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| 6 | <title>zlib Usage Example</title> |
| 7 | <!-- Copyright (c) 2004, 2005 Mark Adler. --> |
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| 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> |