diff --git a/src/liblzma/api/lzma/stream_flags.h b/src/liblzma/api/lzma/stream_flags.h
index 80c5f00f..bb080ac7 100644
--- a/src/liblzma/api/lzma/stream_flags.h
+++ b/src/liblzma/api/lzma/stream_flags.h
@@ -35,9 +35,38 @@
  * Options for encoding and decoding Stream Header and Stream Footer
  */
 typedef struct {
+	/**
+	 * \brief       Stream Flags format version
+	 *
+	 * To prevent API and ABI breakages if new features are needed in
+	 * Stream Header or Stream Footer, a version number is used to
+	 * indicate which fields in this structure are in use. For now,
+	 * version must always be zero. With non-zero version, the
+	 * lzma_stream_header_encode() and lzma_stream_footer_encode()
+	 * will return LZMA_HEADER_ERROR.
+	 *
+	 * lzma_stream_header_decode() and lzma_stream_footer_decode()
+	 * will always set this to the lowest value that supports all the
+	 * features indicated by the Stream Flags field. The application
+	 * must check that the version number set by the decoding functions
+	 * is supported by the application. Otherwise it is possible that
+	 * the application will decode the Stream incorrectly.
+	 */
+	uint32_t version;
+
 	/**
 	 * Backward Size must be a multiple of four bytes. In this Stream
 	 * format version Backward Size is the size of the Index field.
+	 *
+	 * Backward Size isn't actually part of the Stream Flags field, but
+	 * it is convenient to include in this structure anyway. Backward
+	 * Size is present only in the Stream Footer. There is no need to
+	 * initialize backward_size when encoding Stream Header.
+	 *
+	 * lzma_stream_header_decode() always sets backward_size to
+	 * LZMA_VLI_VALUE_UNKNOWN so that it is convenient to use
+	 * lzma_stream_flags_compare() when both Stream Header and Stream
+	 * Footer have been decoded.
 	 */
 	lzma_vli backward_size;
 #	define LZMA_BACKWARD_SIZE_MIN 4
@@ -48,6 +77,34 @@ typedef struct {
 	 */
 	lzma_check check;
 
+	/**
+	 * Reserved space to allow possible future extensions without
+	 * breaking the ABI. You should not touch these, because the
+	 * names of these variables may change.
+	 *
+	 * (We will never be able to use all of these since Stream Flags
+	 * is just two bytes plus Backward Size of four bytes. But it's
+	 * nice to have the proper types when they are needed.)
+	 */
+	lzma_reserved_enum reserved_enum1;
+	lzma_reserved_enum reserved_enum2;
+	lzma_reserved_enum reserved_enum3;
+	lzma_reserved_enum reserved_enum4;
+	lzma_reserved_enum reserved_enum5;
+	lzma_reserved_enum reserved_enum6;
+	lzma_bool reserved_bool1;
+	lzma_bool reserved_bool2;
+	lzma_bool reserved_bool3;
+	lzma_bool reserved_bool4;
+	lzma_bool reserved_bool5;
+	lzma_bool reserved_bool6;
+	lzma_bool reserved_bool7;
+	lzma_bool reserved_bool8;
+	uint32_t reserved_int1;
+	uint32_t reserved_int2;
+	uint32_t reserved_int3;
+	uint32_t reserved_int4;
+
 } lzma_stream_flags;
 
 
@@ -61,6 +118,8 @@ typedef struct {
  *                          need to be initialized.
  *
  * \return      - LZMA_OK: Encoding was successful.
+ *              - LZMA_HEADER_ERROR: options->version is not supported by
+ *                this liblzma version.
  *              - LZMA_PROG_ERROR: Invalid options.
  */
 extern lzma_ret lzma_stream_header_encode(
@@ -76,6 +135,8 @@ extern lzma_ret lzma_stream_header_encode(
  * \param       options     Stream Footer options to be encoded.
  *
  * \return      - LZMA_OK: Encoding was successful.
+ *              - LZMA_HEADER_ERROR: options->version is not supported by
+ *                this liblzma version.
  *              - LZMA_PROG_ERROR: Invalid options.
  */
 extern lzma_ret lzma_stream_footer_encode(
@@ -92,7 +153,7 @@ extern lzma_ret lzma_stream_footer_encode(
  *
  * options->index_size is always set to LZMA_VLI_VALUE_UNKNOWN. This is to
  * help comparing Stream Flags from Stream Header and Stream Footer with
- * lzma_stream_flags_equal().
+ * lzma_stream_flags_compare().
  *
  * \return      - LZMA_OK: Decoding was successful.
  *              - LZMA_FORMAT_ERROR: Magic bytes don't match, thus the given
@@ -121,6 +182,13 @@ extern lzma_ret lzma_stream_header_decode(
  *                is corrupt.
  *              - LZMA_HEADER_ERROR: Unsupported options are present
  *                in the footer.
+ *
+ * \note        If Stream Header was already decoded successfully, but
+ *              decoding Stream Footer returns LZMA_FORMAT_ERROR, the
+ *              application should probably report some other error message
+ *              than "unsupported file format", since the file more likely is
+ *              corrupt (possibly truncated). Stream decoder in liblzma uses
+ *              LZMA_DATA_ERROR in this situation.
  */
 extern lzma_ret lzma_stream_footer_decode(
 		lzma_stream_flags *options, const uint8_t *in)
@@ -130,10 +198,18 @@ extern lzma_ret lzma_stream_footer_decode(
 /**
  * \brief       Compare two lzma_stream_flags structures
  *
- * index_size values are compared only if both are not LZMA_VLI_VALUE_UNKNOWN.
+ * backward_size values are compared only if both are not
+ * LZMA_VLI_VALUE_UNKNOWN.
  *
- * \return      true if both structures are considered equal; false otherwise.
+ * \return      - LZMA_OK: Both are equal. If either had backward_size set
+ *                to LZMA_VLI_VALUE_UNKNOWN, backward_size values were not
+ *                compared or validated.
+ *              - LZMA_DATA_ERROR: The structures differ.
+ *              - LZMA_HEADER_ERROR: version in either structure is greater
+ *                than the maximum supported version (currently zero).
+ *              - LZMA_PROG_ERROR: Invalid value, e.g. invalid check or
+ *                backward_size.
  */
-extern lzma_bool lzma_stream_flags_equal(
+extern lzma_ret lzma_stream_flags_compare(
 		const lzma_stream_flags *a, const lzma_stream_flags *b)
 		lzma_attr_pure;
diff --git a/src/liblzma/common/stream_decoder.c b/src/liblzma/common/stream_decoder.c
index 884c4e9d..5360d87c 100644
--- a/src/liblzma/common/stream_decoder.c
+++ b/src/liblzma/common/stream_decoder.c
@@ -294,9 +294,8 @@ stream_decode(lzma_coder *coder, lzma_allocator *allocator,
 
 		// Compare that the Stream Flags fields are identical in
 		// both Stream Header and Stream Footer.
-		if (!lzma_stream_flags_equal(&coder->stream_flags,
-				&footer_flags))
-			return LZMA_DATA_ERROR;
+		return_if_error(lzma_stream_flags_compare(
+				&coder->stream_flags, &footer_flags));
 
 		if (!coder->concatenated)
 			return LZMA_STREAM_END;
diff --git a/src/liblzma/common/stream_encoder.c b/src/liblzma/common/stream_encoder.c
index b21ee652..8748f52e 100644
--- a/src/liblzma/common/stream_encoder.c
+++ b/src/liblzma/common/stream_encoder.c
@@ -180,6 +180,7 @@ stream_encode(lzma_coder *coder, lzma_allocator *allocator,
 
 		// Encode the Stream Footer into coder->buffer.
 		const lzma_stream_flags stream_flags = {
+			.version = 0,
 			.backward_size = lzma_index_size(coder->index),
 			.check = coder->block_options.check,
 		};
@@ -247,6 +248,7 @@ lzma_stream_encoder_init(lzma_next_coder *next, lzma_allocator *allocator,
 
 	// Encode the Stream Header
 	lzma_stream_flags stream_flags = {
+		.version = 0,
 		.check = check,
 	};
 	return_if_error(lzma_stream_header_encode(
diff --git a/src/liblzma/common/stream_flags_common.c b/src/liblzma/common/stream_flags_common.c
index c44b3ff2..aaa9fe02 100644
--- a/src/liblzma/common/stream_flags_common.c
+++ b/src/liblzma/common/stream_flags_common.c
@@ -24,17 +24,31 @@ const uint8_t lzma_header_magic[6] = { 0xFF, 0x4C, 0x5A, 0x4D, 0x41, 0x00 };
 const uint8_t lzma_footer_magic[2] = { 0x59, 0x5A };
 
 
-extern LZMA_API lzma_bool
-lzma_stream_flags_equal(const lzma_stream_flags *a, const lzma_stream_flags *b)
+extern LZMA_API lzma_ret
+lzma_stream_flags_compare(
+		const lzma_stream_flags *a, const lzma_stream_flags *b)
 {
+	// We can compare only version 0 structures.
+	if (a->version != 0 || b->version != 0)
+		return LZMA_HEADER_ERROR;
+
+	// Check type
+	if ((unsigned int)(a->check) > LZMA_CHECK_ID_MAX
+			|| (unsigned int)(b->check) > LZMA_CHECK_ID_MAX)
+		return LZMA_PROG_ERROR;
+
 	if (a->check != b->check)
-		return false;
+		return LZMA_DATA_ERROR;
 
 	// Backward Sizes are compared only if they are known in both.
 	if (a->backward_size != LZMA_VLI_VALUE_UNKNOWN
-			&& b->backward_size != LZMA_VLI_VALUE_UNKNOWN
-			&& a->backward_size != b->backward_size)
-		return false;
+			&& b->backward_size != LZMA_VLI_VALUE_UNKNOWN) {
+		if (!is_backward_size_valid(a) || !is_backward_size_valid(b))
+			return LZMA_PROG_ERROR;
 
-	return true;
+		if (a->backward_size != b->backward_size)
+			return LZMA_DATA_ERROR;
+	}
+
+	return LZMA_OK;
 }
diff --git a/src/liblzma/common/stream_flags_common.h b/src/liblzma/common/stream_flags_common.h
index 6e57857b..f422b02a 100644
--- a/src/liblzma/common/stream_flags_common.h
+++ b/src/liblzma/common/stream_flags_common.h
@@ -28,4 +28,13 @@
 extern const uint8_t lzma_header_magic[6];
 extern const uint8_t lzma_footer_magic[2];
 
+
+static inline bool
+is_backward_size_valid(const lzma_stream_flags *options)
+{
+	return options->backward_size >= LZMA_BACKWARD_SIZE_MIN
+			&& options->backward_size <= LZMA_BACKWARD_SIZE_MAX
+			&& (options->backward_size & 3) == 0;
+}
+
 #endif
diff --git a/src/liblzma/common/stream_flags_decoder.c b/src/liblzma/common/stream_flags_decoder.c
index ccc1539d..a54ce813 100644
--- a/src/liblzma/common/stream_flags_decoder.c
+++ b/src/liblzma/common/stream_flags_decoder.c
@@ -27,6 +27,7 @@ stream_flags_decode(lzma_stream_flags *options, const uint8_t *in)
 	if (in[0] != 0x00 || (in[1] & 0xF0))
 		return true;
 
+	options->version = 0;
 	options->check = in[1] & 0x0F;
 
 	return false;
@@ -53,7 +54,7 @@ lzma_stream_header_decode(lzma_stream_flags *options, const uint8_t *in)
 		return LZMA_HEADER_ERROR;
 
 	// Set Backward Size to indicate unknown value. That way
-	// lzma_stream_flags_equal can be used to compare Stream Header
+	// lzma_stream_flags_compare() can be used to compare Stream Header
 	// and Stream Footer while keeping it useful also for comparing
 	// two Stream Footers.
 	options->backward_size = LZMA_VLI_VALUE_UNKNOWN;
diff --git a/src/liblzma/common/stream_flags_encoder.c b/src/liblzma/common/stream_flags_encoder.c
index 1d736a8a..88ed81e0 100644
--- a/src/liblzma/common/stream_flags_encoder.c
+++ b/src/liblzma/common/stream_flags_encoder.c
@@ -39,6 +39,9 @@ lzma_stream_header_encode(const lzma_stream_flags *options, uint8_t *out)
 	assert(sizeof(lzma_header_magic) + LZMA_STREAM_FLAGS_SIZE
 			+ 4 == LZMA_STREAM_HEADER_SIZE);
 
+	if (options->version != 0)
+		return LZMA_HEADER_ERROR;
+
 	// Magic
 	memcpy(out, lzma_header_magic, sizeof(lzma_header_magic));
 
@@ -63,10 +66,11 @@ lzma_stream_footer_encode(const lzma_stream_flags *options, uint8_t *out)
 	assert(2 * 4 + LZMA_STREAM_FLAGS_SIZE + sizeof(lzma_footer_magic)
 			== LZMA_STREAM_HEADER_SIZE);
 
+	if (options->version != 0)
+		return LZMA_HEADER_ERROR;
+
 	// Backward Size
-	if (options->backward_size < LZMA_BACKWARD_SIZE_MIN
-			|| options->backward_size > LZMA_BACKWARD_SIZE_MAX
-			|| (options->backward_size & 3))
+	if (!is_backward_size_valid(options))
 		return LZMA_PROG_ERROR;
 
 	integer_write_32(out + 4, options->backward_size / 4 - 1);
diff --git a/tests/test_stream_flags.c b/tests/test_stream_flags.c
index ead75501..abd0296d 100644
--- a/tests/test_stream_flags.c
+++ b/tests/test_stream_flags.c
@@ -28,7 +28,11 @@ static uint8_t buffer[LZMA_STREAM_HEADER_SIZE];
 static bool
 validate(void)
 {
-	return !lzma_stream_flags_equal(&known_flags, &decoded_flags);
+	// TODO: This could require the specific error type as an argument.
+	// We could also test that lzma_stream_flags_compare() gives
+	// the correct return values in different situations.
+	return lzma_stream_flags_compare(&known_flags, &decoded_flags)
+			!= LZMA_OK;
 }
 
 
@@ -44,7 +48,7 @@ test_header_decoder(lzma_ret expected_ret)
 		return false;
 
 	// Header doesn't have Backward Size, so make
-	// lzma_stream_flags_equal() ignore it.
+	// lzma_stream_flags_compare() ignore it.
 	decoded_flags.backward_size = LZMA_VLI_VALUE_UNKNOWN;
 	return validate();
 }