02_decompress.c File Reference

Decompress .xz files to stdout. More...

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

Go to the source code of this file.

Functions
static bool	init_decoder (lzma_stream *strm)
static bool	decompress (lzma_stream *strm, const char *inname, FILE *infile, FILE *outfile)
int	main (int argc, char **argv)

Detailed Description

Decompress .xz files to stdout.

Usage: ./02_decompress INPUT_FILES... > OUTFILE

Example: ./02_decompress foo.xz bar.xz > foobar

Definition in file 02_decompress.c.

Function Documentation

decompress()

static bool decompress ( lzma_stream *  strm, const char *  inname, FILE *  infile, FILE *  outfile  )

static

Definition at line 104 of file 02_decompress.c.

{
    // 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;
        }
    }
}

References test-lz4-speed::action, lzma_stream::avail_in, lzma_stream::avail_out, if(), inbuf, LZMA_BUF_ERROR, LZMA_DATA_ERROR, LZMA_FINISH, LZMA_FORMAT_ERROR, LZMA_MEM_ERROR, LZMA_OK, LZMA_OPTIONS_ERROR, LZMA_RUN, LZMA_STREAM_END, msg, lzma_stream::next_in, lzma_stream::next_out, NULL, outbuf, outfile, and strm.

Referenced by main().

init_decoder()

static bool init_decoder ( lzma_stream *  strm )

static

Definition at line 26 of file 02_decompress.c.

{
    // 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;
}

References LZMA_CONCATENATED, LZMA_MEM_ERROR, LZMA_OK, LZMA_OPTIONS_ERROR, msg, strm, and UINT64_MAX.

Referenced by main().

main()

int main	(	int	argc,
		char **	argv
	)

Definition at line 244 of file 02_decompress.c.

{
    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;
}

References argv, decompress(), benchmark::FILE, i, init_decoder(), LZMA_STREAM_INIT, NULL, and strm.