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LZMA compression


Version: 9.35

This file describes LZMA encoding and decoding functions written in C language.

LZMA is an improved version of famous LZ77 compression algorithm. 

It was improved in way of maximum increasing of compression ratio,

keeping high decompression speed and low memory requirements for 


Note: you can read also LZMA Specification (lzma-specification.txt from LZMA SDK)

Also you can look source code for LZMA encoding and decoding:


LZMA compressed file format


Offset Size Description

  0     1   Special LZMA properties (lc,lp, pb in encoded form)

  1     4   Dictionary size (little endian)

  5     8   Uncompressed size (little endian). -1 means unknown size

 13         Compressed data



Please note that interfaces for ANSI-C code were changed in LZMA SDK 4.58.

If you want to use old interfaces you can download previous version of LZMA SDK

from sourceforge.net site.

To use ANSI-C LZMA Decoder you need the following files:

1) LzmaDec.h + LzmaDec.c + Types.h

Look example code:


Memory requirements for LZMA decoding


Stack usage of LZMA decoding function for local variables is not 

larger than 200-400 bytes.

LZMA Decoder uses dictionary buffer and internal state structure.

Internal state structure consumes

  state_size = (4 + (1.5 << (lc + lp))) KB

by default (lc=3, lp=0), state_size = 16 KB.

How To decompress data


LZMA Decoder (ANSI-C version) now supports 2 interfaces:

1) Single-call Decompressing

2) Multi-call State Decompressing (zlib-like interface)

You must use external allocator:


void *SzAlloc(void *p, size_t size) { p = p; return malloc(size); }

void SzFree(void *p, void *address) { p = p; free(address); }

ISzAlloc alloc = { SzAlloc, SzFree };

You can use p = p; operator to disable compiler warnings.

Single-call Decompressing


When to use: RAM->RAM decompressing

Compile files: LzmaDec.h + LzmaDec.c + Types.h

Compile defines: no defines

Memory Requirements:

  - Input buffer: compressed size

  - Output buffer: uncompressed size

  - LZMA Internal Structures: state_size (16 KB for default settings) 


  int LzmaDecode(Byte *dest, SizeT *destLen, const Byte *src, SizeT *srcLen,

      const Byte *propData, unsigned propSize, ELzmaFinishMode finishMode, 

      ELzmaStatus *status, ISzAlloc *alloc);


    dest     - output data

    destLen  - output data size

    src      - input data

    srcLen   - input data size

    propData - LZMA properties  (5 bytes)

    propSize - size of propData buffer (5 bytes)

    finishMode - It has meaning only if the decoding reaches output limit (*destLen).

         LZMA_FINISH_ANY - Decode just destLen bytes.

         LZMA_FINISH_END - Stream must be finished after (*destLen).

                           You can use LZMA_FINISH_END, when you know that 

                           current output buffer covers last bytes of stream. 

    alloc    - Memory allocator.


    destLen  - processed output size 

    srcLen   - processed input size 







    SZ_ERROR_DATA - Data error

    SZ_ERROR_MEM  - Memory allocation error

    SZ_ERROR_UNSUPPORTED - Unsupported properties

    SZ_ERROR_INPUT_EOF - It needs more bytes in input buffer (src).

  If LZMA decoder sees end_marker before reaching output limit, it returns OK result,

  and output value of destLen will be less than output buffer size limit.

  You can use multiple checks to test data integrity after full decompression:

    1) Check Result and "status" variable.

    2) Check that output(destLen) = uncompressedSize, if you know real uncompressedSize.

    3) Check that output(srcLen) = compressedSize, if you know real compressedSize. 

       You must use correct finish mode in that case. */ 

Multi-call State Decompressing (zlib-like interface)


When to use: file->file decompressing 

Compile files: LzmaDec.h + LzmaDec.c + Types.h

Memory Requirements:

 - Buffer for input stream: any size (for example, 16 KB)

 - Buffer for output stream: any size (for example, 16 KB)

 - LZMA Internal Structures: state_size (16 KB for default settings) 

 - LZMA dictionary (dictionary size is encoded in LZMA properties header)

1) read LZMA properties (5 bytes) and uncompressed size (8 bytes, little-endian) to header:

   unsigned char header[LZMA_PROPS_SIZE + 8];

   ReadFile(inFile, header, sizeof(header)

2) Allocate CLzmaDec structures (state + dictionary) using LZMA properties

  CLzmaDec state;


  res = LzmaDec_Allocate(&state, header, LZMA_PROPS_SIZE, &g_Alloc);

  if (res != SZ_OK)

    return res;

3) Init LzmaDec structure before any new LZMA stream. And call LzmaDec_DecodeToBuf in loop


  for (;;)



    int res = LzmaDec_DecodeToBuf(CLzmaDec *p, Byte *dest, SizeT *destLen, 

        const Byte *src, SizeT *srcLen, ELzmaFinishMode finishMode);



4) Free all allocated structures

  LzmaDec_Free(&state, &g_Alloc);

Look example code:


How To compress data


Compile files: 










Memory Requirements:

  - (dictSize * 11.5 + 6 MB) + state_size

Lzma Encoder can use two memory allocators:

1) alloc - for small arrays.

2) allocBig - for big arrays.

For example, you can use Large RAM Pages (2 MB) in allocBig allocator for 

better compression speed. Note that Windows has bad implementation for 

Large RAM Pages. 

It's OK to use same allocator for alloc and allocBig.

Single-call Compression with callbacks


Look example code:


When to use: file->file compressing 

1) you must implement callback structures for interfaces:





static void *SzAlloc(void *p, size_t size) { p = p; return MyAlloc(size); }

static void SzFree(void *p, void *address) {  p = p; MyFree(address); }

static ISzAlloc g_Alloc = { SzAlloc, SzFree };

  CFileSeqInStream inStream;

  CFileSeqOutStream outStream;

  inStream.funcTable.Read = MyRead;

  inStream.file = inFile;

  outStream.funcTable.Write = MyWrite;

  outStream.file = outFile;

2) Create CLzmaEncHandle object;

  CLzmaEncHandle enc;

  enc = LzmaEnc_Create(&g_Alloc);

  if (enc == 0)

    return SZ_ERROR_MEM;

3) initialize CLzmaEncProps properties;


  Then you can change some properties in that structure.

4) Send LZMA properties to LZMA Encoder

  res = LzmaEnc_SetProps(enc, &props);

5) Write encoded properties to header

    Byte header[LZMA_PROPS_SIZE + 8];

    size_t headerSize = LZMA_PROPS_SIZE;

    UInt64 fileSize;

    int i;

    res = LzmaEnc_WriteProperties(enc, header, &headerSize);

    fileSize = MyGetFileLength(inFile);

    for (i = 0; i < 8; i++)

      header[headerSize++] = (Byte)(fileSize >> (8 * i));

    MyWriteFileAndCheck(outFile, header, headerSize)

6) Call encoding function:

      res = LzmaEnc_Encode(enc, &outStream.funcTable, &inStream.funcTable, 

        NULL, &g_Alloc, &g_Alloc);

7) Destroy LZMA Encoder Object

  LzmaEnc_Destroy(enc, &g_Alloc, &g_Alloc);

If callback function return some error code, LzmaEnc_Encode also returns that code

or it can return the code like SZ_ERROR_READ, SZ_ERROR_WRITE or SZ_ERROR_PROGRESS.

Single-call RAM->RAM Compression


Single-call RAM->RAM Compression is similar to Compression with callbacks,

but you provide pointers to buffers instead of pointers to stream callbacks:

SRes LzmaEncode(Byte *dest, SizeT *destLen, const Byte *src, SizeT srcLen,

    const CLzmaEncProps *props, Byte *propsEncoded, SizeT *propsSize, int writeEndMark, 

    ICompressProgress *progress, ISzAlloc *alloc, ISzAlloc *allocBig);

Return code:

  SZ_OK               - OK

  SZ_ERROR_MEM        - Memory allocation error 

  SZ_ERROR_PARAM      - Incorrect paramater

  SZ_ERROR_OUTPUT_EOF - output buffer overflow

  SZ_ERROR_THREAD     - errors in multithreading functions (only for Mt version)



_LZMA_SIZE_OPT - Enable some optimizations in LZMA Decoder to get smaller executable code.

_LZMA_PROB32   - It can increase the speed on some 32-bit CPUs, but memory usage for 

                 some structures will be doubled in that case.

_LZMA_UINT32_IS_ULONG  - Define it if int is 16-bit on your compiler and long is 32-bit.

_LZMA_NO_SYSTEM_SIZE_T  - Define it if you don't want to use size_t type.

_7ZIP_PPMD_SUPPPORT - Define it if you don't want to support PPMD method in AMSI-C .7z decoder.

C++ LZMA Encoder/Decoder 


C++ LZMA code use COM-like interfaces. So if you want to use it, 

you can study basics of COM/OLE.

C++ LZMA code is just wrapper over ANSI-C code.

C++ Notes


If you use some C++ code folders in 7-Zip (for example, C++ code for .7z handling),

you must check that you correctly work with "new" operator.

7-Zip can be compiled with MSVC 6.0 that doesn't throw "exception" from "new" operator.

So 7-Zip uses "CPP\Common\NewHandler.cpp" that redefines "new" operator:

operator new(size_t size)


  void *p = ::malloc(size);

  if (p == 0)

    throw CNewException();

  return p;


If you use MSCV that throws exception for "new" operator, you can compile without 

"NewHandler.cpp". So standard exception will be used. Actually some code of 

7-Zip catches any exception in internal code and converts it to HRESULT code.

So you don't need to catch CNewException, if you call COM interfaces of 7-Zip.