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[xz/debian.git] / doc / examples / 02_decompress.c

///////////////////////////////////////////////////////////////////////////////
//
/// \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;
}