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