1*9573673dSchristos<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 2*9573673dSchristos "http://www.w3.org/TR/REC-html40/loose.dtd"> 3*9573673dSchristos<html> 4*9573673dSchristos<head> 5*9573673dSchristos<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 6*9573673dSchristos<title>zlib Usage Example</title> 7*9573673dSchristos<!-- Copyright (c) 2004, 2005 Mark Adler. --> 8*9573673dSchristos</head> 9*9573673dSchristos<body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#00A000"> 10*9573673dSchristos<h2 align="center"> zlib Usage Example </h2> 11*9573673dSchristosWe often get questions about how the <tt>deflate()</tt> and <tt>inflate()</tt> functions should be used. 12*9573673dSchristosUsers wonder when they should provide more input, when they should use more output, 13*9573673dSchristoswhat to do with a <tt>Z_BUF_ERROR</tt>, how to make sure the process terminates properly, and 14*9573673dSchristosso on. So for those who have read <tt>zlib.h</tt> (a few times), and 15*9573673dSchristoswould like further edification, below is an annotated example in C of simple routines to compress and decompress 16*9573673dSchristosfrom an input file to an output file using <tt>deflate()</tt> and <tt>inflate()</tt> respectively. The 17*9573673dSchristosannotations are interspersed between lines of the code. So please read between the lines. 18*9573673dSchristosWe hope this helps explain some of the intricacies of <em>zlib</em>. 19*9573673dSchristos<p> 20*9573673dSchristosWithout further adieu, here is the program <a href="zpipe.c"><tt>zpipe.c</tt></a>: 21*9573673dSchristos<pre><b> 22*9573673dSchristos/* zpipe.c: example of proper use of zlib's inflate() and deflate() 23*9573673dSchristos Not copyrighted -- provided to the public domain 24*9573673dSchristos Version 1.4 11 December 2005 Mark Adler */ 25*9573673dSchristos 26*9573673dSchristos/* Version history: 27*9573673dSchristos 1.0 30 Oct 2004 First version 28*9573673dSchristos 1.1 8 Nov 2004 Add void casting for unused return values 29*9573673dSchristos Use switch statement for inflate() return values 30*9573673dSchristos 1.2 9 Nov 2004 Add assertions to document zlib guarantees 31*9573673dSchristos 1.3 6 Apr 2005 Remove incorrect assertion in inf() 32*9573673dSchristos 1.4 11 Dec 2005 Add hack to avoid MSDOS end-of-line conversions 33*9573673dSchristos Avoid some compiler warnings for input and output buffers 34*9573673dSchristos */ 35*9573673dSchristos</b></pre><!-- --> 36*9573673dSchristosWe now include the header files for the required definitions. From 37*9573673dSchristos<tt>stdio.h</tt> we use <tt>fopen()</tt>, <tt>fread()</tt>, <tt>fwrite()</tt>, 38*9573673dSchristos<tt>feof()</tt>, <tt>ferror()</tt>, and <tt>fclose()</tt> for file i/o, and 39*9573673dSchristos<tt>fputs()</tt> for error messages. From <tt>string.h</tt> we use 40*9573673dSchristos<tt>strcmp()</tt> for command line argument processing. 41*9573673dSchristosFrom <tt>assert.h</tt> we use the <tt>assert()</tt> macro. 42*9573673dSchristosFrom <tt>zlib.h</tt> 43*9573673dSchristoswe use the basic compression functions <tt>deflateInit()</tt>, 44*9573673dSchristos<tt>deflate()</tt>, and <tt>deflateEnd()</tt>, and the basic decompression 45*9573673dSchristosfunctions <tt>inflateInit()</tt>, <tt>inflate()</tt>, and 46*9573673dSchristos<tt>inflateEnd()</tt>. 47*9573673dSchristos<pre><b> 48*9573673dSchristos#include <stdio.h> 49*9573673dSchristos#include <string.h> 50*9573673dSchristos#include <assert.h> 51*9573673dSchristos#include "zlib.h" 52*9573673dSchristos</b></pre><!-- --> 53*9573673dSchristosThis is an ugly hack required to avoid corruption of the input and output data on 54*9573673dSchristosWindows/MS-DOS systems. Without this, those systems would assume that the input and output 55*9573673dSchristosfiles are text, and try to convert the end-of-line characters from one standard to 56*9573673dSchristosanother. That would corrupt binary data, and in particular would render the compressed data unusable. 57*9573673dSchristosThis sets the input and output to binary which suppresses the end-of-line conversions. 58*9573673dSchristos<tt>SET_BINARY_MODE()</tt> will be used later on <tt>stdin</tt> and <tt>stdout</tt>, at the beginning of <tt>main()</tt>. 59*9573673dSchristos<pre><b> 60*9573673dSchristos#if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(__CYGWIN__) 61*9573673dSchristos# include <fcntl.h> 62*9573673dSchristos# include <io.h> 63*9573673dSchristos# define SET_BINARY_MODE(file) setmode(fileno(file), O_BINARY) 64*9573673dSchristos#else 65*9573673dSchristos# define SET_BINARY_MODE(file) 66*9573673dSchristos#endif 67*9573673dSchristos</b></pre><!-- --> 68*9573673dSchristos<tt>CHUNK</tt> is simply the buffer size for feeding data to and pulling data 69*9573673dSchristosfrom the <em>zlib</em> routines. Larger buffer sizes would be more efficient, 70*9573673dSchristosespecially for <tt>inflate()</tt>. If the memory is available, buffers sizes 71*9573673dSchristoson the order of 128K or 256K bytes should be used. 72*9573673dSchristos<pre><b> 73*9573673dSchristos#define CHUNK 16384 74*9573673dSchristos</b></pre><!-- --> 75*9573673dSchristosThe <tt>def()</tt> routine compresses data from an input file to an output file. The output data 76*9573673dSchristoswill be in the <em>zlib</em> format, which is different from the <em>gzip</em> or <em>zip</em> 77*9573673dSchristosformats. The <em>zlib</em> format has a very small header of only two bytes to identify it as 78*9573673dSchristosa <em>zlib</em> stream and to provide decoding information, and a four-byte trailer with a fast 79*9573673dSchristoscheck value to verify the integrity of the uncompressed data after decoding. 80*9573673dSchristos<pre><b> 81*9573673dSchristos/* Compress from file source to file dest until EOF on source. 82*9573673dSchristos def() returns Z_OK on success, Z_MEM_ERROR if memory could not be 83*9573673dSchristos allocated for processing, Z_STREAM_ERROR if an invalid compression 84*9573673dSchristos level is supplied, Z_VERSION_ERROR if the version of zlib.h and the 85*9573673dSchristos version of the library linked do not match, or Z_ERRNO if there is 86*9573673dSchristos an error reading or writing the files. */ 87*9573673dSchristosint def(FILE *source, FILE *dest, int level) 88*9573673dSchristos{ 89*9573673dSchristos</b></pre> 90*9573673dSchristosHere are the local variables for <tt>def()</tt>. <tt>ret</tt> will be used for <em>zlib</em> 91*9573673dSchristosreturn codes. <tt>flush</tt> will keep track of the current flushing state for <tt>deflate()</tt>, 92*9573673dSchristoswhich is either no flushing, or flush to completion after the end of the input file is reached. 93*9573673dSchristos<tt>have</tt> is the amount of data returned from <tt>deflate()</tt>. The <tt>strm</tt> structure 94*9573673dSchristosis used to pass information to and from the <em>zlib</em> routines, and to maintain the 95*9573673dSchristos<tt>deflate()</tt> state. <tt>in</tt> and <tt>out</tt> are the input and output buffers for 96*9573673dSchristos<tt>deflate()</tt>. 97*9573673dSchristos<pre><b> 98*9573673dSchristos int ret, flush; 99*9573673dSchristos unsigned have; 100*9573673dSchristos z_stream strm; 101*9573673dSchristos unsigned char in[CHUNK]; 102*9573673dSchristos unsigned char out[CHUNK]; 103*9573673dSchristos</b></pre><!-- --> 104*9573673dSchristosThe first thing we do is to initialize the <em>zlib</em> state for compression using 105*9573673dSchristos<tt>deflateInit()</tt>. This must be done before the first use of <tt>deflate()</tt>. 106*9573673dSchristosThe <tt>zalloc</tt>, <tt>zfree</tt>, and <tt>opaque</tt> fields in the <tt>strm</tt> 107*9573673dSchristosstructure must be initialized before calling <tt>deflateInit()</tt>. Here they are 108*9573673dSchristosset to the <em>zlib</em> constant <tt>Z_NULL</tt> to request that <em>zlib</em> use 109*9573673dSchristosthe default memory allocation routines. An application may also choose to provide 110*9573673dSchristoscustom memory allocation routines here. <tt>deflateInit()</tt> will allocate on the 111*9573673dSchristosorder of 256K bytes for the internal state. 112*9573673dSchristos(See <a href="zlib_tech.html"><em>zlib Technical Details</em></a>.) 113*9573673dSchristos<p> 114*9573673dSchristos<tt>deflateInit()</tt> is called with a pointer to the structure to be initialized and 115*9573673dSchristosthe compression level, which is an integer in the range of -1 to 9. Lower compression 116*9573673dSchristoslevels result in faster execution, but less compression. Higher levels result in 117*9573673dSchristosgreater compression, but slower execution. The <em>zlib</em> constant Z_DEFAULT_COMPRESSION, 118*9573673dSchristosequal to -1, 119*9573673dSchristosprovides a good compromise between compression and speed and is equivalent to level 6. 120*9573673dSchristosLevel 0 actually does no compression at all, and in fact expands the data slightly to produce 121*9573673dSchristosthe <em>zlib</em> format (it is not a byte-for-byte copy of the input). 122*9573673dSchristosMore advanced applications of <em>zlib</em> 123*9573673dSchristosmay use <tt>deflateInit2()</tt> here instead. Such an application may want to reduce how 124*9573673dSchristosmuch memory will be used, at some price in compression. Or it may need to request a 125*9573673dSchristos<em>gzip</em> header and trailer instead of a <em>zlib</em> header and trailer, or raw 126*9573673dSchristosencoding with no header or trailer at all. 127*9573673dSchristos<p> 128*9573673dSchristosWe must check the return value of <tt>deflateInit()</tt> against the <em>zlib</em> constant 129*9573673dSchristos<tt>Z_OK</tt> to make sure that it was able to 130*9573673dSchristosallocate memory for the internal state, and that the provided arguments were valid. 131*9573673dSchristos<tt>deflateInit()</tt> will also check that the version of <em>zlib</em> that the <tt>zlib.h</tt> 132*9573673dSchristosfile came from matches the version of <em>zlib</em> actually linked with the program. This 133*9573673dSchristosis especially important for environments in which <em>zlib</em> is a shared library. 134*9573673dSchristos<p> 135*9573673dSchristosNote that an application can initialize multiple, independent <em>zlib</em> streams, which can 136*9573673dSchristosoperate in parallel. The state information maintained in the structure allows the <em>zlib</em> 137*9573673dSchristosroutines to be reentrant. 138*9573673dSchristos<pre><b> 139*9573673dSchristos /* allocate deflate state */ 140*9573673dSchristos strm.zalloc = Z_NULL; 141*9573673dSchristos strm.zfree = Z_NULL; 142*9573673dSchristos strm.opaque = Z_NULL; 143*9573673dSchristos ret = deflateInit(&strm, level); 144*9573673dSchristos if (ret != Z_OK) 145*9573673dSchristos return ret; 146*9573673dSchristos</b></pre><!-- --> 147*9573673dSchristosWith the pleasantries out of the way, now we can get down to business. The outer <tt>do</tt>-loop 148*9573673dSchristosreads all of the input file and exits at the bottom of the loop once end-of-file is reached. 149*9573673dSchristosThis loop contains the only call of <tt>deflate()</tt>. So we must make sure that all of the 150*9573673dSchristosinput data has been processed and that all of the output data has been generated and consumed 151*9573673dSchristosbefore we fall out of the loop at the bottom. 152*9573673dSchristos<pre><b> 153*9573673dSchristos /* compress until end of file */ 154*9573673dSchristos do { 155*9573673dSchristos</b></pre> 156*9573673dSchristosWe start off by reading data from the input file. The number of bytes read is put directly 157*9573673dSchristosinto <tt>avail_in</tt>, and a pointer to those bytes is put into <tt>next_in</tt>. We also 158*9573673dSchristoscheck 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 159*9573673dSchristos<em>zlib</em> constant <tt>Z_FINISH</tt>, which is later passed to <tt>deflate()</tt> to 160*9573673dSchristosindicate that this is the last chunk of input data to compress. We need to use <tt>feof()</tt> 161*9573673dSchristosto check for end-of-file as opposed to seeing if fewer than <tt>CHUNK</tt> bytes have been read. The 162*9573673dSchristosreason is that if the input file length is an exact multiple of <tt>CHUNK</tt>, we will miss 163*9573673dSchristosthe fact that we got to the end-of-file, and not know to tell <tt>deflate()</tt> to finish 164*9573673dSchristosup the compressed stream. If we are not yet at the end of the input, then the <em>zlib</em> 165*9573673dSchristosconstant <tt>Z_NO_FLUSH</tt> will be passed to <tt>deflate</tt> to indicate that we are still 166*9573673dSchristosin the middle of the uncompressed data. 167*9573673dSchristos<p> 168*9573673dSchristosIf there is an error in reading from the input file, the process is aborted with 169*9573673dSchristos<tt>deflateEnd()</tt> being called to free the allocated <em>zlib</em> state before returning 170*9573673dSchristosthe error. We wouldn't want a memory leak, now would we? <tt>deflateEnd()</tt> can be called 171*9573673dSchristosat any time after the state has been initialized. Once that's done, <tt>deflateInit()</tt> (or 172*9573673dSchristos<tt>deflateInit2()</tt>) would have to be called to start a new compression process. There is 173*9573673dSchristosno point here in checking the <tt>deflateEnd()</tt> return code. The deallocation can't fail. 174*9573673dSchristos<pre><b> 175*9573673dSchristos strm.avail_in = fread(in, 1, CHUNK, source); 176*9573673dSchristos if (ferror(source)) { 177*9573673dSchristos (void)deflateEnd(&strm); 178*9573673dSchristos return Z_ERRNO; 179*9573673dSchristos } 180*9573673dSchristos flush = feof(source) ? Z_FINISH : Z_NO_FLUSH; 181*9573673dSchristos strm.next_in = in; 182*9573673dSchristos</b></pre><!-- --> 183*9573673dSchristosThe inner <tt>do</tt>-loop passes our chunk of input data to <tt>deflate()</tt>, and then 184*9573673dSchristoskeeps calling <tt>deflate()</tt> until it is done producing output. Once there is no more 185*9573673dSchristosnew output, <tt>deflate()</tt> is guaranteed to have consumed all of the input, i.e., 186*9573673dSchristos<tt>avail_in</tt> will be zero. 187*9573673dSchristos<pre><b> 188*9573673dSchristos /* run deflate() on input until output buffer not full, finish 189*9573673dSchristos compression if all of source has been read in */ 190*9573673dSchristos do { 191*9573673dSchristos</b></pre> 192*9573673dSchristosOutput space is provided to <tt>deflate()</tt> by setting <tt>avail_out</tt> to the number 193*9573673dSchristosof available output bytes and <tt>next_out</tt> to a pointer to that space. 194*9573673dSchristos<pre><b> 195*9573673dSchristos strm.avail_out = CHUNK; 196*9573673dSchristos strm.next_out = out; 197*9573673dSchristos</b></pre> 198*9573673dSchristosNow we call the compression engine itself, <tt>deflate()</tt>. It takes as many of the 199*9573673dSchristos<tt>avail_in</tt> bytes at <tt>next_in</tt> as it can process, and writes as many as 200*9573673dSchristos<tt>avail_out</tt> bytes to <tt>next_out</tt>. Those counters and pointers are then 201*9573673dSchristosupdated past the input data consumed and the output data written. It is the amount of 202*9573673dSchristosoutput space available that may limit how much input is consumed. 203*9573673dSchristosHence the inner loop to make sure that 204*9573673dSchristosall of the input is consumed by providing more output space each time. Since <tt>avail_in</tt> 205*9573673dSchristosand <tt>next_in</tt> are updated by <tt>deflate()</tt>, we don't have to mess with those 206*9573673dSchristosbetween <tt>deflate()</tt> calls until it's all used up. 207*9573673dSchristos<p> 208*9573673dSchristosThe parameters to <tt>deflate()</tt> are a pointer to the <tt>strm</tt> structure containing 209*9573673dSchristosthe input and output information and the internal compression engine state, and a parameter 210*9573673dSchristosindicating whether and how to flush data to the output. Normally <tt>deflate</tt> will consume 211*9573673dSchristosseveral K bytes of input data before producing any output (except for the header), in order 212*9573673dSchristosto accumulate statistics on the data for optimum compression. It will then put out a burst of 213*9573673dSchristoscompressed data, and proceed to consume more input before the next burst. Eventually, 214*9573673dSchristos<tt>deflate()</tt> 215*9573673dSchristosmust be told to terminate the stream, complete the compression with provided input data, and 216*9573673dSchristoswrite out the trailer check value. <tt>deflate()</tt> will continue to compress normally as long 217*9573673dSchristosas the flush parameter is <tt>Z_NO_FLUSH</tt>. Once the <tt>Z_FINISH</tt> parameter is provided, 218*9573673dSchristos<tt>deflate()</tt> will begin to complete the compressed output stream. However depending on how 219*9573673dSchristosmuch output space is provided, <tt>deflate()</tt> may have to be called several times until it 220*9573673dSchristoshas provided the complete compressed stream, even after it has consumed all of the input. The flush 221*9573673dSchristosparameter must continue to be <tt>Z_FINISH</tt> for those subsequent calls. 222*9573673dSchristos<p> 223*9573673dSchristosThere are other values of the flush parameter that are used in more advanced applications. You can 224*9573673dSchristosforce <tt>deflate()</tt> to produce a burst of output that encodes all of the input data provided 225*9573673dSchristosso far, even if it wouldn't have otherwise, for example to control data latency on a link with 226*9573673dSchristoscompressed data. You can also ask that <tt>deflate()</tt> do that as well as erase any history up to 227*9573673dSchristosthat point so that what follows can be decompressed independently, for example for random access 228*9573673dSchristosapplications. Both requests will degrade compression by an amount depending on how often such 229*9573673dSchristosrequests are made. 230*9573673dSchristos<p> 231*9573673dSchristos<tt>deflate()</tt> has a return value that can indicate errors, yet we do not check it here. Why 232*9573673dSchristosnot? Well, it turns out that <tt>deflate()</tt> can do no wrong here. Let's go through 233*9573673dSchristos<tt>deflate()</tt>'s return values and dispense with them one by one. The possible values are 234*9573673dSchristos<tt>Z_OK</tt>, <tt>Z_STREAM_END</tt>, <tt>Z_STREAM_ERROR</tt>, or <tt>Z_BUF_ERROR</tt>. <tt>Z_OK</tt> 235*9573673dSchristosis, well, ok. <tt>Z_STREAM_END</tt> is also ok and will be returned for the last call of 236*9573673dSchristos<tt>deflate()</tt>. This is already guaranteed by calling <tt>deflate()</tt> with <tt>Z_FINISH</tt> 237*9573673dSchristosuntil it has no more output. <tt>Z_STREAM_ERROR</tt> is only possible if the stream is not 238*9573673dSchristosinitialized properly, but we did initialize it properly. There is no harm in checking for 239*9573673dSchristos<tt>Z_STREAM_ERROR</tt> here, for example to check for the possibility that some 240*9573673dSchristosother part of the application inadvertently clobbered the memory containing the <em>zlib</em> state. 241*9573673dSchristos<tt>Z_BUF_ERROR</tt> will be explained further below, but 242*9573673dSchristossuffice it to say that this is simply an indication that <tt>deflate()</tt> could not consume 243*9573673dSchristosmore input or produce more output. <tt>deflate()</tt> can be called again with more output space 244*9573673dSchristosor more available input, which it will be in this code. 245*9573673dSchristos<pre><b> 246*9573673dSchristos ret = deflate(&strm, flush); /* no bad return value */ 247*9573673dSchristos assert(ret != Z_STREAM_ERROR); /* state not clobbered */ 248*9573673dSchristos</b></pre> 249*9573673dSchristosNow we compute how much output <tt>deflate()</tt> provided on the last call, which is the 250*9573673dSchristosdifference between how much space was provided before the call, and how much output space 251*9573673dSchristosis still available after the call. Then that data, if any, is written to the output file. 252*9573673dSchristosWe can then reuse the output buffer for the next call of <tt>deflate()</tt>. Again if there 253*9573673dSchristosis a file i/o error, we call <tt>deflateEnd()</tt> before returning to avoid a memory leak. 254*9573673dSchristos<pre><b> 255*9573673dSchristos have = CHUNK - strm.avail_out; 256*9573673dSchristos if (fwrite(out, 1, have, dest) != have || ferror(dest)) { 257*9573673dSchristos (void)deflateEnd(&strm); 258*9573673dSchristos return Z_ERRNO; 259*9573673dSchristos } 260*9573673dSchristos</b></pre> 261*9573673dSchristosThe inner <tt>do</tt>-loop is repeated until the last <tt>deflate()</tt> call fails to fill the 262*9573673dSchristosprovided output buffer. Then we know that <tt>deflate()</tt> has done as much as it can with 263*9573673dSchristosthe provided input, and that all of that input has been consumed. We can then fall out of this 264*9573673dSchristosloop and reuse the input buffer. 265*9573673dSchristos<p> 266*9573673dSchristosThe way we tell that <tt>deflate()</tt> has no more output is by seeing that it did not fill 267*9573673dSchristosthe output buffer, leaving <tt>avail_out</tt> greater than zero. However suppose that 268*9573673dSchristos<tt>deflate()</tt> has no more output, but just so happened to exactly fill the output buffer! 269*9573673dSchristos<tt>avail_out</tt> is zero, and we can't tell that <tt>deflate()</tt> has done all it can. 270*9573673dSchristosAs far as we know, <tt>deflate()</tt> 271*9573673dSchristoshas more output for us. So we call it again. But now <tt>deflate()</tt> produces no output 272*9573673dSchristosat all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call 273*9573673dSchristoswasn't able to do anything, either consume input or produce output, and so it returns 274*9573673dSchristos<tt>Z_BUF_ERROR</tt>. (See, I told you I'd cover this later.) However this is not a problem at 275*9573673dSchristosall. Now we finally have the desired indication that <tt>deflate()</tt> is really done, 276*9573673dSchristosand so we drop out of the inner loop to provide more input to <tt>deflate()</tt>. 277*9573673dSchristos<p> 278*9573673dSchristosWith <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will 279*9573673dSchristoscomplete the output stream. Once that is done, subsequent calls of <tt>deflate()</tt> would return 280*9573673dSchristos<tt>Z_STREAM_ERROR</tt> if the flush parameter is not <tt>Z_FINISH</tt>, and do no more processing 281*9573673dSchristosuntil the state is reinitialized. 282*9573673dSchristos<p> 283*9573673dSchristosSome applications of <em>zlib</em> have two loops that call <tt>deflate()</tt> 284*9573673dSchristosinstead of the single inner loop we have here. The first loop would call 285*9573673dSchristoswithout flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call 286*9573673dSchristos<tt>deflate()</tt> with no more 287*9573673dSchristosdata and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this 288*9573673dSchristosexample, that can be avoided by simply keeping track of the current flush state. 289*9573673dSchristos<pre><b> 290*9573673dSchristos } while (strm.avail_out == 0); 291*9573673dSchristos assert(strm.avail_in == 0); /* all input will be used */ 292*9573673dSchristos</b></pre><!-- --> 293*9573673dSchristosNow we check to see if we have already processed all of the input file. That information was 294*9573673dSchristossaved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so, 295*9573673dSchristosthen we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt> 296*9573673dSchristosfrom the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was 297*9573673dSchristosconsumed and all of the output was generated. 298*9573673dSchristos<pre><b> 299*9573673dSchristos /* done when last data in file processed */ 300*9573673dSchristos } while (flush != Z_FINISH); 301*9573673dSchristos assert(ret == Z_STREAM_END); /* stream will be complete */ 302*9573673dSchristos</b></pre><!-- --> 303*9573673dSchristosThe process is complete, but we still need to deallocate the state to avoid a memory leak 304*9573673dSchristos(or rather more like a memory hemorrhage if you didn't do this). Then 305*9573673dSchristosfinally we can return with a happy return value. 306*9573673dSchristos<pre><b> 307*9573673dSchristos /* clean up and return */ 308*9573673dSchristos (void)deflateEnd(&strm); 309*9573673dSchristos return Z_OK; 310*9573673dSchristos} 311*9573673dSchristos</b></pre><!-- --> 312*9573673dSchristosNow we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt> 313*9573673dSchristosdecompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the 314*9573673dSchristosuncompressed data to the output file. Much of the discussion above for <tt>def()</tt> 315*9573673dSchristosapplies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between 316*9573673dSchristosthe two. 317*9573673dSchristos<pre><b> 318*9573673dSchristos/* Decompress from file source to file dest until stream ends or EOF. 319*9573673dSchristos inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be 320*9573673dSchristos allocated for processing, Z_DATA_ERROR if the deflate data is 321*9573673dSchristos invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and 322*9573673dSchristos the version of the library linked do not match, or Z_ERRNO if there 323*9573673dSchristos is an error reading or writing the files. */ 324*9573673dSchristosint inf(FILE *source, FILE *dest) 325*9573673dSchristos{ 326*9573673dSchristos</b></pre> 327*9573673dSchristosThe local variables have the same functionality as they do for <tt>def()</tt>. The 328*9573673dSchristosonly difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt> 329*9573673dSchristoscan tell from the <em>zlib</em> stream itself when the stream is complete. 330*9573673dSchristos<pre><b> 331*9573673dSchristos int ret; 332*9573673dSchristos unsigned have; 333*9573673dSchristos z_stream strm; 334*9573673dSchristos unsigned char in[CHUNK]; 335*9573673dSchristos unsigned char out[CHUNK]; 336*9573673dSchristos</b></pre><!-- --> 337*9573673dSchristosThe initialization of the state is the same, except that there is no compression level, 338*9573673dSchristosof course, and two more elements of the structure are initialized. <tt>avail_in</tt> 339*9573673dSchristosand <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This 340*9573673dSchristosis because the application has the option to provide the start of the zlib stream in 341*9573673dSchristosorder for <tt>inflateInit()</tt> to have access to information about the compression 342*9573673dSchristosmethod to aid in memory allocation. In the current implementation of <em>zlib</em> 343*9573673dSchristos(up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of 344*9573673dSchristos<tt>inflate()</tt> anyway. However those fields must be initialized since later versions 345*9573673dSchristosof <em>zlib</em> that provide more compression methods may take advantage of this interface. 346*9573673dSchristosIn any case, no decompression is performed by <tt>inflateInit()</tt>, so the 347*9573673dSchristos<tt>avail_out</tt> and <tt>next_out</tt> fields do not need to be initialized before calling. 348*9573673dSchristos<p> 349*9573673dSchristosHere <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to 350*9573673dSchristosindicate that no input data is being provided. 351*9573673dSchristos<pre><b> 352*9573673dSchristos /* allocate inflate state */ 353*9573673dSchristos strm.zalloc = Z_NULL; 354*9573673dSchristos strm.zfree = Z_NULL; 355*9573673dSchristos strm.opaque = Z_NULL; 356*9573673dSchristos strm.avail_in = 0; 357*9573673dSchristos strm.next_in = Z_NULL; 358*9573673dSchristos ret = inflateInit(&strm); 359*9573673dSchristos if (ret != Z_OK) 360*9573673dSchristos return ret; 361*9573673dSchristos</b></pre><!-- --> 362*9573673dSchristosThe outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates 363*9573673dSchristosthat it has reached the end of the compressed data and has produced all of the uncompressed 364*9573673dSchristosoutput. This is in contrast to <tt>def()</tt> which processes all of the input file. 365*9573673dSchristosIf end-of-file is reached before the compressed data self-terminates, then the compressed 366*9573673dSchristosdata is incomplete and an error is returned. 367*9573673dSchristos<pre><b> 368*9573673dSchristos /* decompress until deflate stream ends or end of file */ 369*9573673dSchristos do { 370*9573673dSchristos</b></pre> 371*9573673dSchristosWe read input data and set the <tt>strm</tt> structure accordingly. If we've reached the 372*9573673dSchristosend of the input file, then we leave the outer loop and report an error, since the 373*9573673dSchristoscompressed data is incomplete. Note that we may read more data than is eventually consumed 374*9573673dSchristosby <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream. 375*9573673dSchristosFor applications where <em>zlib</em> streams are embedded in other data, this routine would 376*9573673dSchristosneed to be modified to return the unused data, or at least indicate how much of the input 377*9573673dSchristosdata was not used, so the application would know where to pick up after the <em>zlib</em> stream. 378*9573673dSchristos<pre><b> 379*9573673dSchristos strm.avail_in = fread(in, 1, CHUNK, source); 380*9573673dSchristos if (ferror(source)) { 381*9573673dSchristos (void)inflateEnd(&strm); 382*9573673dSchristos return Z_ERRNO; 383*9573673dSchristos } 384*9573673dSchristos if (strm.avail_in == 0) 385*9573673dSchristos break; 386*9573673dSchristos strm.next_in = in; 387*9573673dSchristos</b></pre><!-- --> 388*9573673dSchristosThe inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to 389*9573673dSchristoskeep calling <tt>inflate()</tt> until has generated all of the output it can with the 390*9573673dSchristosprovided input. 391*9573673dSchristos<pre><b> 392*9573673dSchristos /* run inflate() on input until output buffer not full */ 393*9573673dSchristos do { 394*9573673dSchristos</b></pre> 395*9573673dSchristosJust like in <tt>def()</tt>, the same output space is provided for each call of <tt>inflate()</tt>. 396*9573673dSchristos<pre><b> 397*9573673dSchristos strm.avail_out = CHUNK; 398*9573673dSchristos strm.next_out = out; 399*9573673dSchristos</b></pre> 400*9573673dSchristosNow we run the decompression engine itself. There is no need to adjust the flush parameter, since 401*9573673dSchristosthe <em>zlib</em> format is self-terminating. The main difference here is that there are 402*9573673dSchristosreturn values that we need to pay attention to. <tt>Z_DATA_ERROR</tt> 403*9573673dSchristosindicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format, 404*9573673dSchristoswhich means that either the data is not a <em>zlib</em> stream to begin with, or that the data was 405*9573673dSchristoscorrupted somewhere along the way since it was compressed. The other error to be processed is 406*9573673dSchristos<tt>Z_MEM_ERROR</tt>, which can occur since memory allocation is deferred until <tt>inflate()</tt> 407*9573673dSchristosneeds it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>. 408*9573673dSchristos<p> 409*9573673dSchristosAdvanced applications may use 410*9573673dSchristos<tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the 411*9573673dSchristosfirst 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt> 412*9573673dSchristosrequests that that dictionary be provided before it can start to decompress. Without the dictionary, 413*9573673dSchristoscorrect decompression is not possible. For this routine, we have no idea what the dictionary is, 414*9573673dSchristosso the <tt>Z_NEED_DICT</tt> indication is converted to a <tt>Z_DATA_ERROR</tt>. 415*9573673dSchristos<p> 416*9573673dSchristos<tt>inflate()</tt> can also return <tt>Z_STREAM_ERROR</tt>, which should not be possible here, 417*9573673dSchristosbut could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be 418*9573673dSchristoschecked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be 419*9573673dSchristoschecked for later. 420*9573673dSchristos<pre><b> 421*9573673dSchristos ret = inflate(&strm, Z_NO_FLUSH); 422*9573673dSchristos assert(ret != Z_STREAM_ERROR); /* state not clobbered */ 423*9573673dSchristos switch (ret) { 424*9573673dSchristos case Z_NEED_DICT: 425*9573673dSchristos ret = Z_DATA_ERROR; /* and fall through */ 426*9573673dSchristos case Z_DATA_ERROR: 427*9573673dSchristos case Z_MEM_ERROR: 428*9573673dSchristos (void)inflateEnd(&strm); 429*9573673dSchristos return ret; 430*9573673dSchristos } 431*9573673dSchristos</b></pre> 432*9573673dSchristosThe output of <tt>inflate()</tt> is handled identically to that of <tt>deflate()</tt>. 433*9573673dSchristos<pre><b> 434*9573673dSchristos have = CHUNK - strm.avail_out; 435*9573673dSchristos if (fwrite(out, 1, have, dest) != have || ferror(dest)) { 436*9573673dSchristos (void)inflateEnd(&strm); 437*9573673dSchristos return Z_ERRNO; 438*9573673dSchristos } 439*9573673dSchristos</b></pre> 440*9573673dSchristosThe inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated 441*9573673dSchristosby not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot 442*9573673dSchristosassert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file 443*9573673dSchristosdoes. 444*9573673dSchristos<pre><b> 445*9573673dSchristos } while (strm.avail_out == 0); 446*9573673dSchristos</b></pre><!-- --> 447*9573673dSchristosThe outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the 448*9573673dSchristosend of the input <em>zlib</em> stream, has completed the decompression and integrity 449*9573673dSchristoscheck, and has provided all of the output. This is indicated by the <tt>inflate()</tt> 450*9573673dSchristosreturn value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt> 451*9573673dSchristosequal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end 452*9573673dSchristosof the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the 453*9573673dSchristosloop continues to read more input. 454*9573673dSchristos<pre><b> 455*9573673dSchristos /* done when inflate() says it's done */ 456*9573673dSchristos } while (ret != Z_STREAM_END); 457*9573673dSchristos</b></pre><!-- --> 458*9573673dSchristosAt this point, decompression successfully completed, or we broke out of the loop due to no 459*9573673dSchristosmore data being available from the input file. If the last <tt>inflate()</tt> return value 460*9573673dSchristosis not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error 461*9573673dSchristosis returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt> 462*9573673dSchristosis called first to avoid a memory leak. 463*9573673dSchristos<pre><b> 464*9573673dSchristos /* clean up and return */ 465*9573673dSchristos (void)inflateEnd(&strm); 466*9573673dSchristos return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR; 467*9573673dSchristos} 468*9573673dSchristos</b></pre><!-- --> 469*9573673dSchristosThat ends the routines that directly use <em>zlib</em>. The following routines make this 470*9573673dSchristosa command-line program by running data through the above routines from <tt>stdin</tt> to 471*9573673dSchristos<tt>stdout</tt>, and handling any errors reported by <tt>def()</tt> or <tt>inf()</tt>. 472*9573673dSchristos<p> 473*9573673dSchristos<tt>zerr()</tt> is used to interpret the possible error codes from <tt>def()</tt> 474*9573673dSchristosand <tt>inf()</tt>, as detailed in their comments above, and print out an error message. 475*9573673dSchristosNote that these are only a subset of the possible return values from <tt>deflate()</tt> 476*9573673dSchristosand <tt>inflate()</tt>. 477*9573673dSchristos<pre><b> 478*9573673dSchristos/* report a zlib or i/o error */ 479*9573673dSchristosvoid zerr(int ret) 480*9573673dSchristos{ 481*9573673dSchristos fputs("zpipe: ", stderr); 482*9573673dSchristos switch (ret) { 483*9573673dSchristos case Z_ERRNO: 484*9573673dSchristos if (ferror(stdin)) 485*9573673dSchristos fputs("error reading stdin\n", stderr); 486*9573673dSchristos if (ferror(stdout)) 487*9573673dSchristos fputs("error writing stdout\n", stderr); 488*9573673dSchristos break; 489*9573673dSchristos case Z_STREAM_ERROR: 490*9573673dSchristos fputs("invalid compression level\n", stderr); 491*9573673dSchristos break; 492*9573673dSchristos case Z_DATA_ERROR: 493*9573673dSchristos fputs("invalid or incomplete deflate data\n", stderr); 494*9573673dSchristos break; 495*9573673dSchristos case Z_MEM_ERROR: 496*9573673dSchristos fputs("out of memory\n", stderr); 497*9573673dSchristos break; 498*9573673dSchristos case Z_VERSION_ERROR: 499*9573673dSchristos fputs("zlib version mismatch!\n", stderr); 500*9573673dSchristos } 501*9573673dSchristos} 502*9573673dSchristos</b></pre><!-- --> 503*9573673dSchristosHere is the <tt>main()</tt> routine used to test <tt>def()</tt> and <tt>inf()</tt>. The 504*9573673dSchristos<tt>zpipe</tt> command is simply a compression pipe from <tt>stdin</tt> to <tt>stdout</tt>, if 505*9573673dSchristosno arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other 506*9573673dSchristosarguments are provided, no compression or decompression is performed. Instead a usage 507*9573673dSchristosmessage is displayed. Examples are <tt>zpipe < foo.txt > foo.txt.z</tt> to compress, and 508*9573673dSchristos<tt>zpipe -d < foo.txt.z > foo.txt</tt> to decompress. 509*9573673dSchristos<pre><b> 510*9573673dSchristos/* compress or decompress from stdin to stdout */ 511*9573673dSchristosint main(int argc, char **argv) 512*9573673dSchristos{ 513*9573673dSchristos int ret; 514*9573673dSchristos 515*9573673dSchristos /* avoid end-of-line conversions */ 516*9573673dSchristos SET_BINARY_MODE(stdin); 517*9573673dSchristos SET_BINARY_MODE(stdout); 518*9573673dSchristos 519*9573673dSchristos /* do compression if no arguments */ 520*9573673dSchristos if (argc == 1) { 521*9573673dSchristos ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION); 522*9573673dSchristos if (ret != Z_OK) 523*9573673dSchristos zerr(ret); 524*9573673dSchristos return ret; 525*9573673dSchristos } 526*9573673dSchristos 527*9573673dSchristos /* do decompression if -d specified */ 528*9573673dSchristos else if (argc == 2 && strcmp(argv[1], "-d") == 0) { 529*9573673dSchristos ret = inf(stdin, stdout); 530*9573673dSchristos if (ret != Z_OK) 531*9573673dSchristos zerr(ret); 532*9573673dSchristos return ret; 533*9573673dSchristos } 534*9573673dSchristos 535*9573673dSchristos /* otherwise, report usage */ 536*9573673dSchristos else { 537*9573673dSchristos fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr); 538*9573673dSchristos return 1; 539*9573673dSchristos } 540*9573673dSchristos} 541*9573673dSchristos</b></pre> 542*9573673dSchristos<hr> 543*9573673dSchristos<i>Copyright (c) 2004, 2005 by Mark Adler<br>Last modified 11 December 2005</i> 544*9573673dSchristos</body> 545*9573673dSchristos</html> 546