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Filename : 02_decompress.c
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///////////////////////////////////////////////////////////////////////////////
//
/// \file       02_decompress.c
/// \brief      Decompress .xz files to stdout
///
/// Usage:      ./02_decompress INPUT_FILES... > OUTFILE
///
/// Example:    ./02_decompress foo.xz bar.xz > foobar
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////

#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <lzma.h>


static bool
init_decoder(lzma_stream *strm)
{
	// Initialize a .xz decoder. The decoder supports a memory usage limit
	// and a set of flags.
	//
	// The memory usage of the decompressor depends on the settings used
	// to compress a .xz file. It can vary from less than a megabyte to
	// a few gigabytes, but in practice (at least for now) it rarely
	// exceeds 65 MiB because that's how much memory is required to
	// decompress files created with "xz -9". Settings requiring more
	// memory take extra effort to use and don't (at least for now)
	// provide significantly better compression in most cases.
	//
	// Memory usage limit is useful if it is important that the
	// decompressor won't consume gigabytes of memory. The need
	// for limiting depends on the application. In this example,
	// no memory usage limiting is used. This is done by setting
	// the limit to UINT64_MAX.
	//
	// The .xz format allows concatenating compressed files as is:
	//
	//     echo foo | xz > foobar.xz
	//     echo bar | xz >> foobar.xz
	//
	// When decompressing normal standalone .xz files, LZMA_CONCATENATED
	// should always be used to support decompression of concatenated
	// .xz files. If LZMA_CONCATENATED isn't used, the decoder will stop
	// after the first .xz stream. This can be useful when .xz data has
	// been embedded inside another file format.
	//
	// Flags other than LZMA_CONCATENATED are supported too, and can
	// be combined with bitwise-or. See lzma/container.h
	// (src/liblzma/api/lzma/container.h in the source package or e.g.
	// /usr/include/lzma/container.h depending on the install prefix)
	// for details.
	lzma_ret ret = lzma_stream_decoder(
			strm, UINT64_MAX, LZMA_CONCATENATED);

	// Return successfully if the initialization went fine.
	if (ret == LZMA_OK)
		return true;

	// Something went wrong. The possible errors are documented in
	// lzma/container.h (src/liblzma/api/lzma/container.h in the source
	// package or e.g. /usr/include/lzma/container.h depending on the
	// install prefix).
	//
	// Note that LZMA_MEMLIMIT_ERROR is never possible here. If you
	// specify a very tiny limit, the error will be delayed until
	// the first headers have been parsed by a call to lzma_code().
	const char *msg;
	switch (ret) {
	case LZMA_MEM_ERROR:
		msg = "Memory allocation failed";
		break;

	case LZMA_OPTIONS_ERROR:
		msg = "Unsupported decompressor flags";
		break;

	default:
		// This is most likely LZMA_PROG_ERROR indicating a bug in
		// this program or in liblzma. It is inconvenient to have a
		// separate error message for errors that should be impossible
		// to occur, but knowing the error code is important for
		// debugging. That's why it is good to print the error code
		// at least when there is no good error message to show.
		msg = "Unknown error, possibly a bug";
		break;
	}

	fprintf(stderr, "Error initializing the decoder: %s (error code %u)\n",
			msg, ret);
	return false;
}


static bool
decompress(lzma_stream *strm, const char *inname, FILE *infile, FILE *outfile)
{
	// When LZMA_CONCATENATED flag was used when initializing the decoder,
	// we need to tell lzma_code() when there will be no more input.
	// This is done by setting action to LZMA_FINISH instead of LZMA_RUN
	// in the same way as it is done when encoding.
	//
	// When LZMA_CONCATENATED isn't used, there is no need to use
	// LZMA_FINISH to tell when all the input has been read, but it
	// is still OK to use it if you want. When LZMA_CONCATENATED isn't
	// used, the decoder will stop after the first .xz stream. In that
	// case some unused data may be left in strm->next_in.
	lzma_action action = LZMA_RUN;

	uint8_t inbuf[BUFSIZ];
	uint8_t outbuf[BUFSIZ];

	strm->next_in = NULL;
	strm->avail_in = 0;
	strm->next_out = outbuf;
	strm->avail_out = sizeof(outbuf);

	while (true) {
		if (strm->avail_in == 0 && !feof(infile)) {
			strm->next_in = inbuf;
			strm->avail_in = fread(inbuf, 1, sizeof(inbuf),
					infile);

			if (ferror(infile)) {
				fprintf(stderr, "%s: Read error: %s\n",
						inname, strerror(errno));
				return false;
			}

			// Once the end of the input file has been reached,
			// we need to tell lzma_code() that no more input
			// will be coming. As said before, this isn't required
			// if the LZMA_CONCATENATED flag isn't used when
			// initializing the decoder.
			if (feof(infile))
				action = LZMA_FINISH;
		}

		lzma_ret ret = lzma_code(strm, action);

		if (strm->avail_out == 0 || ret == LZMA_STREAM_END) {
			size_t write_size = sizeof(outbuf) - strm->avail_out;

			if (fwrite(outbuf, 1, write_size, outfile)
					!= write_size) {
				fprintf(stderr, "Write error: %s\n",
						strerror(errno));
				return false;
			}

			strm->next_out = outbuf;
			strm->avail_out = sizeof(outbuf);
		}

		if (ret != LZMA_OK) {
			// Once everything has been decoded successfully, the
			// return value of lzma_code() will be LZMA_STREAM_END.
			//
			// It is important to check for LZMA_STREAM_END. Do not
			// assume that getting ret != LZMA_OK would mean that
			// everything has gone well or that when you aren't
			// getting more output it must have successfully
			// decoded everything.
			if (ret == LZMA_STREAM_END)
				return true;

			// It's not LZMA_OK nor LZMA_STREAM_END,
			// so it must be an error code. See lzma/base.h
			// (src/liblzma/api/lzma/base.h in the source package
			// or e.g. /usr/include/lzma/base.h depending on the
			// install prefix) for the list and documentation of
			// possible values. Many values listen in lzma_ret
			// enumeration aren't possible in this example, but
			// can be made possible by enabling memory usage limit
			// or adding flags to the decoder initialization.
			const char *msg;
			switch (ret) {
			case LZMA_MEM_ERROR:
				msg = "Memory allocation failed";
				break;

			case LZMA_FORMAT_ERROR:
				// .xz magic bytes weren't found.
				msg = "The input is not in the .xz format";
				break;

			case LZMA_OPTIONS_ERROR:
				// For example, the headers specify a filter
				// that isn't supported by this liblzma
				// version (or it hasn't been enabled when
				// building liblzma, but no-one sane does
				// that unless building liblzma for an
				// embedded system). Upgrading to a newer
				// liblzma might help.
				//
				// Note that it is unlikely that the file has
				// accidentally became corrupt if you get this
				// error. The integrity of the .xz headers is
				// always verified with a CRC32, so
				// unintentionally corrupt files can be
				// distinguished from unsupported files.
				msg = "Unsupported compression options";
				break;

			case LZMA_DATA_ERROR:
				msg = "Compressed file is corrupt";
				break;

			case LZMA_BUF_ERROR:
				// Typically this error means that a valid
				// file has got truncated, but it might also
				// be a damaged part in the file that makes
				// the decoder think the file is truncated.
				// If you prefer, you can use the same error
				// message for this as for LZMA_DATA_ERROR.
				msg = "Compressed file is truncated or "
						"otherwise corrupt";
				break;

			default:
				// This is most likely LZMA_PROG_ERROR.
				msg = "Unknown error, possibly a bug";
				break;
			}

			fprintf(stderr, "%s: Decoder error: "
					"%s (error code %u)\n",
					inname, msg, ret);
			return false;
		}
	}
}


extern int
main(int argc, char **argv)
{
	if (argc <= 1) {
		fprintf(stderr, "Usage: %s FILES...\n", argv[0]);
		return EXIT_FAILURE;
	}

	lzma_stream strm = LZMA_STREAM_INIT;

	bool success = true;

	// Try to decompress all files.
	for (int i = 1; i < argc; ++i) {
		if (!init_decoder(&strm)) {
			// Decoder initialization failed. There's no point
			// to retry it so we need to exit.
			success = false;
			break;
		}

		FILE *infile = fopen(argv[i], "rb");

		if (infile == NULL) {
			fprintf(stderr, "%s: Error opening the "
					"input file: %s\n",
					argv[i], strerror(errno));
			success = false;
		} else {
			success &= decompress(&strm, argv[i], infile, stdout);
			fclose(infile);
		}
	}

	// Free the memory allocated for the decoder. This only needs to be
	// done after the last file.
	lzma_end(&strm);

	if (fclose(stdout)) {
		fprintf(stderr, "Write error: %s\n", strerror(errno));
		success = false;
	}

	return success ? EXIT_SUCCESS : EXIT_FAILURE;
}