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, 297 insertions, 0 deletions

diff --git a/doc/examples/01_compress_easy.c b/doc/examples/01_compress_easy.c
new file mode 100644
index 00000000..f79cade1
--- /dev/null
+++ b/doc/examples/01_compress_easy.c
@@ -0,0 +1,297 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+/// \file       01_compress_easy.c
+/// \brief      Compress from stdin to stdout in multi-call mode
+///
+/// Usage:      ./01_compress_easy PRESET < INFILE > OUTFILE
+///
+/// Example:    ./01_compress_easy 6 < foo > foo.xz
+//
+//  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 void
+show_usage_and_exit(const char *argv0)
+{
+	fprintf(stderr, "Usage: %s PRESET < INFILE > OUTFILE\n"
+			"PRESET is a number 0-9 and can optionally be "
+			"by `e' to indicate extreme preset\n",
+			argv0);
+	exit(EXIT_FAILURE);
+}
+
+
+static uint32_t
+get_preset(int argc, char **argv)
+{
+	// One argument whose first char must be 0-9.
+	if (argc != 2 || argv[1][0] < '0' || argv[1][0] > '9')
+		show_usage_and_exit(argv[0]);
+
+	// Calculate the preste level 0-9.
+	uint32_t preset = argv[1][0] - '0';
+
+	// If there is a second char, it must be 'e'. It will set
+	// the LZMA_PRESET_EXTREME flag.
+	if (argv[1][1] != '\0') {
+		if (argv[1][1] != 'e' || argv[1][2] != '\0')
+			show_usage_and_exit(argv[0]);
+
+		preset |= LZMA_PRESET_EXTREME;
+	}
+
+	return preset;
+}
+
+
+static bool
+init_encoder(lzma_stream *strm, uint32_t preset)
+{
+	// Initialize the encoder using a preset. Set the integrity to check
+	// to CRC64, which is the default in the xz command line tool. If
+	// the .xz file needs to be decompressed with XZ Embedded, use
+	// LZMA_CHECK_CRC32 instead.
+	lzma_ret ret = lzma_easy_encoder(strm, preset, LZMA_CHECK_CRC64);
+
+	// 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).
+	const char *msg;
+	switch (ret) {
+	case LZMA_MEM_ERROR:
+		msg = "Memory allocation failed";
+		break;
+
+	case LZMA_OPTIONS_ERROR:
+		msg = "Specified preset is not supported";
+		break;
+
+	case LZMA_UNSUPPORTED_CHECK:
+		msg = "Specified integrity check is not supported";
+		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 encoder: %s (error code %u)\n",
+			msg, ret);
+	return false;
+}
+
+
+static bool
+compress(lzma_stream *strm, FILE *infile, FILE *outfile)
+{
+	// This will be LZMA_RUN until the end of the input file is reached.
+	// This tells lzma_code() when there will be no more input.
+	lzma_action action = LZMA_RUN;
+
+	// Buffers to temporarily hold uncompressed input
+	// and compressed output.
+	uint8_t inbuf[BUFSIZ];
+	uint8_t outbuf[BUFSIZ];
+
+	// Initialize the input and output pointers. Initializing next_in
+	// and avail_in isn't really necessary when we are going to encode
+	// just one file since LZMA_STREAM_INIT takes care of initializing
+	// those already. But it doesn't hurt much and it will be needed
+	// if encoding more than one file like we will in 02_decompress.c.
+	//
+	// While we don't care about strm->total_in or strm->total_out in this
+	// example, it is worth noting that initializing the encoder will
+	// always reset total_in and total_out to zero. But the encoder
+	// initialization doesn't touch next_in, avail_in, next_out, or
+	// avail_out.
+	strm->next_in = NULL;
+	strm->avail_in = 0;
+	strm->next_out = outbuf;
+	strm->avail_out = sizeof(outbuf);
+
+	// Loop until the file has been successfully compressed or until
+	// an error occurs.
+	while (true) {
+		// Fill the input buffer if it is empty.
+		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, "Read error: %s\n",
+						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 and that it should finish the
+			// encoding.
+			if (feof(infile))
+				action = LZMA_FINISH;
+		}
+
+		// Tell liblzma do the actual encoding.
+		//
+		// This reads up to strm->avail_in bytes of input starting
+		// from strm->next_in. avail_in will be decremented and
+		// next_in incremented by an equal amount to match the
+		// number of input bytes consumed.
+		//
+		// Up to strm->avail_out bytes of compressed output will be
+		// written starting from strm->next_out. avail_out and next_out
+		// will be incremented by an equal amount to match the number
+		// of output bytes written.
+		//
+		// The encoder has to do internal buffering, which means that
+		// it may take quite a bit of input before the same data is
+		// available in compressed form in the output buffer.
+		lzma_ret ret = lzma_code(strm, action);
+
+		// If the output buffer is full or if the compression finished
+		// successfully, write the data from the output bufffer to
+		// the output file.
+		if (strm->avail_out == 0 || ret == LZMA_STREAM_END) {
+			// When lzma_code() has returned LZMA_STREAM_END,
+			// the output buffer is likely to be only partially
+			// full. Calculate how much new data there is to
+			// be written to the output file.
+			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;
+			}
+
+			// Reset next_out and avail_out.
+			strm->next_out = outbuf;
+			strm->avail_out = sizeof(outbuf);
+		}
+
+		// Normally the return value of lzma_code() will be LZMA_OK
+		// until everything has been encoded.
+		if (ret != LZMA_OK) {
+			// Once everything has been encoded 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.
+			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. Most values listen in lzma_ret
+			// enumeration aren't possible in this example.
+			const char *msg;
+			switch (ret) {
+			case LZMA_MEM_ERROR:
+				msg = "Memory allocation failed";
+				break;
+
+			case LZMA_DATA_ERROR:
+				// This error is returned if the compressed
+				// or uncompressed size get near 8 EiB
+				// (2^63 bytes) because that's where the .xz
+				// file format size limits currently are.
+				// That is, the possibility of this error
+				// is mostly theoretical unless you are doing
+				// something very unusual.
+				//
+				// Note that strm->total_in and strm->total_out
+				// have nothing to do with this error. Changing
+				// those variables won't increase or decrease
+				// the chance of getting this error.
+				msg = "File size limits exceeded";
+				break;
+
+			default:
+				// This is most likely LZMA_PROG_ERROR, but
+				// if this program is buggy (or liblzma has
+				// a bug), it may be e.g. LZMA_BUF_ERROR or
+				// LZMA_OPTIONS_ERROR too.
+				//
+				// 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, "Encoder error: %s (error code %u)\n",
+					msg, ret);
+			return false;
+		}
+	}
+}
+
+
+extern int
+main(int argc, char **argv)
+{
+	// Get the preset number from the command line.
+	uint32_t preset = get_preset(argc, argv);
+
+	// Initialize a lzma_stream structure. When it is allocated on stack,
+	// it is simplest to use LZMA_STREAM_INIT macro like below. When it
+	// is allocated on heap, using memset(strmptr, 0, sizeof(*strmptr))
+	// works (as long as NULL pointers are represented with zero bits
+	// as they are on practically all computers today).
+	lzma_stream strm = LZMA_STREAM_INIT;
+
+	// Initialize the encoder. If it succeeds, compress from
+	// stdin to stdout.
+	bool success = init_encoder(&strm, preset);
+	if (success)
+		success = compress(&strm, stdin, stdout);
+
+	// Free the memory allocated for the encoder. If we were encoding
+	// multiple files, this would only need to be done after the last
+	// file. See 02_decompress.c for handling of multiple files.
+	//
+	// It is OK to call lzma_end() multiple times or when it hasn't been
+	// actually used except initialized with LZMA_STREAM_INIT.
+	lzma_end(&strm);
+
+	// Close stdout to catch possible write errors that can occur
+	// when pending data is flushed from the stdio buffers.
+	if (fclose(stdout)) {
+		fprintf(stderr, "Write error: %s\n", strerror(errno));
+		success = false;
+	}
+
+	return success ? EXIT_SUCCESS : EXIT_FAILURE;
+}