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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> | |
11 | We often get questions about how the <tt>deflate()</tt> and <tt>inflate()</tt> functions should be used. | |
12 | Users wonder when they should provide more input, when they should use more output, | |
13 | what to do with a <tt>Z_BUF_ERROR</tt>, how to make sure the process terminates properly, and | |
14 | so on. So for those who have read <tt>zlib.h</tt> (a few times), and | |
15 | would like further edification, below is an annotated example in C of simple routines to compress and decompress | |
16 | from an input file to an output file using <tt>deflate()</tt> and <tt>inflate()</tt> respectively. The | |
17 | annotations are interspersed between lines of the code. So please read between the lines. | |
18 | We hope this helps explain some of the intricacies of <em>zlib</em>. | |
19 | <p> | |
20 | Without further adieu, here is the program <a href="zpipe.c"><tt>zpipe.c</tt></a>: | |
21 | <pre><b> | |
22 | /* zpipe.c: example of proper use of zlib's inflate() and deflate() | |
23 | Not copyrighted -- provided to the public domain | |
24 | Version 1.4 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><!-- --> | |
36 | We 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. | |
41 | From <tt>assert.h</tt> we use the <tt>assert()</tt> macro. | |
42 | From <tt>zlib.h</tt> | |
43 | we use the basic compression functions <tt>deflateInit()</tt>, | |
44 | <tt>deflate()</tt>, and <tt>deflateEnd()</tt>, and the basic decompression | |
45 | functions <tt>inflateInit()</tt>, <tt>inflate()</tt>, and | |
46 | <tt>inflateEnd()</tt>. | |
47 | <pre><b> | |
48 | #include <stdio.h> | |
49 | #include <string.h> | |
50 | #include <assert.h> | |
51 | #include "zlib.h" | |
52 | </b></pre><!-- --> | |
53 | This is an ugly hack required to avoid corruption of the input and output data on | |
54 | Windows/MS-DOS systems. Without this, those systems would assume that the input and output | |
55 | files are text, and try to convert the end-of-line characters from one standard to | |
56 | another. That would corrupt binary data, and in particular would render the compressed data unusable. | |
57 | This 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 <fcntl.h> | |
62 | # include <io.h> | |
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 | |
69 | from the <em>zlib</em> routines. Larger buffer sizes would be more efficient, | |
70 | especially for <tt>inflate()</tt>. If the memory is available, buffers sizes | |
71 | on the order of 128K or 256K bytes should be used. | |
72 | <pre><b> | |
73 | #define CHUNK 16384 | |
74 | </b></pre><!-- --> | |
75 | The <tt>def()</tt> routine compresses data from an input file to an output file. The output data | |
76 | will be in the <em>zlib</em> format, which is different from the <em>gzip</em> or <em>zip</em> | |
77 | formats. The <em>zlib</em> format has a very small header of only two bytes to identify it as | |
78 | a <em>zlib</em> stream and to provide decoding information, and a four-byte trailer with a fast | |
79 | check 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. */ | |
87 | int def(FILE *source, FILE *dest, int level) | |
88 | { | |
89 | </b></pre> | |
90 | Here are the local variables for <tt>def()</tt>. <tt>ret</tt> will be used for <em>zlib</em> | |
91 | return codes. <tt>flush</tt> will keep track of the current flushing state for <tt>deflate()</tt>, | |
92 | which 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 | |
94 | is 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><!-- --> | |
104 | The 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>. | |
106 | The <tt>zalloc</tt>, <tt>zfree</tt>, and <tt>opaque</tt> fields in the <tt>strm</tt> | |
107 | structure must be initialized before calling <tt>deflateInit()</tt>. Here they are | |
108 | set to the <em>zlib</em> constant <tt>Z_NULL</tt> to request that <em>zlib</em> use | |
109 | the default memory allocation routines. An application may also choose to provide | |
110 | custom memory allocation routines here. <tt>deflateInit()</tt> will allocate on the | |
111 | order 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 | |
115 | the compression level, which is an integer in the range of -1 to 9. Lower compression | |
116 | levels result in faster execution, but less compression. Higher levels result in | |
117 | greater compression, but slower execution. The <em>zlib</em> constant Z_DEFAULT_COMPRESSION, | |
118 | equal to -1, | |
119 | provides a good compromise between compression and speed and is equivalent to level 6. | |
120 | Level 0 actually does no compression at all, and in fact expands the data slightly to produce | |
121 | the <em>zlib</em> format (it is not a byte-for-byte copy of the input). | |
122 | More advanced applications of <em>zlib</em> | |
123 | may use <tt>deflateInit2()</tt> here instead. Such an application may want to reduce how | |
124 | much 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 | |
126 | encoding with no header or trailer at all. | |
127 | <p> | |
128 | We 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 | |
130 | allocate 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> | |
132 | file came from matches the version of <em>zlib</em> actually linked with the program. This | |
133 | is especially important for environments in which <em>zlib</em> is a shared library. | |
134 | <p> | |
135 | Note that an application can initialize multiple, independent <em>zlib</em> streams, which can | |
136 | operate in parallel. The state information maintained in the structure allows the <em>zlib</em> | |
137 | routines 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(&strm, level); | |
144 | if (ret != Z_OK) | |
145 | return ret; | |
146 | </b></pre><!-- --> | |
147 | With the pleasantries out of the way, now we can get down to business. The outer <tt>do</tt>-loop | |
148 | reads all of the input file and exits at the bottom of the loop once end-of-file is reached. | |
149 | This loop contains the only call of <tt>deflate()</tt>. So we must make sure that all of the | |
150 | input data has been processed and that all of the output data has been generated and consumed | |
151 | before we fall out of the loop at the bottom. | |
152 | <pre><b> | |
153 | /* compress until end of file */ | |
154 | do { | |
155 | </b></pre> | |
156 | We start off by reading data from the input file. The number of bytes read is put directly | |
157 | into <tt>avail_in</tt>, and a pointer to those bytes is put into <tt>next_in</tt>. We also | |
158 | check to see if end-of-file on the input has been reached. If we are at the end of file, then <tt>flush</tt> is set to the | |
159 | <em>zlib</em> constant <tt>Z_FINISH</tt>, which is later passed to <tt>deflate()</tt> to | |
160 | indicate that this is the last chunk of input data to compress. We need to use <tt>feof()</tt> | |
161 | to check for end-of-file as opposed to seeing if fewer than <tt>CHUNK</tt> bytes have been read. The | |
162 | reason is that if the input file length is an exact multiple of <tt>CHUNK</tt>, we will miss | |
163 | the fact that we got to the end-of-file, and not know to tell <tt>deflate()</tt> to finish | |
164 | up the compressed stream. If we are not yet at the end of the input, then the <em>zlib</em> | |
165 | constant <tt>Z_NO_FLUSH</tt> will be passed to <tt>deflate</tt> to indicate that we are still | |
166 | in the middle of the uncompressed data. | |
167 | <p> | |
168 | If 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 | |
170 | the error. We wouldn't want a memory leak, now would we? <tt>deflateEnd()</tt> can be called | |
171 | at 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 | |
173 | no 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(&strm); | |
178 | return Z_ERRNO; | |
179 | } | |
180 | flush = feof(source) ? Z_FINISH : Z_NO_FLUSH; | |
181 | strm.next_in = in; | |
182 | </b></pre><!-- --> | |
183 | The inner <tt>do</tt>-loop passes our chunk of input data to <tt>deflate()</tt>, and then | |
184 | keeps calling <tt>deflate()</tt> until it is done producing output. Once there is no more | |
185 | new 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> | |
192 | Output space is provided to <tt>deflate()</tt> by setting <tt>avail_out</tt> to the number | |
193 | of 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> | |
198 | Now 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 | |
201 | updated past the input data consumed and the output data written. It is the amount of | |
202 | output space available that may limit how much input is consumed. | |
203 | Hence the inner loop to make sure that | |
204 | all of the input is consumed by providing more output space each time. Since <tt>avail_in</tt> | |
205 | and <tt>next_in</tt> are updated by <tt>deflate()</tt>, we don't have to mess with those | |
206 | between <tt>deflate()</tt> calls until it's all used up. | |
207 | <p> | |
208 | The parameters to <tt>deflate()</tt> are a pointer to the <tt>strm</tt> structure containing | |
209 | the input and output information and the internal compression engine state, and a parameter | |
210 | indicating whether and how to flush data to the output. Normally <tt>deflate</tt> will consume | |
211 | several K bytes of input data before producing any output (except for the header), in order | |
212 | to accumulate statistics on the data for optimum compression. It will then put out a burst of | |
213 | compressed data, and proceed to consume more input before the next burst. Eventually, | |
214 | <tt>deflate()</tt> | |
215 | must be told to terminate the stream, complete the compression with provided input data, and | |
216 | write out the trailer check value. <tt>deflate()</tt> will continue to compress normally as long | |
217 | as 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 | |
219 | much output space is provided, <tt>deflate()</tt> may have to be called several times until it | |
220 | has provided the complete compressed stream, even after it has consumed all of the input. The flush | |
221 | parameter must continue to be <tt>Z_FINISH</tt> for those subsequent calls. | |
222 | <p> | |
223 | There are other values of the flush parameter that are used in more advanced applications. You can | |
224 | force <tt>deflate()</tt> to produce a burst of output that encodes all of the input data provided | |
225 | so far, even if it wouldn't have otherwise, for example to control data latency on a link with | |
226 | compressed data. You can also ask that <tt>deflate()</tt> do that as well as erase any history up to | |
227 | that point so that what follows can be decompressed independently, for example for random access | |
228 | applications. Both requests will degrade compression by an amount depending on how often such | |
229 | requests are made. | |
230 | <p> | |
231 | <tt>deflate()</tt> has a return value that can indicate errors, yet we do not check it here. Why | |
232 | not? 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> | |
235 | is, 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> | |
237 | until it has no more output. <tt>Z_STREAM_ERROR</tt> is only possible if the stream is not | |
238 | initialized 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 | |
240 | other 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 | |
242 | suffice it to say that this is simply an indication that <tt>deflate()</tt> could not consume | |
243 | more input or produce more output. <tt>deflate()</tt> can be called again with more output space | |
244 | or more available input, which it will be in this code. | |
245 | <pre><b> | |
246 | ret = deflate(&strm, flush); /* no bad return value */ | |
247 | assert(ret != Z_STREAM_ERROR); /* state not clobbered */ | |
248 | </b></pre> | |
249 | Now we compute how much output <tt>deflate()</tt> provided on the last call, which is the | |
250 | difference between how much space was provided before the call, and how much output space | |
251 | is still available after the call. Then that data, if any, is written to the output file. | |
252 | We can then reuse the output buffer for the next call of <tt>deflate()</tt>. Again if there | |
253 | is 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(&strm); | |
258 | return Z_ERRNO; | |
259 | } | |
260 | </b></pre> | |
261 | The inner <tt>do</tt>-loop is repeated until the last <tt>deflate()</tt> call fails to fill the | |
262 | provided output buffer. Then we know that <tt>deflate()</tt> has done as much as it can with | |
263 | the provided input, and that all of that input has been consumed. We can then fall out of this | |
264 | loop and reuse the input buffer. | |
265 | <p> | |
266 | The way we tell that <tt>deflate()</tt> has no more output is by seeing that it did not fill | |
267 | the 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. | |
270 | As far as we know, <tt>deflate()</tt> | |
271 | has more output for us. So we call it again. But now <tt>deflate()</tt> produces no output | |
272 | at all, and <tt>avail_out</tt> remains unchanged as <tt>CHUNK</tt>. That <tt>deflate()</tt> call | |
273 | wasn'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 | |
275 | all. Now we finally have the desired indication that <tt>deflate()</tt> is really done, | |
276 | and so we drop out of the inner loop to provide more input to <tt>deflate()</tt>. | |
277 | <p> | |
278 | With <tt>flush</tt> set to <tt>Z_FINISH</tt>, this final set of <tt>deflate()</tt> calls will | |
279 | complete 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 | |
281 | until the state is reinitialized. | |
282 | <p> | |
283 | Some applications of <em>zlib</em> have two loops that call <tt>deflate()</tt> | |
284 | instead of the single inner loop we have here. The first loop would call | |
285 | without flushing and feed all of the data to <tt>deflate()</tt>. The second loop would call | |
286 | <tt>deflate()</tt> with no more | |
287 | data and the <tt>Z_FINISH</tt> parameter to complete the process. As you can see from this | |
288 | example, 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><!-- --> | |
293 | Now we check to see if we have already processed all of the input file. That information was | |
294 | saved in the <tt>flush</tt> variable, so we see if that was set to <tt>Z_FINISH</tt>. If so, | |
295 | then we're done and we fall out of the outer loop. We're guaranteed to get <tt>Z_STREAM_END</tt> | |
296 | from the last <tt>deflate()</tt> call, since we ran it until the last chunk of input was | |
297 | consumed 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><!-- --> | |
303 | The 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 | |
305 | finally we can return with a happy return value. | |
306 | <pre><b> | |
307 | /* clean up and return */ | |
308 | (void)deflateEnd(&strm); | |
309 | return Z_OK; | |
310 | } | |
311 | </b></pre><!-- --> | |
312 | Now we do the same thing for decompression in the <tt>inf()</tt> routine. <tt>inf()</tt> | |
313 | decompresses what is hopefully a valid <em>zlib</em> stream from the input file and writes the | |
314 | uncompressed data to the output file. Much of the discussion above for <tt>def()</tt> | |
315 | applies to <tt>inf()</tt> as well, so the discussion here will focus on the differences between | |
316 | the 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. */ | |
324 | int inf(FILE *source, FILE *dest) | |
325 | { | |
326 | </b></pre> | |
327 | The local variables have the same functionality as they do for <tt>def()</tt>. The | |
328 | only difference is that there is no <tt>flush</tt> variable, since <tt>inflate()</tt> | |
329 | can 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><!-- --> | |
337 | The initialization of the state is the same, except that there is no compression level, | |
338 | of course, and two more elements of the structure are initialized. <tt>avail_in</tt> | |
339 | and <tt>next_in</tt> must be initialized before calling <tt>inflateInit()</tt>. This | |
340 | is because the application has the option to provide the start of the zlib stream in | |
341 | order for <tt>inflateInit()</tt> to have access to information about the compression | |
342 | method 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 | |
345 | of <em>zlib</em> that provide more compression methods may take advantage of this interface. | |
346 | In 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> | |
349 | Here <tt>avail_in</tt> is set to zero and <tt>next_in</tt> is set to <tt>Z_NULL</tt> to | |
350 | indicate 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(&strm); | |
359 | if (ret != Z_OK) | |
360 | return ret; | |
361 | </b></pre><!-- --> | |
362 | The outer <tt>do</tt>-loop decompresses input until <tt>inflate()</tt> indicates | |
363 | that it has reached the end of the compressed data and has produced all of the uncompressed | |
364 | output. This is in contrast to <tt>def()</tt> which processes all of the input file. | |
365 | If end-of-file is reached before the compressed data self-terminates, then the compressed | |
366 | data 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> | |
371 | We read input data and set the <tt>strm</tt> structure accordingly. If we've reached the | |
372 | end of the input file, then we leave the outer loop and report an error, since the | |
373 | compressed data is incomplete. Note that we may read more data than is eventually consumed | |
374 | by <tt>inflate()</tt>, if the input file continues past the <em>zlib</em> stream. | |
375 | For applications where <em>zlib</em> streams are embedded in other data, this routine would | |
376 | need to be modified to return the unused data, or at least indicate how much of the input | |
377 | data 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(&strm); | |
382 | return Z_ERRNO; | |
383 | } | |
384 | if (strm.avail_in == 0) | |
385 | break; | |
386 | strm.next_in = in; | |
387 | </b></pre><!-- --> | |
388 | The inner <tt>do</tt>-loop has the same function it did in <tt>def()</tt>, which is to | |
389 | keep calling <tt>inflate()</tt> until has generated all of the output it can with the | |
390 | provided input. | |
391 | <pre><b> | |
392 | /* run inflate() on input until output buffer not full */ | |
393 | do { | |
394 | </b></pre> | |
395 | Just 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> | |
400 | Now we run the decompression engine itself. There is no need to adjust the flush parameter, since | |
401 | the <em>zlib</em> format is self-terminating. The main difference here is that there are | |
402 | return values that we need to pay attention to. <tt>Z_DATA_ERROR</tt> | |
403 | indicates that <tt>inflate()</tt> detected an error in the <em>zlib</em> compressed data format, | |
404 | which means that either the data is not a <em>zlib</em> stream to begin with, or that the data was | |
405 | corrupted 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> | |
407 | needs it, unlike <tt>deflate()</tt>, whose memory is allocated at the start by <tt>deflateInit()</tt>. | |
408 | <p> | |
409 | Advanced applications may use | |
410 | <tt>deflateSetDictionary()</tt> to prime <tt>deflate()</tt> with a set of likely data to improve the | |
411 | first 32K or so of compression. This is noted in the <em>zlib</em> header, so <tt>inflate()</tt> | |
412 | requests that that dictionary be provided before it can start to decompress. Without the dictionary, | |
413 | correct decompression is not possible. For this routine, we have no idea what the dictionary is, | |
414 | so 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, | |
417 | but could be checked for as noted above for <tt>def()</tt>. <tt>Z_BUF_ERROR</tt> does not need to be | |
418 | checked for here, for the same reasons noted for <tt>def()</tt>. <tt>Z_STREAM_END</tt> will be | |
419 | checked for later. | |
420 | <pre><b> | |
421 | ret = inflate(&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(&strm); | |
429 | return ret; | |
430 | } | |
431 | </b></pre> | |
432 | The 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(&strm); | |
437 | return Z_ERRNO; | |
438 | } | |
439 | </b></pre> | |
440 | The inner <tt>do</tt>-loop ends when <tt>inflate()</tt> has no more output as indicated | |
441 | by not filling the output buffer, just as for <tt>deflate()</tt>. In this case, we cannot | |
442 | assert that <tt>strm.avail_in</tt> will be zero, since the deflate stream may end before the file | |
443 | does. | |
444 | <pre><b> | |
445 | } while (strm.avail_out == 0); | |
446 | </b></pre><!-- --> | |
447 | The outer <tt>do</tt>-loop ends when <tt>inflate()</tt> reports that it has reached the | |
448 | end of the input <em>zlib</em> stream, has completed the decompression and integrity | |
449 | check, and has provided all of the output. This is indicated by the <tt>inflate()</tt> | |
450 | return value <tt>Z_STREAM_END</tt>. The inner loop is guaranteed to leave <tt>ret</tt> | |
451 | equal to <tt>Z_STREAM_END</tt> if the last chunk of the input file read contained the end | |
452 | of the <em>zlib</em> stream. So if the return value is not <tt>Z_STREAM_END</tt>, the | |
453 | loop 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><!-- --> | |
458 | At this point, decompression successfully completed, or we broke out of the loop due to no | |
459 | more data being available from the input file. If the last <tt>inflate()</tt> return value | |
460 | is not <tt>Z_STREAM_END</tt>, then the <em>zlib</em> stream was incomplete and a data error | |
461 | is returned. Otherwise, we return with a happy return value. Of course, <tt>inflateEnd()</tt> | |
462 | is called first to avoid a memory leak. | |
463 | <pre><b> | |
464 | /* clean up and return */ | |
465 | (void)inflateEnd(&strm); | |
466 | return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR; | |
467 | } | |
468 | </b></pre><!-- --> | |
469 | That ends the routines that directly use <em>zlib</em>. The following routines make this | |
470 | a 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> | |
474 | and <tt>inf()</tt>, as detailed in their comments above, and print out an error message. | |
475 | Note that these are only a subset of the possible return values from <tt>deflate()</tt> | |
476 | and <tt>inflate()</tt>. | |
477 | <pre><b> | |
478 | /* report a zlib or i/o error */ | |
479 | void 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><!-- --> | |
503 | Here 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 | |
505 | no arguments are given, or it is a decompression pipe if <tt>zpipe -d</tt> is used. If any other | |
506 | arguments are provided, no compression or decompression is performed. Instead a usage | |
507 | message 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 */ | |
511 | int 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 && 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] < source > 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> |