+Without further adieu, here is the program zpipe.c: +
+/* zpipe.c: example of proper use of zlib's inflate() and deflate() + Not copyrighted -- provided to the public domain + Version 1.4 11 December 2005 Mark Adler */ + +/* Version history: + 1.0 30 Oct 2004 First version + 1.1 8 Nov 2004 Add void casting for unused return values + Use switch statement for inflate() return values + 1.2 9 Nov 2004 Add assertions to document zlib guarantees + 1.3 6 Apr 2005 Remove incorrect assertion in inf() + 1.4 11 Dec 2005 Add hack to avoid MSDOS end-of-line conversions + Avoid some compiler warnings for input and output buffers + */ ++We now include the header files for the required definitions. From +stdio.h we use fopen(), fread(), fwrite(), +feof(), ferror(), and fclose() for file i/o, and +fputs() for error messages. From string.h we use +strcmp() for command line argument processing. +From assert.h we use the assert() macro. +From zlib.h +we use the basic compression functions deflateInit(), +deflate(), and deflateEnd(), and the basic decompression +functions inflateInit(), inflate(), and +inflateEnd(). +
+#include <stdio.h> +#include <string.h> +#include <assert.h> +#include "zlib.h" ++This is an ugly hack required to avoid corruption of the input and output data on +Windows/MS-DOS systems. Without this, those systems would assume that the input and output +files are text, and try to convert the end-of-line characters from one standard to +another. That would corrupt binary data, and in particular would render the compressed data unusable. +This sets the input and output to binary which suppresses the end-of-line conversions. +SET_BINARY_MODE() will be used later on stdin and stdout, at the beginning of main(). +
+#if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(__CYGWIN__) +# include <fcntl.h> +# include <io.h> +# define SET_BINARY_MODE(file) setmode(fileno(file), O_BINARY) +#else +# define SET_BINARY_MODE(file) +#endif ++CHUNK is simply the buffer size for feeding data to and pulling data +from the zlib routines. Larger buffer sizes would be more efficient, +especially for inflate(). If the memory is available, buffers sizes +on the order of 128K or 256K bytes should be used. +
+#define CHUNK 16384 ++The def() routine compresses data from an input file to an output file. The output data +will be in the zlib format, which is different from the gzip or zip +formats. The zlib format has a very small header of only two bytes to identify it as +a zlib stream and to provide decoding information, and a four-byte trailer with a fast +check value to verify the integrity of the uncompressed data after decoding. +
+/* Compress from file source to file dest until EOF on source.
+ def() returns Z_OK on success, Z_MEM_ERROR if memory could not be
+ allocated for processing, Z_STREAM_ERROR if an invalid compression
+ level is supplied, Z_VERSION_ERROR if the version of zlib.h and the
+ version of the library linked do not match, or Z_ERRNO if there is
+ an error reading or writing the files. */
+int def(FILE *source, FILE *dest, int level)
+{
+
+Here are the local variables for def(). ret will be used for zlib
+return codes. flush will keep track of the current flushing state for deflate(),
+which is either no flushing, or flush to completion after the end of the input file is reached.
+have is the amount of data returned from deflate(). The strm structure
+is used to pass information to and from the zlib routines, and to maintain the
+deflate() state. in and out are the input and output buffers for
+deflate().
++ int ret, flush; + unsigned have; + z_stream strm; + unsigned char in[CHUNK]; + unsigned char out[CHUNK]; ++The first thing we do is to initialize the zlib state for compression using +deflateInit(). This must be done before the first use of deflate(). +The zalloc, zfree, and opaque fields in the strm +structure must be initialized before calling deflateInit(). Here they are +set to the zlib constant Z_NULL to request that zlib use +the default memory allocation routines. An application may also choose to provide +custom memory allocation routines here. deflateInit() will allocate on the +order of 256K bytes for the internal state. +(See zlib Technical Details.) +
+deflateInit() is called with a pointer to the structure to be initialized and +the compression level, which is an integer in the range of -1 to 9. Lower compression +levels result in faster execution, but less compression. Higher levels result in +greater compression, but slower execution. The zlib constant Z_DEFAULT_COMPRESSION, +equal to -1, +provides a good compromise between compression and speed and is equivalent to level 6. +Level 0 actually does no compression at all, and in fact expands the data slightly to produce +the zlib format (it is not a byte-for-byte copy of the input). +More advanced applications of zlib +may use deflateInit2() here instead. Such an application may want to reduce how +much memory will be used, at some price in compression. Or it may need to request a +gzip header and trailer instead of a zlib header and trailer, or raw +encoding with no header or trailer at all. +
+We must check the return value of deflateInit() against the zlib constant +Z_OK to make sure that it was able to +allocate memory for the internal state, and that the provided arguments were valid. +deflateInit() will also check that the version of zlib that the zlib.h +file came from matches the version of zlib actually linked with the program. This +is especially important for environments in which zlib is a shared library. +
+Note that an application can initialize multiple, independent zlib streams, which can +operate in parallel. The state information maintained in the structure allows the zlib +routines to be reentrant. +
+ /* allocate deflate state */ + strm.zalloc = Z_NULL; + strm.zfree = Z_NULL; + strm.opaque = Z_NULL; + ret = deflateInit(&strm, level); + if (ret != Z_OK) + return ret; ++With the pleasantries out of the way, now we can get down to business. The outer do-loop +reads all of the input file and exits at the bottom of the loop once end-of-file is reached. +This loop contains the only call of deflate(). So we must make sure that all of the +input data has been processed and that all of the output data has been generated and consumed +before we fall out of the loop at the bottom. +
+ /* compress until end of file */
+ do {
+
+We start off by reading data from the input file. The number of bytes read is put directly
+into avail_in, and a pointer to those bytes is put into next_in. We also
+check to see if end-of-file on the input has been reached. If we are at the end of file, then flush is set to the
+zlib constant Z_FINISH, which is later passed to deflate() to
+indicate that this is the last chunk of input data to compress. We need to use feof()
+to check for end-of-file as opposed to seeing if fewer than CHUNK bytes have been read. The
+reason is that if the input file length is an exact multiple of CHUNK, we will miss
+the fact that we got to the end-of-file, and not know to tell deflate() to finish
+up the compressed stream. If we are not yet at the end of the input, then the zlib
+constant Z_NO_FLUSH will be passed to deflate to indicate that we are still
+in the middle of the uncompressed data.
++If there is an error in reading from the input file, the process is aborted with +deflateEnd() being called to free the allocated zlib state before returning +the error. We wouldn't want a memory leak, now would we? deflateEnd() can be called +at any time after the state has been initialized. Once that's done, deflateInit() (or +deflateInit2()) would have to be called to start a new compression process. There is +no point here in checking the deflateEnd() return code. The deallocation can't fail. +
+ strm.avail_in = fread(in, 1, CHUNK, source);
+ if (ferror(source)) {
+ (void)deflateEnd(&strm);
+ return Z_ERRNO;
+ }
+ flush = feof(source) ? Z_FINISH : Z_NO_FLUSH;
+ strm.next_in = in;
+
+The inner do-loop passes our chunk of input data to deflate(), and then
+keeps calling deflate() until it is done producing output. Once there is no more
+new output, deflate() is guaranteed to have consumed all of the input, i.e.,
+avail_in will be zero.
+
+ /* run deflate() on input until output buffer not full, finish
+ compression if all of source has been read in */
+ do {
+
+Output space is provided to deflate() by setting avail_out to the number
+of available output bytes and next_out to a pointer to that space.
++ strm.avail_out = CHUNK; + strm.next_out = out; ++Now we call the compression engine itself, deflate(). It takes as many of the +avail_in bytes at next_in as it can process, and writes as many as +avail_out bytes to next_out. Those counters and pointers are then +updated past the input data consumed and the output data written. It is the amount of +output space available that may limit how much input is consumed. +Hence the inner loop to make sure that +all of the input is consumed by providing more output space each time. Since avail_in +and next_in are updated by deflate(), we don't have to mess with those +between deflate() calls until it's all used up. +
+The parameters to deflate() are a pointer to the strm structure containing +the input and output information and the internal compression engine state, and a parameter +indicating whether and how to flush data to the output. Normally deflate will consume +several K bytes of input data before producing any output (except for the header), in order +to accumulate statistics on the data for optimum compression. It will then put out a burst of +compressed data, and proceed to consume more input before the next burst. Eventually, +deflate() +must be told to terminate the stream, complete the compression with provided input data, and +write out the trailer check value. deflate() will continue to compress normally as long +as the flush parameter is Z_NO_FLUSH. Once the Z_FINISH parameter is provided, +deflate() will begin to complete the compressed output stream. However depending on how +much output space is provided, deflate() may have to be called several times until it +has provided the complete compressed stream, even after it has consumed all of the input. The flush +parameter must continue to be Z_FINISH for those subsequent calls. +
+There are other values of the flush parameter that are used in more advanced applications. You can +force deflate() to produce a burst of output that encodes all of the input data provided +so far, even if it wouldn't have otherwise, for example to control data latency on a link with +compressed data. You can also ask that deflate() do that as well as erase any history up to +that point so that what follows can be decompressed independently, for example for random access +applications. Both requests will degrade compression by an amount depending on how often such +requests are made. +
+deflate() has a return value that can indicate errors, yet we do not check it here. Why +not? Well, it turns out that deflate() can do no wrong here. Let's go through +deflate()'s return values and dispense with them one by one. The possible values are +Z_OK, Z_STREAM_END, Z_STREAM_ERROR, or Z_BUF_ERROR. Z_OK +is, well, ok. Z_STREAM_END is also ok and will be returned for the last call of +deflate(). This is already guaranteed by calling deflate() with Z_FINISH +until it has no more output. Z_STREAM_ERROR is only possible if the stream is not +initialized properly, but we did initialize it properly. There is no harm in checking for +Z_STREAM_ERROR here, for example to check for the possibility that some +other part of the application inadvertently clobbered the memory containing the zlib state. +Z_BUF_ERROR will be explained further below, but +suffice it to say that this is simply an indication that deflate() could not consume +more input or produce more output. deflate() can be called again with more output space +or more available input, which it will be in this code. +
+ ret = deflate(&strm, flush); /* no bad return value */ + assert(ret != Z_STREAM_ERROR); /* state not clobbered */ ++Now we compute how much output deflate() provided on the last call, which is the +difference between how much space was provided before the call, and how much output space +is still available after the call. Then that data, if any, is written to the output file. +We can then reuse the output buffer for the next call of deflate(). Again if there +is a file i/o error, we call deflateEnd() before returning to avoid a memory leak. +
+ have = CHUNK - strm.avail_out;
+ if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
+ (void)deflateEnd(&strm);
+ return Z_ERRNO;
+ }
+
+The inner do-loop is repeated until the last deflate() call fails to fill the
+provided output buffer. Then we know that deflate() has done as much as it can with
+the provided input, and that all of that input has been consumed. We can then fall out of this
+loop and reuse the input buffer.
++The way we tell that deflate() has no more output is by seeing that it did not fill +the output buffer, leaving avail_out greater than zero. However suppose that +deflate() has no more output, but just so happened to exactly fill the output buffer! +avail_out is zero, and we can't tell that deflate() has done all it can. +As far as we know, deflate() +has more output for us. So we call it again. But now deflate() produces no output +at all, and avail_out remains unchanged as CHUNK. That deflate() call +wasn't able to do anything, either consume input or produce output, and so it returns +Z_BUF_ERROR. (See, I told you I'd cover this later.) However this is not a problem at +all. Now we finally have the desired indication that deflate() is really done, +and so we drop out of the inner loop to provide more input to deflate(). +
+With flush set to Z_FINISH, this final set of deflate() calls will +complete the output stream. Once that is done, subsequent calls of deflate() would return +Z_STREAM_ERROR if the flush parameter is not Z_FINISH, and do no more processing +until the state is reinitialized. +
+Some applications of zlib have two loops that call deflate() +instead of the single inner loop we have here. The first loop would call +without flushing and feed all of the data to deflate(). The second loop would call +deflate() with no more +data and the Z_FINISH parameter to complete the process. As you can see from this +example, that can be avoided by simply keeping track of the current flush state. +
+ } while (strm.avail_out == 0); + assert(strm.avail_in == 0); /* all input will be used */ ++Now we check to see if we have already processed all of the input file. That information was +saved in the flush variable, so we see if that was set to Z_FINISH. If so, +then we're done and we fall out of the outer loop. We're guaranteed to get Z_STREAM_END +from the last deflate() call, since we ran it until the last chunk of input was +consumed and all of the output was generated. +
+ /* done when last data in file processed */ + } while (flush != Z_FINISH); + assert(ret == Z_STREAM_END); /* stream will be complete */ ++The process is complete, but we still need to deallocate the state to avoid a memory leak +(or rather more like a memory hemorrhage if you didn't do this). Then +finally we can return with a happy return value. +
+ /* clean up and return */ + (void)deflateEnd(&strm); + return Z_OK; +} ++Now we do the same thing for decompression in the inf() routine. inf() +decompresses what is hopefully a valid zlib stream from the input file and writes the +uncompressed data to the output file. Much of the discussion above for def() +applies to inf() as well, so the discussion here will focus on the differences between +the two. +
+/* Decompress from file source to file dest until stream ends or EOF.
+ inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be
+ allocated for processing, Z_DATA_ERROR if the deflate data is
+ invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and
+ the version of the library linked do not match, or Z_ERRNO if there
+ is an error reading or writing the files. */
+int inf(FILE *source, FILE *dest)
+{
+
+The local variables have the same functionality as they do for def(). The
+only difference is that there is no flush variable, since inflate()
+can tell from the zlib stream itself when the stream is complete.
++ int ret; + unsigned have; + z_stream strm; + unsigned char in[CHUNK]; + unsigned char out[CHUNK]; ++The initialization of the state is the same, except that there is no compression level, +of course, and two more elements of the structure are initialized. avail_in +and next_in must be initialized before calling inflateInit(). This +is because the application has the option to provide the start of the zlib stream in +order for inflateInit() to have access to information about the compression +method to aid in memory allocation. In the current implementation of zlib +(up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of +inflate() anyway. However those fields must be initialized since later versions +of zlib that provide more compression methods may take advantage of this interface. +In any case, no decompression is performed by inflateInit(), so the +avail_out and next_out fields do not need to be initialized before calling. +
+Here avail_in is set to zero and next_in is set to Z_NULL to +indicate that no input data is being provided. +
+ /* allocate inflate state */ + strm.zalloc = Z_NULL; + strm.zfree = Z_NULL; + strm.opaque = Z_NULL; + strm.avail_in = 0; + strm.next_in = Z_NULL; + ret = inflateInit(&strm); + if (ret != Z_OK) + return ret; ++The outer do-loop decompresses input until inflate() indicates +that it has reached the end of the compressed data and has produced all of the uncompressed +output. This is in contrast to def() which processes all of the input file. +If end-of-file is reached before the compressed data self-terminates, then the compressed +data is incomplete and an error is returned. +
+ /* decompress until deflate stream ends or end of file */
+ do {
+
+We read input data and set the strm structure accordingly. If we've reached the
+end of the input file, then we leave the outer loop and report an error, since the
+compressed data is incomplete. Note that we may read more data than is eventually consumed
+by inflate(), if the input file continues past the zlib stream.
+For applications where zlib streams are embedded in other data, this routine would
+need to be modified to return the unused data, or at least indicate how much of the input
+data was not used, so the application would know where to pick up after the zlib stream.
+
+ strm.avail_in = fread(in, 1, CHUNK, source);
+ if (ferror(source)) {
+ (void)inflateEnd(&strm);
+ return Z_ERRNO;
+ }
+ if (strm.avail_in == 0)
+ break;
+ strm.next_in = in;
+
+The inner do-loop has the same function it did in def(), which is to
+keep calling inflate() until has generated all of the output it can with the
+provided input.
+
+ /* run inflate() on input until output buffer not full */
+ do {
+
+Just like in def(), the same output space is provided for each call of inflate().
++ strm.avail_out = CHUNK; + strm.next_out = out; ++Now we run the decompression engine itself. There is no need to adjust the flush parameter, since +the zlib format is self-terminating. The main difference here is that there are +return values that we need to pay attention to. Z_DATA_ERROR +indicates that inflate() detected an error in the zlib compressed data format, +which means that either the data is not a zlib stream to begin with, or that the data was +corrupted somewhere along the way since it was compressed. The other error to be processed is +Z_MEM_ERROR, which can occur since memory allocation is deferred until inflate() +needs it, unlike deflate(), whose memory is allocated at the start by deflateInit(). +
+Advanced applications may use +deflateSetDictionary() to prime deflate() with a set of likely data to improve the +first 32K or so of compression. This is noted in the zlib header, so inflate() +requests that that dictionary be provided before it can start to decompress. Without the dictionary, +correct decompression is not possible. For this routine, we have no idea what the dictionary is, +so the Z_NEED_DICT indication is converted to a Z_DATA_ERROR. +
+inflate() can also return Z_STREAM_ERROR, which should not be possible here, +but could be checked for as noted above for def(). Z_BUF_ERROR does not need to be +checked for here, for the same reasons noted for def(). Z_STREAM_END will be +checked for later. +
+ ret = inflate(&strm, Z_NO_FLUSH);
+ assert(ret != Z_STREAM_ERROR); /* state not clobbered */
+ switch (ret) {
+ case Z_NEED_DICT:
+ ret = Z_DATA_ERROR; /* and fall through */
+ case Z_DATA_ERROR:
+ case Z_MEM_ERROR:
+ (void)inflateEnd(&strm);
+ return ret;
+ }
+
+The output of inflate() is handled identically to that of deflate().
+
+ have = CHUNK - strm.avail_out;
+ if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
+ (void)inflateEnd(&strm);
+ return Z_ERRNO;
+ }
+
+The inner do-loop ends when inflate() has no more output as indicated
+by not filling the output buffer, just as for deflate(). In this case, we cannot
+assert that strm.avail_in will be zero, since the deflate stream may end before the file
+does.
++ } while (strm.avail_out == 0); ++The outer do-loop ends when inflate() reports that it has reached the +end of the input zlib stream, has completed the decompression and integrity +check, and has provided all of the output. This is indicated by the inflate() +return value Z_STREAM_END. The inner loop is guaranteed to leave ret +equal to Z_STREAM_END if the last chunk of the input file read contained the end +of the zlib stream. So if the return value is not Z_STREAM_END, the +loop continues to read more input. +
+ /* done when inflate() says it's done */ + } while (ret != Z_STREAM_END); ++At this point, decompression successfully completed, or we broke out of the loop due to no +more data being available from the input file. If the last inflate() return value +is not Z_STREAM_END, then the zlib stream was incomplete and a data error +is returned. Otherwise, we return with a happy return value. Of course, inflateEnd() +is called first to avoid a memory leak. +
+ /* clean up and return */ + (void)inflateEnd(&strm); + return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR; +} ++That ends the routines that directly use zlib. The following routines make this +a command-line program by running data through the above routines from stdin to +stdout, and handling any errors reported by def() or inf(). +
+zerr() is used to interpret the possible error codes from def() +and inf(), as detailed in their comments above, and print out an error message. +Note that these are only a subset of the possible return values from deflate() +and inflate(). +
+/* report a zlib or i/o error */
+void zerr(int ret)
+{
+ fputs("zpipe: ", stderr);
+ switch (ret) {
+ case Z_ERRNO:
+ if (ferror(stdin))
+ fputs("error reading stdin\n", stderr);
+ if (ferror(stdout))
+ fputs("error writing stdout\n", stderr);
+ break;
+ case Z_STREAM_ERROR:
+ fputs("invalid compression level\n", stderr);
+ break;
+ case Z_DATA_ERROR:
+ fputs("invalid or incomplete deflate data\n", stderr);
+ break;
+ case Z_MEM_ERROR:
+ fputs("out of memory\n", stderr);
+ break;
+ case Z_VERSION_ERROR:
+ fputs("zlib version mismatch!\n", stderr);
+ }
+}
+
+Here is the main() routine used to test def() and inf(). The
+zpipe command is simply a compression pipe from stdin to stdout, if
+no arguments are given, or it is a decompression pipe if zpipe -d is used. If any other
+arguments are provided, no compression or decompression is performed. Instead a usage
+message is displayed. Examples are zpipe < foo.txt > foo.txt.z to compress, and
+zpipe -d < foo.txt.z > foo.txt to decompress.
+
+/* compress or decompress from stdin to stdout */
+int main(int argc, char **argv)
+{
+ int ret;
+
+ /* avoid end-of-line conversions */
+ SET_BINARY_MODE(stdin);
+ SET_BINARY_MODE(stdout);
+
+ /* do compression if no arguments */
+ if (argc == 1) {
+ ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION);
+ if (ret != Z_OK)
+ zerr(ret);
+ return ret;
+ }
+
+ /* do decompression if -d specified */
+ else if (argc == 2 && strcmp(argv[1], "-d") == 0) {
+ ret = inf(stdin, stdout);
+ if (ret != Z_OK)
+ zerr(ret);
+ return ret;
+ }
+
+ /* otherwise, report usage */
+ else {
+ fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr);
+ return 1;
+ }
+}
+
+