Docs: Add new example programs.

These have more comments than the old examples and
human-readable error messages. More tutorial-like examples
are needed but these are a start.

1 files changed, 287 insertions, 0 deletions

diff --git a/doc/examples/02_decompress.c b/doc/examples/02_decompress.c
new file mode 100644
index 00000000..4c0f37cb
--- /dev/null
+++ b/doc/examples/02_decompress.c
@@ -0,0 +1,287 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+/// \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_CONATENATED 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;
+}