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LZ4 is lossless compression algorithm, providing compression speed >500 MB/s per core, scalable with multi-cores CPU. It features an extremely fast decoder, with speed in multiple GB/s per core, typically reaching RAM speed limits on multi-core systems.

The LZ4 compression library provides in-memory compression and decompression functions. It gives full buffer control to user. Compression can be done in:

  • a single step (described as Simple Functions)

  • a single step, reusing a context (described in Advanced Functions)

  • unbounded multiple steps (described as Streaming compression)

lz4.h generates and decodes LZ4-compressed blocks (doc/lz4_Block_format.md). Decompressing such a compressed block requires additional metadata. Exact metadata depends on exact decompression function. For the typical case of LZ4_decompress_safe(), metadata includes block's compressed size, and maximum bound of decompressed size. Each application is free to encode and pass such metadata in whichever way it wants.

lz4.h only handle blocks, it can not generate Frames.

Blocks are different from Frames (doc/lz4_Frame_format.md). Frames bundle both blocks and metadata in a specified manner. Embedding metadata is required for compressed data to be self-contained and portable. Frame format is delivered through a companion API, declared in lz4frame.h. The lz4 CLI can only manage frames.


LZ4_FREESTANDING : When this macro is set to 1, it enables "freestanding mode" that is suitable for typical freestanding environment which doesn't support standard C library.

  • LZ4_FREESTANDING is a compile-time switch.

  • It requires the following macros to be defined: LZ4_memcpy, LZ4_memmove, LZ4_memset.

  • It only enables LZ4/HC functions which don't use heap. All LZ4F_* functions are not supported.

  • See tests/freestanding.c to check its basic setup.


LZ4_MEMORY_USAGE : Memory usage formula : N->2^N Bytes (examples : 10 -> 1KB; 12 -> 4KB ; 16 -> 64KB; 20 -> 1MB; ) Increasing memory usage improves compression ratio, at the cost of speed. Reduced memory usage may improve speed at the cost of ratio, thanks to better cache locality. Default value is 14, for 16KB, which nicely fits into Intel x86 L1 cache


Deprecation warnings

Deprecated functions make the compiler generate a warning when invoked. This is meant to invite users to update their source code. Should deprecation warnings be a problem, it is generally possible to disable them, typically with -Wno-deprecated-declarations for gcc or _CRT_SECURE_NO_WARNINGS in Visual.

Another method is to define LZ4_DISABLE_DEPRECATE_WARNINGS before including the header file.


typedef union LZ4_stream_u LZ4_stream_t
typedef union LZ4_streamDecode_u LZ4_streamDecode_t
typedef signed char LZ4_i8
typedef unsigned char LZ4_byte
typedef unsigned short LZ4_u16
typedef unsigned int LZ4_u32
typedef struct LZ4_stream_t_internal LZ4_stream_t_internal

LZ4_stream_t : Never ever use below internal definitions directly ! These definitions are not API/ABI safe, and may change in future versions. If you need static allocation, declare or allocate an LZ4_stream_t object.


int LZ4_versionNumber(void)

library version number; useful to check dll version; requires v1.3.0+

const char *LZ4_versionString(void)

library version string; useful to check dll version; requires v1.7.5+

int LZ4_compress_default(const char *src, char *dst, int srcSize, int dstCapacity)

LZ4_compress_default() : Compresses 'srcSize' bytes from buffer 'src' into already allocated 'dst' buffer of size 'dstCapacity'. Compression is guaranteed to succeed if 'dstCapacity' >= LZ4_compressBound(srcSize). It also runs faster, so it's a recommended setting. If the function cannot compress 'src' into a more limited 'dst' budget, compression stops immediately, and the function result is zero. In which case, 'dst' content is undefined (invalid). srcSize : max supported value is LZ4_MAX_INPUT_SIZE. dstCapacity : size of buffer 'dst' (which must be already allocated)


: the number of bytes written into buffer 'dst' (necessarily <= dstCapacity) or 0 if compression fails Note : This function is protected against buffer overflow scenarios (never writes outside 'dst' buffer, nor read outside 'source' buffer).

int LZ4_decompress_safe(const char *src, char *dst, int compressedSize, int dstCapacity)

LZ4_decompress_safe() : @compressedSize : is the exact complete size of the compressed block. @dstCapacity : is the size of destination buffer (which must be already allocated), is an upper bound of decompressed size.


: the number of bytes decompressed into destination buffer (necessarily <= dstCapacity) If destination buffer is not large enough, decoding will stop and output an error code (negative value). If the source stream is detected malformed, the function will stop decoding and return a negative result. Note 1 : This function is protected against malicious data packets : it will never writes outside 'dst' buffer, nor read outside 'source' buffer, even if the compressed block is maliciously modified to order the decoder to do these actions. In such case, the decoder stops immediately, and considers the compressed block malformed. Note 2 : compressedSize and dstCapacity must be provided to the function, the compressed block does not contain them. The implementation is free to send / store / derive this information in whichever way is most beneficial. If there is a need for a different format which bundles together both compressed data and its metadata, consider looking at lz4frame.h instead.

int LZ4_compressBound(int inputSize)

LZ4_compressBound() : Provides the maximum size that LZ4 compression may output in a "worst case" scenario (input data not compressible) This function is primarily useful for memory allocation purposes (destination buffer size). Macro LZ4_COMPRESSBOUND() is also provided for compilation-time evaluation (stack memory allocation for example). Note that LZ4_compress_default() compresses faster when dstCapacity is >= LZ4_compressBound(srcSize) inputSize : max supported value is LZ4_MAX_INPUT_SIZE return : maximum output size in a "worst case" scenario or 0, if input size is incorrect (too large or negative)

int LZ4_compress_fast(const char *src, char *dst, int srcSize, int dstCapacity, int acceleration)

LZ4_compress_fast() : Same as LZ4_compress_default(), but allows selection of "acceleration" factor. The larger the acceleration value, the faster the algorithm, but also the lesser the compression. It's a trade-off. It can be fine tuned, with each successive value providing roughly +~3% to speed. An acceleration value of "1" is the same as regular LZ4_compress_default() Values <= 0 will be replaced by LZ4_ACCELERATION_DEFAULT (currently == 1, see lz4.c). Values > LZ4_ACCELERATION_MAX will be replaced by LZ4_ACCELERATION_MAX (currently == 65537, see lz4.c).

int LZ4_sizeofState(void)

LZ4_compress_fast_extState() : Same as LZ4_compress_fast(), using an externally allocated memory space for its state. Use LZ4_sizeofState() to know how much memory must be allocated, and allocate it on 8-bytes boundaries (using malloc() typically). Then, provide this buffer as void* state to compression function.

int LZ4_compress_fast_extState(void *state, const char *src, char *dst, int srcSize, int dstCapacity, int acceleration)
int LZ4_compress_destSize(const char *src, char *dst, int *srcSizePtr, int targetDstSize)

LZ4_compress_destSize() : Reverse the logic : compresses as much data as possible from 'src' buffer into already allocated buffer 'dst', of size >= 'targetDestSize'. This function either compresses the entire 'src' content into 'dst' if it's large enough, or fill 'dst' buffer completely with as much data as possible from 'src'. note: acceleration parameter is fixed to "default".

*srcSizePtr : will be modified to indicate how many bytes where read from 'src' to fill 'dst'. New value is necessarily <= input value.

Note : from v1.8.2 to v1.9.1, this function had a bug (fixed in v1.9.2+): the produced compressed content could, in specific circumstances, require to be decompressed into a destination buffer larger by at least 1 byte than the content to decompress. If an application uses

LZ4_compress_destSize(), it's highly recommended to update liblz4 to v1.9.2 or better. If this can't be done or ensured, the receiving decompression function should provide a dstCapacity which is > decompressedSize, by at least 1 byte. See https://github.com/lz4/lz4/issues/859 for details


: Nb bytes written into 'dst' (necessarily <= targetDestSize) or 0 if compression fails.

int LZ4_decompress_safe_partial(const char *src, char *dst, int srcSize, int targetOutputSize, int dstCapacity)

LZ4_decompress_safe_partial() : Decompress an LZ4 compressed block, of size 'srcSize' at position 'src', into destination buffer 'dst' of size 'dstCapacity'. Up to 'targetOutputSize' bytes will be decoded. The function stops decoding on reaching this objective. This can be useful to boost performance whenever only the beginning of a block is required.

Note 1 :

Note 2 : targetOutputSize must be <= dstCapacity

Note 3 : this function effectively stops decoding on reaching targetOutputSize, so dstCapacity is kind of redundant. This is because in older versions of this function, decoding operation would still write complete sequences. Therefore, there was no guarantee that it would stop writing at exactly targetOutputSize, it could write more bytes, though only up to dstCapacity. Some "margin" used to be required for this operation to work properly. Thankfully, this is no longer necessary. The function nonetheless keeps the same signature, in an effort to preserve API compatibility.

Note 4 : If srcSize is the exact size of the block, then targetOutputSize can be any value, including larger than the block's decompressed size. The function will, at most, generate block's decompressed size.

Note 5 : If srcSize is larger than block's compressed size, then targetOutputSize MUST be <= block's decompressed size. Otherwise, silent corruption will occur.


: the number of bytes decoded in dst (necessarily <= targetOutputSize) If source stream is detected malformed, function returns a negative result.


can be < targetOutputSize, if compressed block contains less data.

void LZ4_resetStream_fast(LZ4_stream_t *streamPtr)

Note about RC_INVOKED

  • RC_INVOKED is predefined symbol of rc.exe (the resource compiler which is part of MSVC/Visual Studio). https://docs.microsoft.com/en-us/windows/win32/menurc/predefined-macros

  • Since rc.exe is a legacy compiler, it truncates long symbol (> 30 chars) and reports warning "RC4011: identifier truncated".

  • To eliminate the warning, we surround long preprocessor symbol with "#if !defined(RC_INVOKED) ... #endif" block that means "skip this block when rc.exe is trying to read it".

LZ4_resetStream_fast() : v1.9.0+ Use this to prepare an LZ4_stream_t for a new chain of dependent blocks (e.g., LZ4_compress_fast_continue()).

An LZ4_stream_t must be initialized once before usage. This is automatically done when created by LZ4_createStream(). However, should the LZ4_stream_t be simply declared on stack (for example), it's necessary to initialize it first, using LZ4_initStream().

After init, start any new stream with LZ4_resetStream_fast(). A same LZ4_stream_t can be re-used multiple times consecutively and compress multiple streams, provided that it starts each new stream with LZ4_resetStream_fast().

LZ4_resetStream_fast() is much faster than LZ4_initStream(), but is not compatible with memory regions containing garbage data.

Note: it's only useful to call LZ4_resetStream_fast() in the context of streaming compression. The extState functions perform their own resets. Invoking LZ4_resetStream_fast() before is redundant, and even counterproductive.

int LZ4_loadDict(LZ4_stream_t *streamPtr, const char *dictionary, int dictSize)

LZ4_loadDict() : Use this function to reference a static dictionary into LZ4_stream_t. The dictionary must remain available during compression. LZ4_loadDict() triggers a reset, so any previous data will be forgotten. The same dictionary will have to be loaded on decompression side for successful decoding. Dictionary are useful for better compression of small data (KB range). While LZ4 accept any input as dictionary, results are generally better when using Zstandard's Dictionary Builder. Loading a size of 0 is allowed, and is the same as reset.


: loaded dictionary size, in bytes (necessarily <= 64 KB)

int LZ4_compress_fast_continue(LZ4_stream_t *streamPtr, const char *src, char *dst, int srcSize, int dstCapacity, int acceleration)

LZ4_compress_fast_continue() : Compress 'src' content using data from previously compressed blocks, for better compression ratio. 'dst' buffer must be already allocated. If dstCapacity >= LZ4_compressBound(srcSize), compression is guaranteed to succeed, and runs faster.

Note 1 : Each invocation to LZ4_compress_fast_continue() generates a new block. Each block has precise boundaries. Each block must be decompressed separately, calling LZ4_decompress_*() with relevant metadata. It's not possible to append blocks together and expect a single invocation of LZ4_decompress_*() to decompress them together.

Note 2 : The previous 64KB of source data is assumed to remain present, unmodified, at same address in memory !

Note 3 : When input is structured as a double-buffer, each buffer can have any size, including < 64 KB. Make sure that buffers are separated, by at least one byte. This construction ensures that each block only depends on previous block.

Note 4 : If input buffer is a ring-buffer, it can have any size, including < 64 KB.

Note 5 : After an error, the stream status is undefined (invalid), it can only be reset or freed.


: size of compressed block or 0 if there is an error (typically, cannot fit into 'dst').

int LZ4_saveDict(LZ4_stream_t *streamPtr, char *safeBuffer, int maxDictSize)

LZ4_saveDict() : If last 64KB data cannot be guaranteed to remain available at its current memory location, save it into a safer place (char* safeBuffer). This is schematically equivalent to a memcpy() followed by LZ4_loadDict(), but is much faster, because LZ4_saveDict() doesn't need to rebuild tables.


: saved dictionary size in bytes (necessarily <= maxDictSize), or 0 if error.

int LZ4_setStreamDecode(LZ4_streamDecode_t *LZ4_streamDecode, const char *dictionary, int dictSize)

LZ4_createStreamDecode() and LZ4_freeStreamDecode() : creation / destruction of streaming decompression tracking context. A tracking context can be re-used multiple times.

LZ4_setStreamDecode() : An LZ4_streamDecode_t context can be allocated once and re-used multiple times. Use this function to start decompression of a new stream of blocks. A dictionary can optionally be set. Use NULL or size 0 for a reset order. Dictionary is presumed stable : it must remain accessible and unmodified during next decompression.


: 1 if OK, 0 if error

int LZ4_decoderRingBufferSize(int maxBlockSize)

LZ4_decoderRingBufferSize() : v1.8.2+ Note : in a ring buffer scenario (optional), blocks are presumed decompressed next to each other up to the moment there is not enough remaining space for next block (remainingSize < maxBlockSize), at which stage it resumes from beginning of ring buffer. When setting such a ring buffer for streaming decompression, provides the minimum size of this ring buffer to be compatible with any source respecting maxBlockSize condition.


: minimum ring buffer size, or 0 if there is an error (invalid maxBlockSize).

int LZ4_decompress_safe_continue(LZ4_streamDecode_t *LZ4_streamDecode, const char *src, char *dst, int srcSize, int dstCapacity)

LZ4_decompress_safe_continue() : This decoding function allows decompression of consecutive blocks in "streaming" mode. The difference with the usual independent blocks is that new blocks are allowed to find references into former blocks. A block is an unsplittable entity, and must be presented entirely to the decompression function. LZ4_decompress_safe_continue() only accepts one block at a time. It's modeled after LZ4_decompress_safe() and behaves similarly.

@LZ4_streamDecode : decompression state, tracking the position in memory of past data @compressedSize : exact complete size of one compressed block. @dstCapacity : size of destination buffer (which must be already allocated), must be an upper bound of decompressed size.

The last 64KB of previously decoded data

must remain available and unmodified at the memory position where they were previously decoded. If less than 64KB of data has been decoded, all the data must be present.

Special : if decompression side sets a ring buffer, it must respect one of the following conditions :

  • Decompression buffer size is at least LZ4_decoderRingBufferSize(maxBlockSize). maxBlockSize is the maximum size of any single block. It can have any value > 16 bytes. In which case, encoding and decoding buffers do not need to be synchronized. Actually, data can be produced by any source compliant with LZ4 format specification, and respecting maxBlockSize.

  • Synchronized mode : Decompression buffer size is exactly the same as compression buffer size, and follows exactly same update rule (block boundaries at same positions), and decoding function is provided with exact decompressed size of each block (exception for last block of the stream), then decoding & encoding ring buffer can have any size, including small ones ( < 64 KB).

  • Decompression buffer is larger than encoding buffer, by a minimum of maxBlockSize more bytes. In which case, encoding and decoding buffers do not need to be synchronized, and encoding ring buffer can have any size, including small ones ( < 64 KB).

Whenever these conditions are not possible, save the last 64KB of decoded data into a safe buffer where it can't be modified during decompression, then indicate where this data is saved using LZ4_setStreamDecode(), before decompressing next block.


: number of bytes decompressed into destination buffer (necessarily <= dstCapacity) If destination buffer is not large enough, decoding will stop and output an error code (negative value). If the source stream is detected malformed, the function will stop decoding and return a negative result.

int LZ4_decompress_safe_usingDict(const char *src, char *dst, int srcSize, int dstCapacity, const char *dictStart, int dictSize)

LZ4_decompress_safe_usingDict() : Works the same as a combination of LZ4_setStreamDecode() followed by LZ4_decompress_safe_continue() However, it's stateless: it doesn't need any LZ4_streamDecode_t state. Dictionary is presumed stable : it must remain accessible and unmodified during decompression. Performance tip : Decompression speed can be substantially increased when dst == dictStart + dictSize.

int LZ4_decompress_safe_partial_usingDict(const char *src, char *dst, int compressedSize, int targetOutputSize, int maxOutputSize, const char *dictStart, int dictSize)

LZ4_decompress_safe_partial_usingDict() : Behaves the same as LZ4_decompress_safe_partial() with the added ability to specify a memory segment for past data. Performance tip : Decompression speed can be substantially increased when dst == dictStart + dictSize.

LZ4_stream_t *LZ4_initStream(void *buffer, size_t size)

LZ4_initStream() : v1.9.0+ An LZ4_stream_t structure must be initialized at least once. This is automatically done when invoking LZ4_createStream(), but it's not when the structure is simply declared on stack (for example).

Use LZ4_initStream() to properly initialize a newly declared LZ4_stream_t. It can also initialize any arbitrary buffer of sufficient size, and will

Note : initialization fails if size and alignment conditions are not respected. In which case, the function will


a pointer of proper type upon initialization.


NULL. Note2: An LZ4_stream_t structure guarantees correct alignment and size. Note3: Before v1.9.0, use LZ4_resetStream() instead

int LZ4_compress(const char *src, char *dest, int srcSize)

Obsolete compression functions (since v1.7.3)

int LZ4_compress_limitedOutput(const char *src, char *dest, int srcSize, int maxOutputSize)
int LZ4_compress_withState(void *state, const char *source, char *dest, int inputSize)
int LZ4_compress_limitedOutput_withState(void *state, const char *source, char *dest, int inputSize, int maxOutputSize)
int LZ4_compress_continue(LZ4_stream_t *LZ4_streamPtr, const char *source, char *dest, int inputSize)
int LZ4_compress_limitedOutput_continue(LZ4_stream_t *LZ4_streamPtr, const char *source, char *dest, int inputSize, int maxOutputSize)
int LZ4_uncompress(const char *source, char *dest, int outputSize)

Obsolete decompression functions (since v1.8.0)

int LZ4_uncompress_unknownOutputSize(const char *source, char *dest, int isize, int maxOutputSize)
void *LZ4_create(char *inputBuffer)
int LZ4_sizeofStreamState(void)
int LZ4_resetStreamState(void *state, char *inputBuffer)
char *LZ4_slideInputBuffer(void *state)
int LZ4_decompress_safe_withPrefix64k(const char *src, char *dst, int compressedSize, int maxDstSize)

Obsolete streaming decoding functions (since v1.7.0)

int LZ4_decompress_fast_withPrefix64k(const char *src, char *dst, int originalSize)
int LZ4_decompress_fast(const char *src, char *dst, int originalSize)

Obsolete LZ4_decompress_fast variants (since v1.9.0) : These functions used to be faster than LZ4_decompress_safe(), but this is no longer the case. They are now slower. This is because LZ4_decompress_fast() doesn't know the input size, and therefore must progress more cautiously into the input buffer to not read beyond the end of block. On top of that LZ4_decompress_fast() is not protected vs malformed or malicious inputs, making it a security liability. As a consequence, LZ4_decompress_fast() is strongly discouraged, and deprecated.

The last remaining LZ4_decompress_fast() specificity is that it can decompress a block without knowing its compressed size. Such functionality can be achieved in a more secure manner by employing LZ4_decompress_safe_partial().

Parameters: originalSize : is the uncompressed size to regenerate. dst must be already allocated, its size must be >= 'originalSize' bytes.


: number of bytes read from source buffer (== compressed size). The function expects to finish at block's end exactly. If the source stream is detected malformed, the function stops decoding and returns a negative result. note : LZ4_decompress_fast*() requires originalSize. Thanks to this information, it never writes past the output buffer. However, since it doesn't know its 'src' size, it may read an unknown amount of input, past input buffer bounds. Also, since match offsets are not validated, match reads from 'src' may underflow too. These issues never happen if input (compressed) data is correct. But they may happen if input data is invalid (error or intentional tampering). As a consequence, use these functions in trusted environments with trusted data only.

int LZ4_decompress_fast_continue(LZ4_streamDecode_t *LZ4_streamDecode, const char *src, char *dst, int originalSize)
int LZ4_decompress_fast_usingDict(const char *src, char *dst, int originalSize, const char *dictStart, int dictSize)
void LZ4_resetStream(LZ4_stream_t *streamPtr)

LZ4_resetStream() : An LZ4_stream_t structure must be initialized at least once. This is done with LZ4_initStream(), or LZ4_resetStream(). Consider switching to LZ4_initStream(), invoking LZ4_resetStream() will trigger deprecation warnings in the future.

struct LZ4_stream_t_internal

Public Members

LZ4_u32 hashTable[(1 << (14 - 2))]
const LZ4_byte *dictionary
const LZ4_stream_t_internal *dictCtx
LZ4_u32 currentOffset
LZ4_u32 tableType
LZ4_u32 dictSize
union LZ4_stream_u

Public Members

char minStateSize[((1UL << 14) + 32)]
LZ4_stream_t_internal internal_donotuse
struct LZ4_streamDecode_t_internal
#include <lz4.h>

LZ4_streamDecode_t : Never ever use below internal definitions directly ! These definitions are not API/ABI safe, and may change in future versions. If you need static allocation, declare or allocate an LZ4_streamDecode_t object.

Public Members

const LZ4_byte *externalDict
const LZ4_byte *prefixEnd
size_t extDictSize
size_t prefixSize
union LZ4_streamDecode_u

Public Members

char minStateSize[32]
LZ4_streamDecode_t_internal internal_donotuse