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| author | Lasse Collin <lasse.collin@tukaani.org> | 2008-06-17 15:03:46 +0300 |
|---|---|---|
| committer | Lasse Collin <lasse.collin@tukaani.org> | 2008-06-17 15:03:46 +0300 |
| commit | bf6348d1a3ff09fdc06940468f318f75ffa6af11 (patch) | |
| tree | 60db0660cd88e208997d1133a8bf089c83ab2ec8 /doc/file-format.txt | |
| parent | Fix uninitialized variable in LZMA encoder. This was (diff) | |
| download | xz-bf6348d1a3ff09fdc06940468f318f75ffa6af11.tar.xz | |
Update the file format specification draft. The new one is
a lot simpler than the previous versions, but it also means
that the existing code will change a lot.
Diffstat (limited to '')
| -rw-r--r-- | doc/file-format.txt | 1794 |
1 files changed, 508 insertions, 1286 deletions
diff --git a/doc/file-format.txt b/doc/file-format.txt index 2c8cd486..49c9a75f 100644 --- a/doc/file-format.txt +++ b/doc/file-format.txt @@ -3,82 +3,54 @@ The .lzma File Format --------------------- 0. Preface - 0.1. Copyright Notices - 0.2. Changes + 0.1. Copyright Notices + 0.2. Changes 1. Conventions - 1.1. Byte and Its Representation - 1.2. Multibyte Integers - 2. Stream - 2.1. Stream Types - 2.1.1. Single-Block Stream - 2.1.2. Multi-Block Stream - 2.2. Stream Header - 2.2.1. Header Magic Bytes - 2.2.2. Stream Flags - 2.2.3. CRC32 + 1.1. Byte and Its Representation + 1.2. Multibyte Integers + 2. Overall Structure of .lzma File + 2.1. Stream + 2.1.1. Stream Header + 2.1.1.1. Header Magic Bytes + 2.1.1.2. Stream Flags + 2.1.1.3. CRC32 + 2.1.2. Stream Footer + 2.1.2.1. CRC32 + 2.1.2.2. Backward Size + 2.1.2.3. Stream Flags + 2.1.2.4. Footer Magic Bytes + 2.2. Stream Padding 3. Block - 3.1. Block Header - 3.1.1. Block Flags - 3.1.2. Compressed Size - 3.1.3. Uncompressed Size - 3.1.4. List of Filter Flags - 3.1.4.1. Misc - 3.1.4.2. External ID - 3.1.4.3. External Size of Properties - 3.1.4.4. Filter Properties - 3.1.5. CRC32 - 3.1.6. Header Padding - 3.2. Compressed Data - 3.3. Block Footer - 3.3.1. Check - 3.3.2. Stream Footer - 3.3.2.1. Uncompressed Size - 3.3.2.2. Backward Size - 3.3.2.3. Stream Flags - 3.3.2.4. Footer Magic Bytes - 3.3.3. Footer Padding - 4. Filters - 4.1. Detecting when All Data Has Been Decoded - 4.1.1. With Uncompressed Size - 4.1.2. With End of Input - 4.1.3. With End of Payload Marker - 4.2. Alignment - 4.3. Filters - 4.3.1. Copy - 4.3.2. Subblock - 4.3.2.1. Format of the Encoded Output - 4.3.3. Delta - 4.3.3.1. Format of the Encoded Output - 4.3.4. LZMA - 4.3.4.1. LZMA Properties - 4.3.4.2. Dictionary Flags - 4.3.5. Branch/Call/Jump Filters for Executables - 5. Metadata - 5.1. Metadata Flags - 5.2. Size of Header Metadata Block - 5.3. Total Size - 5.4. Uncompressed Size - 5.5. Index - 5.5.1. Number of Data Blocks - 5.5.2. Total Sizes - 5.5.3. Uncompressed Sizes - 5.6. Extra - 5.6.1. 0x00: Dummy/Padding - 5.6.2. 0x01: OpenPGP Signature - 5.6.3. 0x02: Filter Information - 5.6.4. 0x03: Comment - 5.6.5. 0x04: List of Checks - 5.6.6. 0x05: Original Filename - 5.6.7. 0x07: Modification Time - 5.6.8. 0x09: High-Resolution Modification Time - 5.6.9. 0x0B: MIME Type - 5.6.10. 0x0D: Homepage URL - 6. Custom Filter and Extra Record IDs - 6.1. Reserved Custom Filter ID Ranges - 7. Cyclic Redundancy Checks - 8. References - 8.1. Normative References - 8.2. Informative References + 3.1. Block Header + 3.1.1. Block Header Size + 3.1.2. Block Flags + 3.1.3. Compressed Size + 3.1.4. Uncompressed Size + 3.1.5. List of Filter Flags + 3.1.6. Header Padding + 3.1.7. CRC32 + 3.2. Compressed Data + 3.3. Check + 4. Index + 4.1. Index Indicator + 4.2. Number of Records + 4.3. List of Records + 4.3.1. Total Size + 4.3.2. Uncompressed Size + 4.4. Index Padding + 4.5. CRC32 + 5. Filter Chains + 5.1. Alignment + 5.2. Security + 5.3. Filters + 5.3.1. LZMA2 + 5.3.2. Branch/Call/Jump Filters for Executables + 5.3.3. Delta + 5.3.3.1. Format of the Encoded Output + 5.4. Custom Filter IDs + 5.4.1. Reserved Custom Filter ID Ranges + 6. Cyclic Redundancy Checks + 7. References 0. Preface @@ -95,7 +67,7 @@ The .lzma File Format 0.1. Copyright Notices - Copyright (C) 2006, 2007 Lasse Collin <lasse.collin@tukaani.org> + Copyright (C) 2006-2008 Lasse Collin <lasse.collin@tukaani.org> Copyright (C) 2006 Ville Koskinen <w-ber@iki.fi> Copying and distribution of this file, with or without @@ -106,13 +78,14 @@ The .lzma File Format All source code examples given in this document are put into the public domain by the authors of this document. - Thanks for helping with this document goes to Igor Pavlov, - Mark Adler and Mikko Pouru. + Special thanks for helping with this document goes to + Igor Pavlov. Thanks for helping with this document goes to + Mark Adler, H. Peter Anvin, and Mikko Pouru. 0.2. Changes - Last modified: 2008-02-01 19:25+0200 + Last modified: 2008-06-17 14:10+0300 (A changelog will be kept once the first official version is made.) @@ -161,7 +134,7 @@ The .lzma File Format In this document, a boxed byte or a byte sequence declared using this notation is called `a field'. The example field - above would be called called `the Foo field' or plain `Foo'. + above would be called `the Foo field' or plain `Foo'. 1.2. Multibyte Integers @@ -170,39 +143,22 @@ The .lzma File Format are stored in little endian byte order (least significant byte first). - When smaller values are more likely than bigger values (e.g. - file sizes), multibyte integers are encoded in a simple + When smaller values are more likely than bigger values (for + example file sizes), multibyte integers are encoded in a variable-length representation: - Numbers in the range [0, 127] are copied as is, and take one byte of space. - - Bigger numbers will occupy two or more bytes. The lowest - seven bits of every byte are used for data; the highest - (eighth) bit indicates either that - 0) the byte is in the middle of the byte sequence, or - 1) the byte is the first or the last byte. + - Bigger numbers will occupy two or more bytes. All but the + last byte of the multibyte representation have the highest + (eighth) bit set. For now, the value of the variable-length integers is limited to 63 bits, which limits the encoded size of the integer to nine bytes. These limits may be increased in future if needed. - Note that the encoding is not as optimal as it could be. For - example, it is possible to encode the number 42 using any - number of bytes between one and nine. This is convenient - for non-streamed encoders, that write Compressed Size or - Uncompressed Size fields to the Block Header (see Section 3.1) - after the Compressed Data field is written to the disk. - - In several situations, the decoder needs to compare that two - fields contain identical information. When comparing fields - using the encoding described in this Section, the decoder must - consider two fields identical if their decoded values are - identical; it does not matter if the encoded variable-length - representations differ. - - The following C code illustrates encoding and decoding 63-bit - variables; the highest bit of uint64_t must be unset. The - functions return the number of bytes occupied by the integer - (1-9), or zero on error. + The following C code illustrates encoding and decoding of + variable-length integers. The functions return the number of + bytes occupied by the integer (1-9), or zero on error. #include <sys/types.h> #include <inttypes.h> @@ -210,20 +166,18 @@ The .lzma File Format size_t encode(uint8_t buf[static 9], uint64_t num) { - if (num >= (UINT64_C(1) << (9 * 7))) + if (num >= UINT64_MAX / 2) return 0; - if (num <= 0x7F) { - buf[0] = num; - return 1; - } - buf[0] = (num & 0x7F) | 0x80; - num >>= 7; - size_t i = 1; + + size_t i = 0; + while (num >= 0x80) { - buf[i++] = num & 0x7F; + buf[i++] = (uint8_t)(num) | 0x80; num >>= 7; } - buf[i++] = num | 0x80; + + buf[i++] = (uint8_t)(num); + return i; } @@ -232,46 +186,29 @@ The .lzma File Format { if (size_max == 0) return 0; + if (size_max > 9) size_max = 9; + *num = buf[0] & 0x7F; - if (!(buf[0] & 0x80)) - return 1; - size_t i = 1; - do { - if (i == size_max) - return 0; - *num |= (uint64_t)(buf[i] & 0x7F) << (7 * i); - } while (!(buf[i++] & 0x80)); - return i; - } + size_t i = 0; - size_t - decode_reverse(const uint8_t buf[], size_t size_max, - uint64_t *num) - { - if (size_max == 0) - return 0; - const size_t end = size_max > 9 ? size_max - 9 : 0; - size_t i = size_max - 1; - *num = buf[i] & 0x7F; - if (!(buf[i] & 0x80)) - return 1; - do { - if (i-- == end) + while (buf[i++] & 0x80) { + if (i > size_max || buf[i] == 0x00) return 0; - *num <<= 7; - *num |= buf[i] & 0x7F; - } while (!(buf[i] & 0x80)); - return size_max - i; + + *num |= (uint64_t)(buf[i] & 0x7F) << (i * 7); + } + + return i; } -2. Stream +2. Overall Structure of .lzma File - +========+========+========+ - | Stream | Stream | Stream | ... - +========+========+========+ + +========+================+========+================+ + | Stream | Stream Padding | Stream | Stream Padding | ... + +========+================+========+================+ A file contains usually only one Stream. However, it is possible to concatenate multiple Streams together with no @@ -280,53 +217,44 @@ The .lzma File Format Stream once the end of the first Stream has been reached. -2.1. Stream Types +2.1. Stream - There are two types of Streams: Single-Block Streams and - Multi-Block Streams. Decoders conforming to this specification - must support at least Single-Block Streams. Supporting - Multi-Block Streams is optional. If the decoder supports only - Single-Block Streams, the documentation of the decoder should - mention this fact clearly. + +-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+ + | Stream Header | Block | Block | ... | Block | + +-+-+-+-+-+-+-+-+-+-+-+-+=======+=======+ +=======+ + +=======+-+-+-+-+-+-+-+-+-+-+-+-+ + ---> | Index | Stream Footer | + +=======+-+-+-+-+-+-+-+-+-+-+-+-+ -2.1.1. Single-Block Stream + All the above fields have a size that is a multiple of four. If + Stream is used as an internal part of another file format, it + is recommended to make the Stream start at an offset that is + a multiple of four bytes. - +===============+============+ - | Stream Header | Data Block | - +===============+============+ + Stream Header, Index, and Stream Footer are always present in + a Stream. The maximum size of the Index field is 16 GiB (2^34). - As the name says, a Single-Block Stream has exactly one Block. - The Block must be a Data Block; Metadata Blocks are not allowed - in Single-Block Streams. + There are zero or more Blocks. The maximum number of Blocks is + limited only by the maximum size of the Index field. + Total size of a Stream must be less than 8 EiB (2^63 bytes). + The same limit applies to the total amount of uncompressed + data stored in a Stream. -2.1.2. Multi-Block Stream + If an implementation supports handling .lzma files with + multiple concatenated Streams, it may apply the above limits + to the file as a whole instead of limiting per Stream basis. - +===============+=======================+ - | Stream Header | Header Metadata Block | - +===============+=======================+ - - +============+ +============+=======================+ - ---> | Data Block | ... | Data Block | Footer Metadata Block | - +============+ +============+=======================+ - - Notes: - - Stream Header is mandatory. - - Header Metadata Block is optional. - - Each Multi-Block Stream has at least one Data Block. The - maximum number of Data Blocks is not limited. - - Footer Metadata Block is mandatory. +2.1.1. Stream Header -2.2. Stream Header - - +---+---+---+---+---+---+--------------+--+--+--+--+ + +---+---+---+---+---+---+-------+------+--+--+--+--+ | Header Magic Bytes | Stream Flags | CRC32 | - +---+---+---+---+---+---+--------------+--+--+--+--+ + +---+---+---+---+---+---+-------+------+--+--+--+--+ -2.2.1. Header Magic Bytes +2.1.1.1. Header Magic Bytes The first six (6) bytes of the Stream are so called Header Magic Bytes. They can be used to identify the file type. @@ -341,33 +269,47 @@ The .lzma File Format Notes: - The first byte (0xFF) was chosen so that the files cannot be erroneously detected as being in LZMA_Alone format, in - which the first byte is in the the range [0x00, 0xE0]. + which the first byte is in the range [0x00, 0xE0]. - The sixth byte (0x00) was chosen to prevent applications from misdetecting the file as a text file. + If the Header Magic Bytes don't match, the decoder must + indicate an error. + + +2.1.1.2. Stream Flags + + The first byte of Stream Flags is always a nul byte. In future + this byte may be used to indicate new Stream version or other + Stream properties. + + The second byte of Stream Flags is a bit field: -2.2.2. Stream Flags - - Bit(s) Mask Description - 0-2 0x07 Type of Check (see Section 3.3.1): - ID Size Check name - 0x00 0 bytes None - 0x01 4 bytes CRC32 - 0x02 4 bytes (Reserved) - 0x03 8 bytes CRC64 - 0x04 16 bytes (Reserved) - 0x05 32 bytes SHA-256 - 0x06 32 bytes (Reserved) - 0x07 64 bytes (Reserved) - 3 0x08 The CRC32 field is present in Block Headers. - 4 0x10 If unset, this is a Single-Block Stream; if set, - this is a Multi-Block Stream. - 5-7 0xE0 Reserved for future use; must be zero for now. + Bit(s) Mask Description + 0-3 0x0F Type of Check (see Section 3.3): + ID Size Check name + 0x00 0 bytes None + 0x01 4 bytes CRC32 + 0x02 4 bytes (Reserved) + 0x03 4 bytes (Reserved) + 0x04 8 bytes CRC64 + 0x05 8 bytes (Reserved) + 0x06 8 bytes (Reserved) + 0x07 16 bytes (Reserved) + 0x08 16 bytes (Reserved) + 0x09 16 bytes (Reserved) + 0x0A 32 bytes SHA-256 + 0x0B 32 bytes (Reserved) + 0x0C 32 bytes (Reserved) + 0x0D 64 bytes (Reserved) + 0x0E 64 bytes (Reserved) + 0x0F 64 bytes (Reserved) + 4-7 0xF0 Reserved for future use; must be zero for now. Implementations must support at least the Check IDs 0x00 (None) - and 0x01 (CRC32). Supporting other Check IDs is optional. If an - unsupported Check is used, the decoder must indicate a warning - or error. + and 0x01 (CRC32). Supporting other Check IDs is optional. If + an unsupported Check is used, the decoder should indicate a + warning or error. If any reserved bit is set, the decoder must indicate an error. It is possible that there is a new field present which the @@ -375,256 +317,259 @@ The .lzma File Format incorrectly. -2.2.3. CRC32 +2.1.1.3. CRC32 The CRC32 is calculated from the Stream Flags field. It is stored as an unsigned 32-bit little endian integer. If the calculated value does not match the stored one, the decoder must indicate an error. - Note that this field is always present; the bit in Stream Flags - controls only presence of CRC32 in Block Headers. + The idea is that Stream Flags would always be two bytes, even + if new features are needed. This way old decoders will be able + to verify the CRC32 calculated from Stream Flags, and thus + distinguish between corrupt files (CRC32 doesn't match) and + files that the decoder doesn't support (CRC32 matches but + Stream Flags has reserved bits set). -3. Block +2.1.2. Stream Footer - +==============+=================+==============+ - | Block Header | Compressed Data | Block Footer | - +==============+=================+==============+ + +-+-+-+-+---+---+---+---+-------+------+----------+---------+ + | CRC32 | Backward Size | Stream Flags | Footer Magic Bytes | + +-+-+-+-+---+---+---+---+-------+------+----------+---------+ - There are two types of Blocks: - - Data Blocks hold the actual compressed data. - - Metadata Blocks hold the Index, Extra, and a few other - non-data fields (see Section 5). - The type of the Block is indicated by the corresponding bit - in the Block Flags field (see Section 3.1.1). +2.1.2.1. CRC32 + The CRC32 is calculated from the Backward Size and Stream Flags + fields. It is stored as an unsigned 32-bit little endian + integer. If the calculated value does not match the stored one, + the decoder must indicate an error. -3.1. Block Header + The reason to have the CRC32 field before the Backward Size and + Stream Flags fields is to keep the four-byte fields aligned to + a multiple of four bytes. - +------+------+=================+===================+ - | Block Flags | Compressed Size | Uncompressed Size | - +------+------+=================+===================+ - +======================+--+--+--+--+================+ - ---> | List of Filter Flags | CRC32 | Header Padding | - +======================+--+--+--+--+================+ +2.1.2.2. Backward Size + Backward Size is stored as a 32-bit little endian integer, + which indicates the size of the Index field as multiple of + four bytes, minimum value being four bytes: -3.1.1. Block Flags + real_backward_size = (stored_backward_size + 1) * 4; - The first byte of the Block Flags field is a bit field: + Using a fixed-size integer to store this value makes it + slightly simpler to parse the Stream Footer when the + application needs to parse the Stream backwards. - Bit(s) Mask Description - 0-2 0x07 Number of filters (0-7) - 3 0x08 Use End of Payload Marker (even if - Uncompressed Size is stored to Block Header). - 4 0x10 The Compressed Size field is present. - 5 0x20 The Uncompressed Size field is present. - 6 0x40 Reserved for future use; must be zero for now. - 7 0x80 This is a Metadata Block. - The second byte of the Block Flags field is also a bit field: +2.1.2.3. Stream Flags - Bit(s) Mask Description - 0-4 0x1F Size of the Header Padding field (0-31 bytes) - 5-7 0xE0 Reserved for future use; must be zero for now. + This is a copy of the Stream Flags field from the Stream + Header. The information stored to Stream Flags is needed + when parsing the Stream backwards. The decoder must compare + the Stream Flags fields in both Stream Header and Stream + Footer, and indicate an error if they are not identical. - The decoder must indicate an error if End of Payload Marker - is not used and Uncompressed Size is not stored to the Block - Header. Because of this, the first byte of Block Flags can - never be a nul byte. This is useful when detecting beginning - of the Block after Footer Padding (see Section 3.3.3). - If any reserved bit is set, the decoder must indicate an error. - It is possible that there is a new field present which the - decoder is not aware of, and can thus parse the Block Header - incorrectly. +2.1.2.4. Footer Magic Bytes + As the last step of the decoding process, the decoder must + verify the existence of Footer Magic Bytes. If they don't + match, an error must be indicated. -3.1.2. Compressed Size + Using a C array and ASCII: + const uint8_t FOOTER_MAGIC[2] = { 'Y', 'Z' }; - This field is present only if the appropriate bit is set in - the Block Flags field (see Section 3.1.1). + In hexadecimal: + 59 5A - This field contains the size of the Compressed Data field. - The size is stored using the encoding described in Section 1.2. - If the Compressed Size does not match the real size of the - Compressed Data field, the decoder must indicate an error. + The primary reason to have Footer Magic Bytes is to make + it easier to detect incomplete files quickly, without + uncompressing. If the file does not end with Footer Magic Bytes + (excluding Stream Padding described in Section 2.2), it cannot + be undamaged, unless someone has intentionally appended garbage + after the end of the Stream. - Having the Compressed Size field in the Block Header can be - useful for multithreaded decoding when seeking is not possible. - If the Blocks are small enough, the decoder can read multiple - Blocks into its internal buffer, and decode the Blocks in - parallel. - Compressed Size can also be useful when seeking forwards to - a specific location in streamed mode: the decoder can quickly - skip over irrelevant Blocks, without decoding them. +2.2. Stream Padding + Only the decoders that support decoding of concatenated Streams + must support Stream Padding. -3.1.3. Uncompressed Size + Stream Padding must contain only nul bytes. Any non-nul byte + should be considered as the beginning of a new Stream. To + preserve the four-byte alignment of consecutive Streams, the + size of Stream Padding must be a multiple of four bytes. Empty + Stream Padding is allowed. - This field is present only if the appropriate bit is set in - the Block Flags field (see Section 3.1.1). + Note that non-empty Stream Padding is allowed at the end of the + file; there doesn't need to be a new Stream after non-empty + Stream Padding. This can be convenient in certain situations + [GNU-tar]. - The Uncompressed Size field contains the size of the Block - after uncompressing. + The possibility of Padding should be taken into account when + designing an application that parses the Stream backwards. - Storing Uncompressed Size serves several purposes: - - The decoder will know when all of the data has been - decoded without an explicit End of Payload Marker. - - The decoder knows how much memory it needs to allocate - for a temporary buffer in multithreaded mode. - - Simple error detection: wrong size indicates a broken file. - - Sometimes it is useful to know the file size without - uncompressing the file. - It should be noted that the only reliable way to find out what - the real uncompressed size is is to uncompress the Block, - because the Block Header and Metadata Block fields may contain - (intentionally or unintentionally) invalid information. +3. Block - Uncompressed Size is stored using the encoding described in - Section 1.2. If the Uncompressed Size does not match the - real uncompressed size, the decoder must indicate an error. + +==============+=================+=======+ + | Block Header | Compressed Data | Check | + +==============+=================+=======+ -3.1.4. List of Filter Flags +3.1. Block Header - +================+================+ +================+ - | Filter 0 Flags | Filter 1 Flags | ... | Filter n Flags | - +================+================+ +================+ + +-------------------+-------------+=================+ + | Block Header Size | Block Flags | Compressed Size | + +-------------------+-------------+=================+ - The number of Filter Flags fields is stored in the Block Flags - field (see Section 3.1.1). As a special case, if the number of - Filter Flags fields is zero, it is equivalent to having the - Copy filter as the only filter. + +===================+======================+ + ---> | Uncompressed Size | List of Filter Flags | + +===================+======================+ - The format of each Filter Flags field is as follows: + +================+--+--+--+--+ + ---> | Header Padding | CRC32 | + +================+--+--+--+--+ - +------+=============+=============================+ - | Misc | External ID | External Size of Properties | - +------+=============+=============================+ - +===================+ - ---> | Filter Properties | - +===================+ +3.1.1. Block Header Size - The list of officially defined Filter IDs and the formats of - their Filter Properties are described in Section 4.3. + This field overlaps with the Index Indicator field (see + Section 4.1). + This field contains the size of the Block Header field, + including the Block Header Size field itself. Valid values are + in the range [0x01, 0xFF], which indicate the size of the Block + Header as multiples of four bytes, minimum size being eight + bytes: -3.1.4.1. Misc + real_header_size = (encoded_header_size + 1) * 4; - To save space, the most commonly used Filter IDs and the - Size of Filter Properties are encoded in a single byte. - Depending on the contents of the Misc field, Filter ID is - the value of the Misc or External ID field. + If bigger Block Header is needed in future, a new field can be + added between the current Block Header and Compressed Data + fields. The presence of this new field would be indicated in + the Block Header. - Value Filter ID Size of Filter Properties - 0x00 - 0x1F Misc 0 bytes - 0x20 - 0x3F Misc 1 byte - 0x40 - 0x5F Misc 2 bytes - 0x60 - 0x7F Misc 3 bytes - 0x80 - 0x9F Misc 4 bytes - 0xA0 - 0xBF Misc 5 bytes - 0xC0 - 0xDF Misc 6 bytes - 0xE0 - 0xFE External ID 0-30 bytes - 0xFF External ID External Size of Properties - The following code demonstrates parsing the Misc field and, - when needed, the External ID and External Size of Properties - fields. +3.1.2. Block Flags - uint64_t id; - uint64_t properties_size; - uint8_t misc = read_byte(); + The first byte of the Block Flags field is a bit field: - if (misc >= 0xE0) { - id = read_variable_length_integer(); + Bit(s) Mask Description + 0-1 0x03 Number of filters (1-4) + 2-5 0x3C Reserved for future use; must be zero for now. + 6 0x40 The Compressed Size field is present. + 7 0x80 The Uncompressed Size field is present. - if (misc == 0xFF) - properties_size = read_variable_length_integer(); - else - properties_size = misc - 0xE0; + If any reserved bit is set, the decoder must indicate an error. + It is possible that there is a new field present which the + decoder is not aware of, and can thus parse the Block Header + incorrectly. - } else { - id = misc; - properties_size = misc / 0x20; - } +3.1.3. Compressed Size -3.1.4.2. External ID + This field is present only if the appropriate bit is set in + the Block Flags field (see Section 3.1.2). - This field is present only if the Misc field contains a value - that indicates usage of External ID. The External ID is stored - using the encoding described in Section 1.2. + This field contains the size of the Compressed Data field as + multiple of four bytes, minimum value being four bytes: + real_compressed_size = (stored_compressed_size + 1) * 4; -3.1.4.3. External Size of Properties + The size is stored using the encoding described in Section 1.2. + If the Compressed Size does not match the real size of the + Compressed Data field, the decoder must indicate an error. - This field is present only if the Misc field contains a value - that indicates usage of External Size of Properties. The size - of Filter Properties is stored using the encoding described in - Section 1.2. +3.1.4. Uncompressed Size -3.1.4.4. Filter Properties + This field is present only if the appropriate bit is set in + the Block Flags field (see Section 3.1.2). - Size of this field depends on the Misc field (Section 3.1.4.1) - and, if present, External Size of Properties field (Section - 3.1.4.3). The format of this field is depends on the selected - filter; see Section 4.3 for details. + The Uncompressed Size field contains the size of the Block + after uncompressing. Uncompressed Size is stored using the + encoding described in Section 1.2. If the Uncompressed Size + does not match the real uncompressed size, the decoder must + indicate an error. + Storing the Compressed Size and Uncompressed Size fields serves + several purposes: + - The decoder knows how much memory it needs to allocate + for a temporary buffer in multithreaded mode. + - Simple error detection: wrong size indicates a broken file. + - Seeking forwards to a specific location in streamed mode. -3.1.5. CRC32 + It should be noted that the only reliable way to determine + the real uncompressed size is to uncompress the Block, + because the Block Header and Index fields may contain + (intentionally or unintentionally) invalid information. - This field is present only if the appropriate bit is set in - the Stream Flags field (see Section 2.2.2). - The CRC32 is calculated over everything in the Block Header - field except the Header Padding field and the CRC32 field - itself. It is stored as an unsigned 32-bit little endian - integer. If the calculated value does not match the stored - one, the decoder must indicate an error. +3.1.5. List of Filter Flags + + +================+================+ +================+ + | Filter 0 Flags | Filter 1 Flags | ... | Filter n Flags | + +================+================+ +================+ + + The number of Filter Flags fields is stored in the Block Flags + field (see Section 3.1.2). + + The format of each Filter Flags field is as follows: + + +===========+====================+===================+ + | Filter ID | Size of Properties | Filter Properties | + +===========+====================+===================+ + + Both Filter ID and Size of Properties are stored using the + encoding described in Section 1.2. Size of Properties indicates + the size of the Filter Properties field as bytes. The list of + officially defined Filter IDs and the formats of their Filter + Properties are described in Section 5.3. 3.1.6. Header Padding - This field contains as many nul bytes as indicated by the value - stored in the Header Flags field. If the Header Padding field - contains any non-nul bytes, the decoder must indicate an error. + This field contains as many nul byte as it is needed to make + the Block Header have the size specified in Block Header Size. + If any of the bytes are not nul bytes, the decoder must + indicate an error. It is possible that there is a new field + present which the decoder is not aware of, and can thus parse + the Block Header incorrectly. - The intent of the Header Padding field is to allow alignment - of Compressed Data. The usefulness of alignment is described - in Section 4.3. +3.1.7. CRC32 -3.2. Compressed Data + The CRC32 is calculated over everything in the Block Header + field except the CRC32 field itself. It is stored as an + unsigned 32-bit little endian integer. If the calculated + value does not match the stored one, the decoder must indicate + an error. - The format of Compressed Data depends on Block Flags and List - of Filter Flags. Excluding the descriptions of the simplest - filters in Section 4, the format of the filter-specific encoded - data is out of scope of this document. + By verifying the CRC32 of the Block Header before parsing the + actual contents allows the decoder to distinguish between + corrupt and unsupported files. - Note a special case: if End of Payload Marker (see Section - 3.1.1) is not used and Uncompressed Size is zero, the size - of the Compressed Data field is always zero. +3.2. Compressed Data -3.3. Block Footer + The format of Compressed Data depends on Block Flags and List + of Filter Flags. Excluding the descriptions of the simplest + filters in Section 5.3, the format of the filter-specific + encoded data is out of scope of this document. - +=======+===============+================+ - | Check | Stream Footer | Footer Padding | - +=======+===============+================+ + If the natural size of Compressed Data is not a multiple of + four bytes, it must be padded with 1-3 nul bytes to make it + a multiple of four bytes. -3.3.1. Check +3.3. Check The type and size of the Check field depends on which bits - are set in the Stream Flags field (see Section 2.2.2). + are set in the Stream Flags field (see Section 2.1.1.2). The Check, when used, is calculated from the original uncompressed data. If the calculated Check does not match the @@ -633,101 +578,106 @@ The .lzma File Format a warning or error. -3.3.2. Stream Footer +4. Index - +===================+===============+--------------+ - | Uncompressed Size | Backward Size | Stream Flags | - +===================+===============+--------------+ + +-----------------+=========================+ + | Index Indicator | Number of Index Records | + +-----------------+=========================+ - +----------+---------+ - ---> | Footer Magic Bytes | - +----------+---------+ + +=================+=========+-+-+-+-+ + ---> | List of Records | Padding | CRC32 | + +=================+=========+-+-+-+-+ - Stream Footer is present only in - - Data Block of a Single-Block Stream; and - - Footer Metadata Block of a Multi-Block Stream. + Index serves several purporses. Using it, one can + - verify that all Blocks in a Stream have been processed; + - find out the uncompressed size of a Stream; and + - quickly access the beginning of any Block (random access). - The Stream Footer field is placed inside Block Footer, because - no padding is allowed between Check and Stream Footer. +4.1. Index Indicator -3.3.2.1. Uncompressed Size + This field overlaps with the Block Header Size field (see + Section 3.1.1). The value of Index Indicator is always 0x00. - This field is present only in the Data Block of a Single-Block - Stream if Uncompressed Size is not stored to the Block Header - (see Section 3.1.1). Without the Uncompressed Size field in - Stream Footer it would not be possible to quickly find out - the Uncompressed Size of the Stream in all cases. - Uncompressed Size is stored using the encoding described in - Section 1.2. If the stored value does not match the real - uncompressed size of the Single-Block Stream, the decoder must - indicate an error. +4.2. Number of Records + This field indicates how many Records there are in the List + of Records field, and thus how many Blocks there are in the + Stream. The value is stored using the encoding described in + Section 1.2. If the decoder has decoded all the Blocks of the + Stream, and then notices that the Number of Records doesn't + match the real number of Blocks, the decoder must indicate an + error. -3.3.2.2. Backward Size - This field contains the total size of the Block Header, - Compressed Data, Check, and Uncompressed Size fields. The - value is stored using the encoding described in Section 1.2. - If the Backward Size does not match the real total size of - the appropriate fields, the decoder must indicate an error. +4.3. List of Records - Implementations reading the Stream backwards should notice - that the value in this field can never be zero. + List of Records consists of as many Records as indicated by the + Number of Records field: + +========+========+ + | Record | Record | ... + +========+========+ -3.3.2.3. Stream Flags + Each Record contains two fields: - This is a copy of the Stream Flags field from the Stream - Header. The information stored to Stream Flags is needed - when parsing the Stream backwards. + +============+===================+ + | Total Size | Uncompressed Size | + +============+===================+ + If the decoder has decoded all the Blocks of the Stream, it + must verify that the contents of the Records match the real + Total Size and Uncompressed Size of the respective Blocks. -3.3.2.4. Footer Magic Bytes + Implementation hint: It is possible to verify the Index with + constant memory usage by calculating for example SHA256 of both + the real size values and the List of Records, then comparing + the check values. Implementing this using non-cryptographic + check like CRC32 should be avoided unless small code size is + important. - As the last step of the decoding process, the decoder must - verify the existence of Footer Magic Bytes. If they are not - found, an error must be indicated. + If the decoder supports random-access reading, it must verify + that Total Size and Uncompressed Size of every completely + decoded Block match the sizes stored in the Index. If only + partial Block is decoded, the decoder must verify that the + processed sizes don't exceed the sizes stored in the Index. - Using a C array and ASCII: - const uint8_t FOOTER_MAGIC[2] = { 'Y', 'Z' }; - In hexadecimal: - 59 5A +4.3.1. Total Size - The primary reason to have Footer Magic Bytes is to make - it easier to detect incomplete files quickly, without - uncompressing. If the file does not end with Footer Magic Bytes - (excluding Footer Padding described in Section 3.3.3), it - cannot be undamaged, unless someone has intentionally appended - garbage after the end of the Stream. (Appending garbage at the - end of the file does not prevent uncompressing the file, but - may give a warning or error depending on the decoder - implementation.) + This field indicates the encoded size of the respective Block + as multiples of four bytes, minimum value being four bytes: + real_total_size = (stored_total_size + 1) * 4; -3.3.3. Footer Padding + The value is stored using the encoding described in Section + 1.2. - In certain situations it is convenient to be able to pad - Blocks or Streams to be multiples of, for example, 512 bytes. - Footer Padding makes this possible. Note that this is in no - way required to enforce alignment in the way described in - Section 4.3; the Header Padding field is enough for that. - When Footer Padding is used, it must contain only nul bytes. - Any non-nul byte should be considered as the beginning of - a new Block or Stream. +4.3.2. Uncompressed Size - The possibility of Padding should be taken into account when - designing an application that wants to find out information - about a Stream by parsing Footer Metadata Block. + This field indicates the Uncompressed Size of the respective + Block as bytes. The value is stored using the encoding + described in Section 1.2. - Support for Padding was inspired by a related note in - [GNU-tar]. +4.4. Index Padding -4. Filters + This field must contain 0-3 nul bytes to pad the Index to + a multiple of four bytes. + + +4.5. CRC32 + + The CRC32 is calculated over everything in the Index field + except the CRC32 field itself. The CRC32 is stored as an + unsigned 32-bit little endian integer. If the calculated + value does not match the stored one, the decoder must indicate + an error. + + +5. Filter Chains The Block Flags field defines how many filters are used. When more than one filter is used, the filters are chained; that is, @@ -737,116 +687,11 @@ The .lzma File Format v Uncompressed Data ^ | Filter 0 | Encoder | Filter 1 | Decoder - | ... | | Filter n | v Compressed Data ^ - The filters are independent from each other, except that they - must cooperate a little to make it possible, in all cases, to - detect when all of the data has been decoded. In addition, the - filters should cooperate in the encoder to keep the alignment - optimal. - - -4.1. Detecting when All Data Has Been Decoded - - There must be a way for the decoder to detect when all of the - Compressed Data has been decoded. This is simple when only - one filter is used, but a bit more complex when multiple - filters are chained. - This file format supports three methods to detect when all of - the data has been decoded: - - Uncompressed size - - End of Input - - End of Payload Marker - - In both encoder and decoder, filters are initialized starting - from the first filter in the chain. For each filter, one of - these three methods is used. - - -4.1.1. With Uncompressed Size - - This method is the only method supported by all filters. - It must be used when uncompressed size is known by the - filter-specific encoder or decoder. In practice this means - that Uncompressed Size has been stored to the Block Header. - - In case of the first filter in the chain, the uncompressed size - given to the filter-specific encoder or decoder equals the - Uncompressed Size stored in the Block Header. For the rest of - the filters in the chain, uncompressed size is the size of the - output data of the previous filter in the chain. - - Note that when Use End of Payload Marker bit is set in Block - Flags, Uncompressed Size is considered to be unknown even if - it was present in the Block Header. Thus, if End of Payload - Marker is used, uncompressed size of all of the filters in - the chain is unknown, and can never be used to detect when - all of the data has been decoded. - - Once the correct number of bytes has been written out, the - filter-specific decoder indicates to its caller that all of - the data has been decoded. If the filter-specific decoder - detects End of Input or End of Payload Marker before the - correct number of bytes is decoded, the decoder must indicate - an error. - - -4.1.2. With End of Input - - Most filters will know that all of the data has been decoded - when the End of Input data has been reached. Once the filter - knows that it has received the input data in its entirety, - it finishes its job, and indicates to its caller that all of - the data has been decoded. The filter-specific decoder must - indicate an error if it detects End of Payload Marker. - - Note that this method can work only when the filter is not - the last filter in the chain, because only another filter - can indicate the End of Input data. In practice this means, - that a filter later in the chain must support embedding - End of Payload Marker. - - When a filter that cannot embed End of Payload Marker is the - last filter in the chain, Subblock filter is appended to the - chain as an implicit filter. In the simplest case, this occurs - when no filters are specified, and the End of Payload Marker - bit is set in Block Flags. - - -4.1.3. With End of Payload Marker - - End of Payload Marker is a filter-specific bit sequence that - indicates the end of data. It is supported by only a few - filters. It is used when uncompressed size is unknown, and - the filter - - doesn't support End of Input; or - - is the last filter in the chain. - - End of Payload Marker is embedded at the end of the encoded - data by the filter-specific encoder. When the filter-specific - decoder detects the embedded End of Payload Marker, the decoder - knows that all of the data has been decoded. Then it finishes - its job, and indicates to its caller that all of the data has - been decoded. If the filter-specific decoder detects End of - Input before End of Payload Marker, the decoder must indicate - an error. - - If the filter supports both End of Input and End of Payload - Marker, the former is used, unless the filter is the last - filter in the chain. - - -4.2. Alignment - - Some filters give better compression ratio or are faster - when the input or output data is aligned. For optimal results, - the encoder should try to enforce proper alignment when - possible. Not enforcing alignment in the encoder is not - an error. Thus, the decoder must be able to handle files with - suboptimal alignment. +5.1. Alignment Alignment of uncompressed input data is usually the job of the application producing the data. For example, to get the @@ -866,8 +711,9 @@ The .lzma File Format four-byte-aligned input data. The output of the last filter in the chain is stored to the - Compressed Data field. Aligning Compressed Data appropriately - can increase + Compressed Data field, which is is guaranteed to be aligned + to a multiple of four bytes relative to the beginning of the + Stream. This can increase - speed, if the filtered data is handled multiple bytes at a time by the filter-specific encoder and decoder, because accessing aligned data in computer memory is @@ -875,308 +721,67 @@ The .lzma File Format - compression ratio, if the output data is later compressed with an external compression tool. - Compressed Data in a Stream can be aligned by using the Header - Padding field in the Block Header. - - -4.3. Filters - -4.3.1. Copy - - This is a dummy filter that simply copies all data from input - to output unmodified. - - Filter ID: 0x00 - Size of Filter Properties: 0 bytes - Changes size of data: No - - Detecting when all of the data has been decoded: - Uncompressed size: Yes - End of Payload Marker: No - End of Input: Yes - - Preferred alignment: - Input data: 1 byte - Output data: 1 byte - - -4.3.2. Subblock - - The Subblock filter can be used to - - embed End of Payload Marker when the otherwise last - filter in the chain does not support embedding it; and - - apply additional filters in the middle of a Block. - Filter ID: 0x01 - Size of Filter Properties: 0 bytes - Changes size of data: Yes, unpredictably +5.2. Security - Detecting when all of the data has been decoded: - Uncompressed size: Yes - End of Payload Marker: Yes - End of Input: Yes + If filters would be allowed to be chained freely, it would be + possible to create malicious files, that would be very slow to + decode. Such files could be used to create denial of service + attacks. - Preferred alignment: - Input data: 1 byte - Output data: Freely adjustable + Slow files could occur when multiple filters are chained: + v Compressed input data + | Filter 1 decoder (last filter) + | Filter 0 decoder (non-last filter) + v Uncompressed output data -4.3.2.1. Format of the Encoded Output + The decoder of the last filter in the chain produces a lot of + output from little input. Another filter in the chain takes the + output of the last filter, and produces very little output + while consuming a lot of input. As a result, a lot of data is + moved inside the filter chain, but the filter chain as a whole + gets very little work done. - The encoded data from the Subblock filter consist of zero or - more Subblocks: + To prevent this kind of slow files, there are restrictions on + how the filters can be chained. These restrictions must be + taken into account when designing new filters. - +==========+==========+ - | Subblock | Subblock | ... - +==========+==========+ + The maximum number of filters in the chain has been limited to + four, thus there can be at maximum of three non-last filters. + Of these three non-last filters, only two are allowed to change + the size of the data. - Each Subblock contains two fields: + The non-last filters, that change the size of the data, must + have a limit how much the decoder can compress the data: the + decoder should produce at least n bytes of output when the + filter is given 2n bytes of input. This limit is not + absolute, but significant deviations must be avoided. - +----------------+===============+ - | Subblock Flags | Subblock Data | - +----------------+===============+ + The above limitations guarantee that if the last filter in the + chain produces 4n bytes of output, the chain as a whole will + produce at least n bytes of output. - Subblock Flags is a bitfield: - Bits Mask Description - 0-3 0x0F The interpretation of these bits depend on - the Subblock Type: - - 0x20 Bits 0-3 for Size - - 0x30 Bits 0-3 for Repeat Count - - Other These bits must be zero. - 4-7 0xF0 Subblock Type: - - 0x00: Padding - - 0x10: End of Payload Marker - - 0x20: Data - - 0x30: Repeating Data - - 0x40: Set Subfilter - - 0x50: Unset Subfilter - If some other value is detected, the decoder - must indicate an error. - - The format of the Subblock Data field depends on Subblock Type. - - Subblocks with the Subblock Type 0x00 (Padding) don't have a - Subblock Data field. These Subblocks can be useful for fixing - alignment. There can be at maximum of 31 consecutive Subblocks - with this Subblock Type; if there are more, the decoder must - indicate an error. +5.3. Filters - Subblock with the Subblock Type 0x10 (End of Payload Marker) - doesn't have a Subblock Data field. The decoder must indicate - an error if this Subblock Type is detected when Subfilter is - enabled, or when the Subblock filter is not supposed to embed - the End of Payload Marker. - - Subblocks with the Subblock Type 0x20 (Data) contain the rest - of the Size, which is followed by Size + 1 bytes in the Data - field (that is, Data can never be empty): - - +------+------+------+======+ - | Bits 4-27 for Size | Data | - +------+------+------+======+ - - Subblocks with the Subblock Type 0x30 (Repeating Data) contain - the rest of the Repeat Count, the Size of the Data, and finally - the actual Data to be repeated: - - +---------+---------+--------+------+======+ - | Bits 4-27 for Repeat Count | Size | Data | - +---------+---------+--------+------+======+ - - The size of the Data field is Size + 1. It is repeated Repeat - Count + 1 times. That is, the minimum size of Data is one byte; - the maximum size of Data is 256 bytes. The minimum number of - repeats is one; the maximum number of repeats is 2^28. - - If Subfilter is not used, the Data field of Subblock Types 0x20 - and 0x30 is the output of the decoded Subblock filter. If - Subfilter is used, Data is the input of the Subfilter, and the - decoded output of the Subfilter is the decoded output of the - Subblock filter. - - Subblocks with the Subblock Type 0x40 (Set Subfilter) contain - a Filter Flags field in Subblock Data: - - +==============+ - | Filter Flags | - +==============+ - - It is an error to set the Subfilter to Filter ID 0x00 (Copy) - or 0x01 (Subblock). All the other Filter IDs are allowed. - The decoder must indicate an error if this Subblock Type is - detected when a Subfilter is already enabled. - - Subblocks with the Subblock Type 0x50 (Unset Subfilter) don't - have a Subblock Data field. There must be at least one Subblock - with Subblock Type 0x20 or 0x30 between Subblocks with Subblock - Type 0x40 and 0x50; if there isn't, the decoder must indicate - an error. - - Subblock Types 0x40 and 0x50 are always used as a pair: If the - Subblock filter has been enabled with Subblock Type 0x40, it - must always be disabled later with Subblock Type 0x50. - Disabling must be done even if the Subfilter used End of - Payload Marker; after the Subfilter has detected End of Payload - Marker, the next Subblock that is not Padding must unset the - Subfilter. - - When the Subblock filter is used as an implicit filter to embed - End of Payload marker, the Subblock Types 0x40 and 0x50 (Set or - Unset Subfilter) must not be used. The decoder must indicate an - error if it detects any of these Subblock Types in an implicit - Subblock filter. - - The following code illustrates the basic structure of a - Subblock decoder. - - uint32_t consecutive_padding = 0; - bool got_output_with_subfilter = false; - - while (true) { - uint32_t size; - uint32_t repeat; - uint8_t flags = read_byte(); - - if (flags != 0) - consecutive_padding = 0; - - switch (flags >> 4) { - case 0: - // Padding - if (flags & 0x0F) - return DATA_ERROR; - if (++consecutive_padding == 32) - return DATA_ERROR; - break; - - case 1: - // End of Payload Marker - if (flags & 0x0F) - return DATA_ERROR; - if (subfilter_enabled || !allow_eopm) - return DATA_ERROR; - break; - - case 2: - // Data - size = flags & 0x0F; - for (size_t i = 4; i < 28; i += 8) - size |= (uint32_t)(read_byte()) << i; - - // If any output is produced, this will - // set got_output_with_subfilter to true. - copy_data(size); - break; - - case 3: - // Repeating Data - repeat = flags & 0x0F; - for (size_t i = 4; i < 28; i += 8) - repeat |= (uint32_t)(read_byte()) << i; - size = read_byte(); - - // If any output is produced, this will - // set got_output_with_subfilter to true. - copy_repeating_data(size, repeat); - break; - - case 4: - // Set Subfilter - if (flags & 0x0F) - return DATA_ERROR; - if (subfilter_enabled) - return DATA_ERROR; - got_output_with_subfilter = false; - set_subfilter(); - break; - - case 5: - // Unset Subfilter - if (flags & 0x0F) - return DATA_ERROR; - if (!subfilter_enabled) - return DATA_ERROR; - if (!got_output_with_subfilter) - return DATA_ERROR; - unset_subfilter(); - break; - - default: - return DATA_ERROR; - } - } - - -4.3.3. Delta - - The Delta filter may increase compression ratio when the value - of the next byte correlates with the value of an earlier byte - at specified distance. - - Filter ID: 0x20 - Size of Filter Properties: 1 byte - Changes size of data: No - - Detecting when all of the data has been decoded: - Uncompressed size: Yes - End of Payload Marker: No - End of Input: Yes - - Preferred alignment: - Input data: 1 byte - Output data: Same as the original input data - - The Properties byte indicates the delta distance, which can be - 1-256 bytes backwards from the current byte: 0x00 indicates - distance of 1 byte and 0xFF distance of 256 bytes. - - -4.3.3.1. Format of the Encoded Output - - The code below illustrates both encoding and decoding with - the Delta filter. - - // Distance is in the range [1, 256]. - const unsigned int distance = get_properties_byte() + 1; - uint8_t pos = 0; - uint8_t delta[256]; - - memset(delta, 0, sizeof(delta)); - - while (1) { - const int byte = read_byte(); - if (byte == EOF) - break; - - uint8_t tmp = delta[(uint8_t)(distance + pos)]; - if (is_encoder) { - tmp = (uint8_t)(byte) - tmp; - delta[pos] = (uint8_t)(byte); - } else { - tmp = (uint8_t)(byte) + tmp; - delta[pos] = tmp; - } - - write_byte(tmp); - --pos; - } - - -4.3.4. LZMA +5.3.1. LZMA2 LZMA (Lempel-Ziv-Markov chain-Algorithm) is a general-purporse compression algorithm with high compression ratio and fast - decompression. LZMA based on LZ77 and range coding algorithms. + decompression. LZMA is based on LZ77 and range coding + algorithms. - Filter ID: 0x40 - Size of Filter Properties: 2 bytes - Changes size of data: Yes, unpredictably + LZMA2 uses LZMA internally, but adds support for uncompressed + chunks, eases stateful decoder implementations, and improves + support for multithreading. Thus, the plain LZMA will not be + supported in this file format. - Detecting when all of the data has been decoded: - Uncompressed size: Yes - End of Payload Marker: Yes - End of Input: No + Filter ID: 0x21 + Size of Filter Properties: 1 byte + Changes size of data: Yes + Allow as a non-last filter: No + Allow as the last filter: Yes Preferred alignment: Input data: Adjustable to 1/2/4/8/16 byte(s) @@ -1188,88 +793,45 @@ The .lzma File Format a separate document, because including the documentation here would lengthen this document considerably. - The format of the Filter Properties field is as follows: - - +-----------------+------------------+ - | LZMA Properties | Dictionary Flags | - +-----------------+------------------+ - - -4.3.4.1. LZMA Properties - - The LZMA Properties field contains three properties. An - abbreviation is given in parentheses, followed by the value - range of the property. The field consists of - - 1) the number of literal context bits (lc, [0, 8]); - 2) the number of literal position bits (lp, [0, 4]); and - 3) the number of position bits (pb, [0, 4]). - - They are encoded using the following formula: - - LZMA Properties = (pb * 5 + lp) * 9 + lc - - The following C code illustrates a straightforward way to - decode the properties: - - uint8_t lc, lp, pb; - uint8_t prop = get_lzma_properties() & 0xFF; - if (prop > (4 * 5 + 4) * 9 + 8) - return LZMA_PROPERTIES_ERROR; - - pb = prop / (9 * 5); - prop -= pb * 9 * 5; - lp = prop / 9; - lc = prop - lp * 9; - - -4.3.4.2. Dictionary Flags - - Currently the lowest six bits of the Dictionary Flags field - are in use: + The format of the one-byte Filter Properties field is as + follows: Bits Mask Description 0-5 0x3F Dictionary Size 6-7 0xC0 Reserved for future use; must be zero for now. Dictionary Size is encoded with one-bit mantissa and five-bit - exponent. To avoid wasting space, one-byte dictionary has its - own special value. + exponent. The smallest dictionary size is 4 KiB and the biggest + is 4 GiB. Raw value Mantissa Exponent Dictionary size - 0 1 0 1 byte - 1 2 0 2 bytes - 2 3 0 3 bytes - 3 2 1 4 bytes - 4 3 1 6 bytes - 5 2 2 8 bytes - 6 3 2 12 bytes - 7 2 3 16 bytes - 8 3 3 24 bytes - 9 2 4 32 bytes + 0 2 11 4 KiB + 1 3 11 6 KiB + 2 2 12 8 KiB + 3 3 12 12 KiB + 4 2 13 16 KiB + 5 3 13 24 KiB + 6 2 14 32 KiB ... ... ... ... - 61 2 30 2 GiB - 62 3 30 3 GiB - 63 2 31 4 GiB (*) - - (*) The real maximum size of the dictionary is one byte - less than 4 GiB, because the distance of 4 GiB is - reserved for End of Payload Marker. + 35 3 27 768 MiB + 36 2 28 1024 MiB + 37 3 29 1536 MiB + 38 2 30 2048 MiB + 39 3 30 3072 MiB + 40 2 31 4096 MiB Instead of having a table in the decoder, the dictionary size can be decoded using the following C code: - uint64_t dictionary_size; const uint8_t bits = get_dictionary_flags() & 0x3F; - if (bits == 0) { - dictionary_size = 1; - } else { - dictionary_size = 2 | ((bits + 1) & 1); - dictionary_size = dictionary_size << ((bits - 1) / 2); - } + if (bits > 40) + return DICTIONARY_TOO_BIG; // Bigger than 4 GiB + + uint32_t dictionary_size = 2 | (bits & 1); + dictionary_size <<= bits / 2 + 11; -4.3.5. Branch/Call/Jump Filters for Executables +5.3.2. Branch/Call/Jump Filters for Executables These filters convert relative branch, call, and jump instructions to their absolute counterparts in executable @@ -1278,6 +840,8 @@ The .lzma File Format Size of Filter Properties: 0 or 4 bytes Changes size of data: No + Allow as a non-last filter: Yes + Allow as the last filter: No Detecting when all of the data has been decoded: Uncompressed size: Yes @@ -1307,378 +871,63 @@ The .lzma File Format the Subblock filter. -5. Metadata - - Metadata is stored in Metadata Blocks, which can be in the - beginning or at the end of a Multi-Block Stream. Because of - Blocks, it is possible to compress Metadata in the same way - as the actual data is compressed. This Section describes the - format of the data stored in Metadata Blocks. - - +----------------+===============================+ - | Metadata Flags | Size of Header Metadata Block | - +----------------+===============================+ - - +============+===================+=======+=======+ - ---> | Total Size | Uncompressed Size | Index | Extra | - +============+===================+=======+=======+ - - Stream must be parseable backwards. That is, there must be - a way to locate the beginning of the Stream by starting from - the end of the Stream. Thus, the Footer Metadata Block must - contain the Total Size field or the Index field. If the Stream - has Header Metadata Block, also the Size of Header Metadata - Block field must be present in Footer Metadata Block. - - It must be possible to quickly locate the Blocks in - non-streamed mode. Thus, the Index field must be present - at least in one Metadata Block. - - If the above conditions are not met, the decoder must indicate - an error. - - There should be no additional data after the last field. If - there is, the the decoder should indicate an error. - - -5.1. Metadata Flags - - This field describes which fields are present in a Metadata - Block: - - Bit(s) Mask Desription - 0 0x01 Size of Header Metadata Block is present. - 1 0x02 Total Size is present. - 2 0x04 Uncompressed Size is present. - 3 0x08 Index is present. - 4-6 0x70 Reserve for future use; must be zero for now. - 7 0x80 Extra is present. - - If any reserved bit is set, the decoder must indicate an error. - It is possible that there is a new field present which the - decoder is not aware of, and can thus parse the Metadata - incorrectly. - - -5.2. Size of Header Metadata Block - - This field is present only if the appropriate bit is set in - the Metadata Flags field (see Section 5.1). - - Size of Header Metadata Block is needed to make it possible to - parse the Stream backwards. The size is stored using the - encoding described in Section 1.2. The decoder must verify that - that the value stored in this field is non-zero. In Footer - Metadata Block, the decoder must also verify that the stored - size matches the real size of Header Metadata Block. In the - Header Meatadata Block, the value of this field is ignored as - long as it is not zero. - - -5.3. Total Size - - This field is present only if the appropriate bit is set in the - Metadata Flags field (see Section 5.1). - - This field contains the total size of the Data Blocks in the - Stream. Total Size is stored using the encoding described in - Section 1.2. If the stored value does not match the real total - size of the Data Blocks, the decoder must indicate an error. - The value of this field must be non-zero. - - Total Size can be used to quickly locate the beginning or end - of the Stream. This can be useful for example when doing - random-access reading, and the Index field is not in the - Metadata Block currently being read. - - It is useless to have both Total Size and Index in the same - Metadata Block, because Total Size can be calculated from the - Index field. - - -5.4. Uncompressed Size - - This field is present only if the appropriate bit is set in the - Metadata Flags field (see Section 5.1). - - This field contains the total uncompressed size of the Data - Blocks in the Stream. Uncompresssed Size is stored using the - encoding described in Section 1.2. If the stored value does not - match the real uncompressed size of the Data Blocks, the - decoder must indicate an error. - - It is useless to have both Uncompressed Size and Index in - the same Metadata Block, because Uncompressed Size can be - calculated from the Index field. - - -5.5. Index - - +=======================+=============+====================+ - | Number of Data Blocks | Total Sizes | Uncompressed Sizes | - +=======================+=============+====================+ - - Index serves several purporses. Using it, one can - - verify that all Blocks in a Stream have been processed; - - find out the Uncompressed Size of a Stream; and - - quickly access the beginning of any Block (random access). - - -5.5.1. Number of Data Blocks - - This field contains the number of Data Blocks in the Stream. - The value is stored using the encoding described in Section - 1.2. If the decoder has decoded all the Data Blocks of the - Stream, and then notices that the Number of Records doesn't - match the real number of Data Blocks, the decoder must - indicate an error. The value of this field must be non-zero. - - -5.5.2. Total Sizes - - +============+============+ - | Total Size | Total Size | ... - +============+============+ - - This field lists the Total Sizes of every Data Block in the - Stream. There are as many Total Size fields as indicated by - the Number of Data Blocks field. - - Total Size is the size of Block Header, Compressed Data, and - Block Footer. It is stored using the encoding described in - Section 1.2. If the Total Sizes do not match the real sizes - of respective Blocks, the decoder should indicate an error. - All the Total Size fields must have a non-zero value. - - -5.5.3. Uncompressed Sizes - - +===================+===================+ - | Uncompressed Size | Uncompressed Size | ... - +===================+===================+ - - This field lists the Uncompressed Sizes of every Data Block - in the Stream. There are as many Uncompressed Size fields as - indicated by the Number of Records field. - - Uncompressed Sizes are stored using the encoding described - in Section 1.2. If the Uncompressed Sizes do not match the - real sizes of respective Blocks, the decoder shoud indicate - an error. - - -5.6. Extra - - This field is present only if the appropriate bit is set in the - Metadata Flags field (see Section 5.1). Note that the bit does - not indicate that there is any data in the Extra field; it only - indicates that Extra may be non-empty. - - The Extra field contains only information that is not required - to properly uncompress the Stream or to do random-access - reading. Supporting the Extra field is optional. In case the - decoder doesn't support the Extra field, it should silently - ignore it. - - Extra consists of zero or more Records: - - +========+========+ - | Record | Record | ... - +========+========+ - - Excluding Records with Record ID 0x00, each Record contains - three fields: - - +==========+==============+======+ - | Reord ID | Size of Data | Data | - +==========+==============+======+ - - The Record ID and Size of Data are stored using the encoding - described in Section 1.2. Data can be binary or UTF-8 - [RFC-3629] strings. Non-UTF-8 strings should be avoided. - Because the Size of Data is known, there is no need to - terminate strings with a nul byte, although doing so should - not be considered an error. - - The Record IDs are divided in two categories: - - Safe-to-Copy Records may be preserved as is when the - Stream is modified in ways that don't change the actual - uncompressed data. Examples of such operatings include - recompressing and adding, modifying, or deleting unrelated - Extra Records. - - Unsafe-to-Copy Records should be removed (and possibly - recreated) when any kind of changes are made to the Stream. - - When the actual uncompressed data is modified, all Records - should be removed (and possibly recreated), unless the - application knows that the Data stored to the Record(s) is - still valid. - - The following subsections describe the standard Record IDs and - the format of their Data fields. Safe-to-Copy Records have an - odd ID, while Unsafe-to-Copy Records have an even ID. - - -5.6.1. 0x00: Dummy/Padding - - This Record is special, because it doesn't have the Size of - Data or Data fields. - - Dummy Records can be used, for example, to fill Metadata Block - when a few bytes of extra space has been reserved for it. There - can be any number of Dummy Records. - - -5.6.2. 0x01: OpenPGP Signature - - OpenPGP signature is computed from uncompressed data. The - signature can be used to verify that the contents of a Stream - has been created by a trustworthy source. - - If the decoder supports decoding concatenated Streams, it - must indicate an error when verifying OpenPGP signatures if - there is more than one Stream. - - OpenPGP format is documented in [RFC-2440]. - - -5.6.3. 0x02: Filter Information - - The Filter Information Record contains information about the - filters used in the Stream. This field can be used to quickly - - display which filters are used in each Block; - - check if all the required filters are supported by the - current decoder version; and - - check how much memory is required to decode each Block. - - The format of the Filter Information field is as follows: - - +=================+=================+ - | Block 0 Filters | Block 1 Filters | ... - +=================+=================+ - - There can be at maximum of as many Block Filters fields as - there are Data Blocks in the Stream. The format of the Block - Filters field is as follows: - - +------------------+======================+============+ - | Block Properties | List of Filter Flags | Subfilters | - +------------------+======================+============+ - - Block Properties is a bitfield: - - Bit(s) Mask Description - 0-2 0x07 Number of filters (0-7) - 3 0x08 End of Payload Marker is used. - 4 0x10 The Subfilters field is present. - 5-7 0xE0 Reserved for future use; must be zero for now. - - The contents of the List of Filter Flags field must match the - List of Filter Flags field in the respective Block Header. - - The Subfilters field may be present only if the List of Filter - Flags contains a Filter Flags field for a Subblock filter. The - format of the Subfilters field is as follows: - - +======================+=========================+ - | Number of Subfilters | List of Subfilter Flags | - +======================+=========================+ - - The value stored in the Number of Subfilters field is stored - using the encoding described in Section 1.2. The List of - Subfilter Flags field contains as many Filter Flags fields - as indicated by the Number of Subfilters field. These Filter - Flags fields list some or all the Subfilters used via the - Subblock filter. The order of the listed Subfilters is not - significant. - - Decoders supporting this Record should indicate a warning or - error if this Record contains Filter Flags that are not - actually used by the respective Blocks. - - -5.6.4. 0x03: Comment - - Free-form comment is stored in UTF-8 [RFC-3629] encoding. - - The beginning of a new line should be indicated using the - ASCII Line Feed character (0x0A). When the Line Feed character - is not the native way to indicate new line in the underlying - operating system, the encoder and decoder should convert the - newline characters to and from Line Feeds. - - -5.6.5. 0x04: List of Checks - - +=======+=======+ - | Check | Check | ... - +=======+=======+ - - There are as many Check fields as there are Blocks in the - Stream. The size of Check fields depend on Stream Flags - (see Section 2.2.2). - - Decoders supporting this Record should indicate a warning or - error if the Checks don't match the respective Blocks. - - -5.6.6. 0x05: Original Filename - - Original filename is stored in UTF-8 [RFC-3629] encoding. - - The filename must not include any path, only the filename - itself. Special care must be taken to prevent directory - traversal vulnerabilities. - - When files are moved between different operating systems, it - is possible that filename valid in the source system is not - valid in the target system. It is implementation defined how - the decoder handles this kind of situations. - - -5.6.7. 0x07: Modification Time - - Modification time is stored as POSIX time, as an unsigned - little endian integer. The number of bits depends on the - Size of Data field. Note that the usage of unsigned integer - limits the earliest representable time to 1970-01-01T00:00:00. +5.3.3. Delta + The Delta filter may increase compression ratio when the value + of the next byte correlates with the value of an earlier byte + at specified distance. -5.6.8. 0x09: High-Resolution Modification Time + Filter ID: 0x03 + Size of Filter Properties: 1 byte + Changes size of data: No + Allow as a non-last filter: Yes + Allow as the last filter: No - This Record extends the `0x04: Modification time' Record with - a subsecond time information. There are two supported formats - of this field, which can be distinguished by looking at the - Size of Data field. + Preferred alignment: + Input data: 1 byte + Output data: Same as the original input data - Size Data - 3 [0; 9,999,999] times 100 nanoseconds - 4 [0; 999,999,999] nanoseconds + The Properties byte indicates the delta distance, which can be + 1-256 bytes backwards from the current byte: 0x00 indicates + distance of 1 byte and 0xFF distance of 256 bytes. - The value is stored as an unsigned 24-bit or 32-bit little - endian integer. +5.3.3.1. Format of the Encoded Output -5.6.9. 0x0B: MIME Type + The code below illustrates both encoding and decoding with + the Delta filter. - MIME type of the uncompressed Stream. This can be used to - detect the content type. [IANA-MIME] + // Distance is in the range [1, 256]. + const unsigned int distance = get_properties_byte() + 1; + uint8_t pos = 0; + uint8_t delta[256]; + memset(delta, 0, sizeof(delta)); -5.6.10. 0x0D: Homepage URL + while (1) { + const int byte = read_byte(); + if (byte == EOF) + break; - This field can be used, for example, when distributing software - packages (sources or binaries). The field would indicate the - homepage of the program. + uint8_t tmp = delta[(uint8_t)(distance + pos)]; + if (is_encoder) { + tmp = (uint8_t)(byte) - tmp; + delta[pos] = (uint8_t)(byte); + } else { + tmp = (uint8_t)(byte) + tmp; + delta[pos] = tmp; + } - For details on how to encode URLs, see [RFC-1738]. + write_byte(tmp); + --pos; + } -6. Custom Filter and Extra Record IDs +5.4. Custom Filter IDs - If a developer wants to use custom Filter or Extra Record IDs, - he has two choices. The first choice is to contact Lasse Collin - and ask him to allocate a range of IDs for the developer. + If a developer wants to use custom Filter IDs, he has two + choices. The first choice is to contact Lasse Collin and ask + him to allocate a range of IDs for the developer. The second choice is to generate a 40-bit random integer, which the developer can use as his personal Developer ID. @@ -1690,10 +939,10 @@ The .lzma File Format dd if=/dev/urandom bs=5 count=1 | hexdump The developer can then use his Developer ID to create unique - (well, hopefully unique) Filter and Extra Record IDs. + (well, hopefully unique) Filter IDs. Bits Mask Description - 0-15 0x0000_0000_0000_FFFF Filter or Extra Record ID + 0-15 0x0000_0000_0000_FFFF Filter ID 16-55 0x00FF_FFFF_FFFF_0000 Developer ID 56-62 0x7F00_0000_0000_0000 Static prefix: 0x7F @@ -1702,21 +951,15 @@ The .lzma File Format a shorter ID, see the beginning of this Section how to request a custom ID range. - Note that Filter and Metadata Record IDs are in their own - namespaces. That is, you can use the same ID value as Filter ID - and Metadata Record ID, and the meanings of the IDs do not need - to be related to each other. - -6.1. Reserved Custom Filter ID Ranges +5.4.1. Reserved Custom Filter ID Ranges Range Description - 0x0000_0000 - 0x0000_00DF IDs fitting into the Misc field 0x0002_0000 - 0x0007_FFFF Reserved to ease .7z compatibility 0x0200_0000 - 0x07FF_FFFF Reserved to ease .7z compatibility -7. Cyclic Redundancy Checks +6. Cyclic Redundancy Checks There are several incompatible variations to calculate CRC32 and CRC64. For simplicity and clarity, complete examples are @@ -1811,32 +1054,7 @@ The .lzma File Format } -8. References - -8.1. Normative References - - [RFC-1738] - Uniform Resource Locators (URL) - http://www.ietf.org/rfc/rfc1738.txt - - [RFC-2119] - Key words for use in RFCs to Indicate Requirement Levels - http://www.ietf.org/rfc/rfc2119.txt - - [RFC-2440] - OpenPGP Message Format - http://www.ietf.org/rfc/rfc2440.txt - - [RFC-3629] - UTF-8, a transformation format of ISO 10646 - http://www.ietf.org/rfc/rfc3629.txt - - [IANA-MIME] - MIME Media Types - http://www.iana.org/assignments/media-types/ - - -8.2. Informative References +7. References LZMA SDK - The original LZMA implementation http://7-zip.org/sdk.html @@ -1849,6 +1067,10 @@ The .lzma File Format http://www.ietf.org/rfc/rfc1952.txt - Notation of byte boxes in section `2.1. Overall conventions' + [RFC-2119] + Key words for use in RFCs to Indicate Requirement Levels + http://www.ietf.org/rfc/rfc2119.txt + [GNU-tar] GNU tar 1.16.1 manual http://www.gnu.org/software/tar/manual/html_node/Blocking-Factor.html |