are |
|
401
|
return values that we need to pay attention to. <tt>Z_DATA_ERROR</tt> |
|
402
|
indicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format, |
|
403
|
which means that either the data is not a <em>zlib</em> stream to begin with, or that the data was |
|
404
|
corrupted somewhere along the way since it was compressed. The other error to be processed is |
|
405
|
<tt>Z_MEM_ERROR</tt>, which can occur since memory allocation is deferred until <tt>inflate()</tt> |
|
406
|
needs it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>. |
|
407
|
<p> |
|
408
|
Advanced applications may use |
|
409
|
<tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the |
|
410
|
first 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt> |
|
411
|
requests that that dictionary be provided before it can start to decompress. Without the dictionary, |
|
412
|
correct decompression is not possible. For this routine, we have no idea what the dictionary is, |
|
413
|
so the <tt>Z_NEED_DICT</tt> indication is converted to a <tt>Z_DATA_ERROR</tt>. |
|
414
|
<p> |
|
415
|
<tt>inflate()</tt> can also return <tt>Z_STREAM_ERROR</tt>, which should not be possible here, |
|
416
|
but could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be |
|
417
|
checked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be |
|
418
|
checked for later. |
|
419
|
<pre><b> |
|
420
|
ret = inflate(&strm, Z_NO_FLUSH); |
|
421
|
assert(ret != Z_STREAM_ERROR); /* state not clobbered */ |
|
422
|
switch (ret) { |
|
423
|
case Z_NEED_DICT: |
|
424
|
ret = Z_DATA_ERROR; /* and fall through */ |
|
425
|
case Z_DATA_ERROR: |
|
426
|
case Z_MEM_ERROR: |
|
427
|
(void)inflateEnd(&strm); |
|
428
|
return ret; |
|
429
|
} |
|
430
|
</b></pre> |
|
431
|
The output of <tt>inflate()</tt> is handled identically to that of <tt>deflate()</tt>. |
|
432
|
<pre><b> |
|
433
|
have = CHUNK - strm.avail_out; |
|
434
|
if (fwrite(out, 1, have, dest) != have || ferror(dest)) { |
|
435
|
(void)inflateEnd(&strm); |
|
436
|
return Z_ERRNO; |
|
437
|
} |
|
438
|
</b></pre> |
|
439
|
The inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated |
|
440
|
by not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot |
|
441
|
assert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file |
|
442
|
does. |
|
443
|
<pre><b> |
|
444
|
} while (strm.avail_out == 0); |
|
445
|
</b></pre><!-- --> |
|
446
|
The outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the |
|
447
|
end of the input <em>zlib</em> stream, has completed the decompression and integrity |
|
448
|
check, and has provided all of the output. This is indicated by the <tt>inflate()</tt> |
|
449
|
return value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt> |
|
450
|
equal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end |
|
451
|
of the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the |
|
452
|
loop continues to read more input. |
|
453
|
<pre><b> |
|
454
|
/* done when inflate() says it's done */ |
|
455
|
} while (ret != Z_STREAM_END); |
|
456
|
</b></pre><!-- --> |
|
457
|
At this point, decompression successfully completed, or we broke out of the loop due to no |
|
458
|
more data being available from the input file. If the last <tt>inflate()</tt> return value |
|
459
|
is not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error |
|
460
|
is returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt> |
|
461
|
is called first to avoid a memory leak. |
|
462
|
<pre><b> |
|
463
|
/* clean up and return */ |
|
464
|
(void)inflateEnd(&strm); |
|
465
|
return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR; |
|
466
|
} |
|
467
|
</b></pre><!-- --> |
|
468
|
That ends the routines that directly use <em>zlib</em>. The following routines make this |
|
469
|
a command-line program by running data through the above routines from <tt>stdin</tt> to |
|
470
|
<tt>stdout</tt>, and handling any errors reported by <tt>def()</tt> or <tt>inf()</tt>. |
|
471
|
<p> |
|
472
|
<tt>zerr()</tt> is used to interpret the possible error codes from <tt>def()</tt> |
|
473
|
and <tt>inf()</tt>, as detailed in their comments above, and print out an error message. |
|
474
|
Note that these are only a subset of the possible return values from <tt>deflate()</tt> |
|
475
|
and <tt>inflate()</tt>. |
|
476
|
<pre><b> |
|
477
|
/* report a zlib or i/o error */ |
|
478
|
void zerr(int ret) |
|
479
|
{ |
|
480
|
fputs("zpipe: ", stderr); |
|
481
|
switch (ret) { |
|
482
|
case Z_ERRNO: |
|
483
|
if (ferror(stdin)) |
|
484
|
fputs("error reading stdin\n", stderr); |
|
485
|
if (ferror(stdout)) |
|
486
|
fputs("error writing stdout\n", stderr); |
|
487
|
break; |
|
488
|
case Z_STREAM_ERROR: |
|
489
|
fputs("invalid compression level\n", stderr); |
|
490
|
break; |
|
491
|
case Z_DATA_ERROR: |
|
492
|
fputs("invalid or incomplete deflate data\n", stderr); |
|
493
|
break; |
|
494
|
case Z_MEM_ERROR: |
|
495
|
fputs("out of memory\n", stderr); |
|
496
|
break; |
|
497
|
case Z_VERSION_ERROR: |
|
498
|
fputs("zlib version mismatch!\n", stderr); |
|
499
|
} |
|
500
|
} |
|
501
|
</b></pre><!-- --> |
|
502
|
Here is the <tt>main()</tt> routine used to test <tt>def()</tt> and <tt>inf()</tt>. The |
|
503
|
<tt>zpipe</tt> command is simply a compression pipe from <tt>stdin</tt> to <tt>stdout</tt>, if |
|
504
|
no arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other |
|
505
|
arguments are provided, no compression or decompression is performed. Instead a usage |
|
506
|
message is displayed. Examples are <tt>zpipe < foo.txt > foo.txt.z</tt> to compress, and |
|
507
|
<tt>zpipe -d < foo.txt.z > foo.txt</tt> to decompress. |
|
508
|
<pre><b> |
|
509
|
/* compress or decompress from stdin to stdout */ |
|
510
|
int main(int argc, char **argv) |
|
511
|
{ |
|
512
|
int ret; |
|
513
|
|
|
514
|
/* avoid end-of-line conversions */ |
|
515
|
SET_BINARY_MODE(stdin); |
|
516
|
SET_BINARY_MODE(stdout); |
|
517
|
|
|
518
|
/* do compression if no arguments */ |
|
519
|
if (argc == 1) { |
|
520
|
ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION); |
|
521
|
if (ret != Z_OK) |
|
522
|
zerr(ret); |
|
523
|
return ret; |
|
524
|
} |
|
525
|
|
|
526
|
/* do decompression if -d specified */ |
|
527
|
else if (argc == 2 && strcmp(argv[1], "-d") == 0) { |
|
528
|
ret = inf(stdin, stdout); |
|
529
|
if (ret != Z_OK) |
|
530
|
zerr(ret); |
|
531
|
return ret; |
|
532
|
} |
|
533
|
|
|
534
|
/* otherwise, report usage */ |
|
535
|
else { |
|
536
|
fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr); |
|
537
|
return 1; |
|
538
|
} |
|
539
|
} |
|
540
|
</b></pre> |
|
541
|
<hr> |
|
542
|
<i>Last modified 12 February 2026<br> |
|
543
|
Copyright © 2004-2026 Mark Adler</i><br> |
|
544
|
<a rel="license" href="https://creativecommons.org/licenses/by-nd/4.0/"> |
|
545
|
<img alt="Creative Commons License" style="border-width:0" |
|
546
|
src="https://i.creativecommons.org/l/by-nd/4.0/88x31.png"></a> |
|
547
|
<a rel="license" href="https://creativecommons.org/licenses/by-nd/4.0/"> |
|
548
|
Creative Commons Attribution-NoDerivatives 4.0 International License</a>. |
|
549
|
</body> |
|
550
|
</html> |
|
551
|
|