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