zstd, short for Zstandard, is a fast lossless compression algorithm, targeting real-time compression scenarios
at zlib-level and better compression ratios. The zstd compression library provides in-memory compression and
decompression functions. The library supports compression levels from 1 up to ZSTD_maxCLevel() which is 22.
Levels >= 20, labelled `--ultra`, should be used with caution, as they require more memory.
Compression can be done in:
- a single step (described as Simple API)
- a single step, reusing a context (described as Explicit memory management)
- unbounded multiple steps (described as Streaming compression)
The compression ratio achievable on small data can be highly improved using compression with a dictionary in:
- a single step (described as Simple dictionary API)
- a single step, reusing a dictionary (described as Fast dictionary API)
Advanced experimental functions can be accessed using #define ZSTD_STATIC_LINKING_ONLY before including zstd.h.
These APIs shall never be used with a dynamic library.
They are not "stable", their definition may change in the future. Only static linking is allowed.
unsigned ZSTD_versionNumber(void); /**< library version number; to be used when checking dll version */
size_t ZSTD_compress( void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
int compressionLevel);
Compresses `src` content as a single zstd compressed frame into already allocated `dst`.
Hint : compression runs faster if `dstCapacity` >= `ZSTD_compressBound(srcSize)`.
@return : compressed size written into `dst` (<= `dstCapacity),
or an error code if it fails (which can be tested using ZSTD_isError()).
size_t ZSTD_decompress( void* dst, size_t dstCapacity,
const void* src, size_t compressedSize);
`compressedSize` : must be the _exact_ size of a single compressed frame.
`dstCapacity` is an upper bound of originalSize.
If user cannot imply a maximum upper bound, it's better to use streaming mode to decompress data.
@return : the number of bytes decompressed into `dst` (<= `dstCapacity`),
or an errorCode if it fails (which can be tested using ZSTD_isError()).
unsigned long long ZSTD_getDecompressedSize(const void* src, size_t srcSize);'src' is the start of a zstd compressed frame. @return : content size to be decompressed, as a 64-bits value _if known_, 0 otherwise. note 1 : decompressed size is an optional field, that may not be present, especially in streaming mode. When `return==0`, data to decompress could be any size. In which case, it's necessary to use streaming mode to decompress data. Optionally, application can still use ZSTD_decompress() while relying on implied limits. (For example, data may be necessarily cut into blocks <= 16 KB). note 2 : decompressed size is always present when compression is done with ZSTD_compress() note 3 : decompressed size can be very large (64-bits value), potentially larger than what local system can handle as a single memory segment. In which case, it's necessary to use streaming mode to decompress data. note 4 : If source is untrusted, decompressed size could be wrong or intentionally modified. Always ensure result fits within application's authorized limits. Each application can set its own limits. note 5 : when `return==0`, if precise failure cause is needed, use ZSTD_getFrameParams() to know more.
int ZSTD_maxCLevel(void);/*!< maximum compression level available */ size_t ZSTD_compressBound(size_t srcSize); /*!< maximum compressed size in worst case scenario */ unsigned ZSTD_isError(size_t code); /*!< tells if a `size_t` function result is an error code */ const char* ZSTD_getErrorName(size_t code); /*!< provides readable string from an error code */
When compressing many times, it is recommended to allocate a context just once, and re-use it for each successive compression operation. This will make workload friendlier for system's memory. Use one context per thread for parallel execution in multi-threaded environments.
typedef struct ZSTD_CCtx_s ZSTD_CCtx; ZSTD_CCtx* ZSTD_createCCtx(void); size_t ZSTD_freeCCtx(ZSTD_CCtx* cctx);
size_t ZSTD_compressCCtx(ZSTD_CCtx* ctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize, int compressionLevel);Same as ZSTD_compress(), requires an allocated ZSTD_CCtx (see ZSTD_createCCtx()).
typedef struct ZSTD_DCtx_s ZSTD_DCtx; ZSTD_DCtx* ZSTD_createDCtx(void); size_t ZSTD_freeDCtx(ZSTD_DCtx* dctx);
size_t ZSTD_decompressDCtx(ZSTD_DCtx* ctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);Same as ZSTD_decompress(), requires an allocated ZSTD_DCtx (see ZSTD_createDCtx()).
size_t ZSTD_compress_usingDict(ZSTD_CCtx* ctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const void* dict,size_t dictSize,
int compressionLevel);
Compression using a predefined Dictionary (see dictBuilder/zdict.h).
Note : This function loads the dictionary, resulting in significant startup delay.
Note : When `dict == NULL || dictSize < 8` no dictionary is used.
size_t ZSTD_decompress_usingDict(ZSTD_DCtx* dctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const void* dict,size_t dictSize);
Decompression using a predefined Dictionary (see dictBuilder/zdict.h).
Dictionary must be identical to the one used during compression.
Note : This function loads the dictionary, resulting in significant startup delay.
Note : When `dict == NULL || dictSize < 8` no dictionary is used.
ZSTD_CDict* ZSTD_createCDict(const void* dict, size_t dictSize, int compressionLevel);When compressing multiple messages / blocks with the same dictionary, it's recommended to load it just once. ZSTD_createCDict() will create a digested dictionary, ready to start future compression operations without startup delay. ZSTD_CDict can be created once and used by multiple threads concurrently, as its usage is read-only. `dict` can be released after ZSTD_CDict creation.
size_t ZSTD_freeCDict(ZSTD_CDict* CDict);Function frees memory allocated by ZSTD_createCDict().
size_t ZSTD_compress_usingCDict(ZSTD_CCtx* cctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const ZSTD_CDict* cdict);
Compression using a digested Dictionary.
Faster startup than ZSTD_compress_usingDict(), recommended when same dictionary is used multiple times.
Note that compression level is decided during dictionary creation.
ZSTD_DDict* ZSTD_createDDict(const void* dict, size_t dictSize);Create a digested dictionary, ready to start decompression operation without startup delay. `dict` can be released after creation.
size_t ZSTD_freeDDict(ZSTD_DDict* ddict);Function frees memory allocated with ZSTD_createDDict()
size_t ZSTD_decompress_usingDDict(ZSTD_DCtx* dctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const ZSTD_DDict* ddict);
Decompression using a digested Dictionary.
Faster startup than ZSTD_decompress_usingDict(), recommended when same dictionary is used multiple times.
typedef struct ZSTD_inBuffer_s {
const void* src; /**< start of input buffer */
size_t size; /**< size of input buffer */
size_t pos; /**< position where reading stopped. Will be updated. Necessarily 0 <= pos <= size */
} ZSTD_inBuffer;
typedef struct ZSTD_outBuffer_s {
void* dst; /**< start of output buffer */
size_t size; /**< size of output buffer */
size_t pos; /**< position where writing stopped. Will be updated. Necessarily 0 <= pos <= size */
} ZSTD_outBuffer;
A ZSTD_CStream object is required to track streaming operation.
Use ZSTD_createCStream() and ZSTD_freeCStream() to create/release resources.
ZSTD_CStream objects can be reused multiple times on consecutive compression operations.
It is recommended to re-use ZSTD_CStream in situations where many streaming operations will be achieved consecutively,
since it will play nicer with system's memory, by re-using already allocated memory.
Use one separate ZSTD_CStream per thread for parallel execution.
Start a new compression by initializing ZSTD_CStream.
Use ZSTD_initCStream() to start a new compression operation.
Use ZSTD_initCStream_usingDict() for a compression which requires a dictionary.
Use ZSTD_compressStream() repetitively to consume input stream.
The function will automatically update both `pos` fields.
Note that it may not consume the entire input, in which case `pos < size`,
and it's up to the caller to present again remaining data.
@return : a size hint, preferred nb of bytes to use as input for next function call
(it's just a hint, to help latency a little, any other value will work fine)
(note : the size hint is guaranteed to be <= ZSTD_CStreamInSize() )
or an error code, which can be tested using ZSTD_isError().
At any moment, it's possible to flush whatever data remains within buffer, using ZSTD_flushStream().
`output->pos` will be updated.
Note some content might still be left within internal buffer if `output->size` is too small.
@return : nb of bytes still present within internal buffer (0 if it's empty)
or an error code, which can be tested using ZSTD_isError().
ZSTD_endStream() instructs to finish a frame.
It will perform a flush and write frame epilogue.
The epilogue is required for decoders to consider a frame completed.
Similar to ZSTD_flushStream(), it may not be able to flush the full content if `output->size` is too small.
In which case, call again ZSTD_endStream() to complete the flush.
@return : nb of bytes still present within internal buffer (0 if it's empty)
or an error code, which can be tested using ZSTD_isError().
typedef struct ZSTD_CStream_s ZSTD_CStream; ZSTD_CStream* ZSTD_createCStream(void); size_t ZSTD_freeCStream(ZSTD_CStream* zcs); size_t ZSTD_initCStream(ZSTD_CStream* zcs, int compressionLevel); size_t ZSTD_compressStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output, ZSTD_inBuffer* input); size_t ZSTD_flushStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output); size_t ZSTD_endStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output);
size_t ZSTD_CStreamInSize(void); /**< recommended size for input buffer */
size_t ZSTD_CStreamOutSize(void); /**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block in all circumstances. */
A ZSTD_DStream object is required to track streaming operations.
Use ZSTD_createDStream() and ZSTD_freeDStream() to create/release resources.
ZSTD_DStream objects can be re-used multiple times.
Use ZSTD_initDStream() to start a new decompression operation,
or ZSTD_initDStream_usingDict() if decompression requires a dictionary.
@return : recommended first input size
Use ZSTD_decompressStream() repetitively to consume your input.
The function will update both `pos` fields.
If `input.pos < input.size`, some input has not been consumed.
It's up to the caller to present again remaining data.
If `output.pos < output.size`, decoder has flushed everything it could.
@return : 0 when a frame is completely decoded and fully flushed,
an error code, which can be tested using ZSTD_isError(),
any other value > 0, which means there is still some decoding to do to complete current frame.
The return value is a suggested next input size (a hint to improve latency) that will never load more than the current frame.
typedef struct ZSTD_DStream_s ZSTD_DStream; ZSTD_DStream* ZSTD_createDStream(void); size_t ZSTD_freeDStream(ZSTD_DStream* zds); size_t ZSTD_initDStream(ZSTD_DStream* zds); size_t ZSTD_decompressStream(ZSTD_DStream* zds, ZSTD_outBuffer* output, ZSTD_inBuffer* input);
size_t ZSTD_DStreamInSize(void); /*!< recommended size for input buffer */
size_t ZSTD_DStreamOutSize(void); /*!< recommended size for output buffer. Guarantee to successfully flush at least one complete block in all circumstances. */
The definitions in this section are considered experimental. They should never be used with a dynamic library, as they may change in the future. They are provided for advanced usages. Use them only in association with static linking.
typedef enum { ZSTD_fast, ZSTD_dfast, ZSTD_greedy, ZSTD_lazy, ZSTD_lazy2, ZSTD_btlazy2, ZSTD_btopt, ZSTD_btopt2 } ZSTD_strategy; /* from faster to stronger */
typedef struct {
unsigned windowLog; /**< largest match distance : larger == more compression, more memory needed during decompression */
unsigned chainLog; /**< fully searched segment : larger == more compression, slower, more memory (useless for fast) */
unsigned hashLog; /**< dispatch table : larger == faster, more memory */
unsigned searchLog; /**< nb of searches : larger == more compression, slower */
unsigned searchLength; /**< match length searched : larger == faster decompression, sometimes less compression */
unsigned targetLength; /**< acceptable match size for optimal parser (only) : larger == more compression, slower */
ZSTD_strategy strategy;
} ZSTD_compressionParameters;
typedef struct {
unsigned contentSizeFlag; /**< 1: content size will be in frame header (if known). */
unsigned checksumFlag; /**< 1: will generate a 22-bits checksum at end of frame, to be used for error detection by decompressor */
unsigned noDictIDFlag; /**< 1: no dict ID will be saved into frame header (if dictionary compression) */
} ZSTD_frameParameters;
typedef struct {
ZSTD_compressionParameters cParams;
ZSTD_frameParameters fParams;
} ZSTD_parameters;
typedef void* (*ZSTD_allocFunction) (void* opaque, size_t size);
typedef void (*ZSTD_freeFunction) (void* opaque, void* address);
typedef struct { ZSTD_allocFunction customAlloc; ZSTD_freeFunction customFree; void* opaque; } ZSTD_customMem;
size_t ZSTD_estimateCCtxSize(ZSTD_compressionParameters cParams);Gives the amount of memory allocated for a ZSTD_CCtx given a set of compression parameters. `frameContentSize` is an optional parameter, provide `0` if unknown
ZSTD_CCtx* ZSTD_createCCtx_advanced(ZSTD_customMem customMem);Create a ZSTD compression context using external alloc and free functions
size_t ZSTD_sizeof_CCtx(const ZSTD_CCtx* cctx);Gives the amount of memory used by a given ZSTD_CCtx
ZSTD_CDict* ZSTD_createCDict_advanced(const void* dict, size_t dictSize,
ZSTD_parameters params, ZSTD_customMem customMem);
Create a ZSTD_CDict using external alloc and free, and customized compression parameters
size_t ZSTD_sizeof_CDict(const ZSTD_CDict* cdict);Gives the amount of memory used by a given ZSTD_sizeof_CDict
ZSTD_parameters ZSTD_getParams(int compressionLevel, unsigned long long srcSize, size_t dictSize);same as ZSTD_getCParams(), but @return a full `ZSTD_parameters` object instead of a `ZSTD_compressionParameters`. All fields of `ZSTD_frameParameters` are set to default (0)
ZSTD_compressionParameters ZSTD_getCParams(int compressionLevel, unsigned long long srcSize, size_t dictSize);@return ZSTD_compressionParameters structure for a selected compression level and srcSize. `srcSize` value is optional, select 0 if not known
size_t ZSTD_checkCParams(ZSTD_compressionParameters params);Ensure param values remain within authorized range
ZSTD_compressionParameters ZSTD_adjustCParams(ZSTD_compressionParameters cPar, unsigned long long srcSize, size_t dictSize);optimize params for a given `srcSize` and `dictSize`. both values are optional, select `0` if unknown.
size_t ZSTD_compress_advanced (ZSTD_CCtx* ctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const void* dict,size_t dictSize,
ZSTD_parameters params);
Same as ZSTD_compress_usingDict(), with fine-tune control of each compression parameter
unsigned ZSTD_isFrame(const void* buffer, size_t size);Tells if the content of `buffer` starts with a valid Frame Identifier. Note : Frame Identifier is 4 bytes. If `size < 4`, @return will always be 0. Note 2 : Legacy Frame Identifiers are considered valid only if Legacy Support is enabled. Note 3 : Skippable Frame Identifiers are considered valid.
size_t ZSTD_estimateDCtxSize(void);Gives the potential amount of memory allocated to create a ZSTD_DCtx
ZSTD_DCtx* ZSTD_createDCtx_advanced(ZSTD_customMem customMem);Create a ZSTD decompression context using external alloc and free functions
size_t ZSTD_sizeof_DCtx(const ZSTD_DCtx* dctx);Gives the amount of memory used by a given ZSTD_DCtx
size_t ZSTD_sizeof_DDict(const ZSTD_DDict* ddict);Gives the amount of memory used by a given ZSTD_DDict
unsigned ZSTD_getDictID_fromDict(const void* dict, size_t dictSize);Provides the dictID stored within dictionary. if @return == 0, the dictionary is not conformant with Zstandard specification. It can still be loaded, but as a content-only dictionary.
unsigned ZSTD_getDictID_fromDDict(const ZSTD_DDict* ddict);Provides the dictID of the dictionary loaded into `ddict`. If @return == 0, the dictionary is not conformant to Zstandard specification, or empty. Non-conformant dictionaries can still be loaded, but as content-only dictionaries.
unsigned ZSTD_getDictID_fromFrame(const void* src, size_t srcSize);Provides the dictID required to decompressed the frame stored within `src`. If @return == 0, the dictID could not be decoded. This could for one of the following reasons : - The frame does not require a dictionary to be decoded (most common case). - The frame was built with dictID intentionally removed. Whatever dictionary is necessary is a hidden information. Note : this use case also happens when using a non-conformant dictionary. - `srcSize` is too small, and as a result, the frame header could not be decoded (only possible if `srcSize < ZSTD_FRAMEHEADERSIZE_MAX`). - This is not a Zstandard frame. When identifying the exact failure cause, it's possible to used ZSTD_getFrameParams(), which will provide a more precise error code.
ZSTD_CStream* ZSTD_createCStream_advanced(ZSTD_customMem customMem);
size_t ZSTD_initCStream_usingDict(ZSTD_CStream* zcs, const void* dict, size_t dictSize, int compressionLevel);
size_t ZSTD_initCStream_advanced(ZSTD_CStream* zcs, const void* dict, size_t dictSize,
ZSTD_parameters params, unsigned long long pledgedSrcSize); /**< pledgedSrcSize is optional and can be zero == unknown */
size_t ZSTD_initCStream_usingCDict(ZSTD_CStream* zcs, const ZSTD_CDict* cdict); /**< note : cdict will just be referenced, and must outlive compression session */
size_t ZSTD_resetCStream(ZSTD_CStream* zcs, unsigned long long pledgedSrcSize); /**< re-use compression parameters from previous init; skip dictionary loading stage; zcs must be init at least once before */
size_t ZSTD_sizeof_CStream(const ZSTD_CStream* zcs);
typedef enum { ZSTDdsp_maxWindowSize } ZSTD_DStreamParameter_e;
ZSTD_DStream* ZSTD_createDStream_advanced(ZSTD_customMem customMem);
size_t ZSTD_initDStream_usingDict(ZSTD_DStream* zds, const void* dict, size_t dictSize);
size_t ZSTD_setDStreamParameter(ZSTD_DStream* zds, ZSTD_DStreamParameter_e paramType, unsigned paramValue);
size_t ZSTD_initDStream_usingDDict(ZSTD_DStream* zds, const ZSTD_DDict* ddict); /**< note : ddict will just be referenced, and must outlive decompression session */
size_t ZSTD_resetDStream(ZSTD_DStream* zds); /**< re-use decompression parameters from previous init; saves dictionary loading */
size_t ZSTD_sizeof_DStream(const ZSTD_DStream* zds);
This is an advanced API, giving full control over buffer management, for users which need direct control over memory. But it's also a complex one, with many restrictions (documented below). Prefer using normal streaming API for an easier experience
A ZSTD_CCtx object is required to track streaming operations.
Use ZSTD_createCCtx() / ZSTD_freeCCtx() to manage resource.
ZSTD_CCtx object can be re-used multiple times within successive compression operations.
Start by initializing a context.
Use ZSTD_compressBegin(), or ZSTD_compressBegin_usingDict() for dictionary compression,
or ZSTD_compressBegin_advanced(), for finer parameter control.
It's also possible to duplicate a reference context which has already been initialized, using ZSTD_copyCCtx()
Then, consume your input using ZSTD_compressContinue().
There are some important considerations to keep in mind when using this advanced function :
- ZSTD_compressContinue() has no internal buffer. It uses externally provided buffer only.
- Interface is synchronous : input is consumed entirely and produce 1+ (or more) compressed blocks.
- Caller must ensure there is enough space in `dst` to store compressed data under worst case scenario.
Worst case evaluation is provided by ZSTD_compressBound().
ZSTD_compressContinue() doesn't guarantee recover after a failed compression.
- ZSTD_compressContinue() presumes prior input ***is still accessible and unmodified*** (up to maximum distance size, see WindowLog).
It remembers all previous contiguous blocks, plus one separated memory segment (which can itself consists of multiple contiguous blocks)
- ZSTD_compressContinue() detects that prior input has been overwritten when `src` buffer overlaps.
In which case, it will "discard" the relevant memory section from its history.
Finish a frame with ZSTD_compressEnd(), which will write the last block(s) and optional checksum.
It's possible to use a NULL,0 src content, in which case, it will write a final empty block to end the frame,
Without last block mark, frames will be considered unfinished (broken) by decoders.
You can then reuse `ZSTD_CCtx` (ZSTD_compressBegin()) to compress some new frame.
size_t ZSTD_compressBegin(ZSTD_CCtx* cctx, int compressionLevel); size_t ZSTD_compressBegin_usingDict(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, int compressionLevel); size_t ZSTD_compressBegin_advanced(ZSTD_CCtx* cctx, const void* dict, size_t dictSize, ZSTD_parameters params, unsigned long long pledgedSrcSize); size_t ZSTD_copyCCtx(ZSTD_CCtx* cctx, const ZSTD_CCtx* preparedCCtx, unsigned long long pledgedSrcSize); size_t ZSTD_compressContinue(ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize); size_t ZSTD_compressEnd(ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
A ZSTD_DCtx object is required to track streaming operations.
Use ZSTD_createDCtx() / ZSTD_freeDCtx() to manage it.
A ZSTD_DCtx object can be re-used multiple times.
First typical operation is to retrieve frame parameters, using ZSTD_getFrameParams().
It fills a ZSTD_frameParams structure which provide important information to correctly decode the frame,
such as the minimum rolling buffer size to allocate to decompress data (`windowSize`),
and the dictionary ID used.
(Note : content size is optional, it may not be present. 0 means : content size unknown).
Note that these values could be wrong, either because of data malformation, or because an attacker is spoofing deliberate false information.
As a consequence, check that values remain within valid application range, especially `windowSize`, before allocation.
Each application can set its own limit, depending on local restrictions. For extended interoperability, it is recommended to support at least 8 MB.
Frame parameters are extracted from the beginning of the compressed frame.
Data fragment must be large enough to ensure successful decoding, typically `ZSTD_frameHeaderSize_max` bytes.
@result : 0 : successful decoding, the `ZSTD_frameParams` structure is correctly filled.
>0 : `srcSize` is too small, please provide at least @result bytes on next attempt.
errorCode, which can be tested using ZSTD_isError().
Start decompression, with ZSTD_decompressBegin() or ZSTD_decompressBegin_usingDict().
Alternatively, you can copy a prepared context, using ZSTD_copyDCtx().
Then use ZSTD_nextSrcSizeToDecompress() and ZSTD_decompressContinue() alternatively.
ZSTD_nextSrcSizeToDecompress() tells how many bytes to provide as 'srcSize' to ZSTD_decompressContinue().
ZSTD_decompressContinue() requires this _exact_ amount of bytes, or it will fail.
@result of ZSTD_decompressContinue() is the number of bytes regenerated within 'dst' (necessarily <= dstCapacity).
It can be zero, which is not an error; it just means ZSTD_decompressContinue() has decoded some metadata item.
It can also be an error code, which can be tested with ZSTD_isError().
ZSTD_decompressContinue() needs previous data blocks during decompression, up to `windowSize`.
They should preferably be located contiguously, prior to current block.
Alternatively, a round buffer of sufficient size is also possible. Sufficient size is determined by frame parameters.
ZSTD_decompressContinue() is very sensitive to contiguity,
if 2 blocks don't follow each other, make sure that either the compressor breaks contiguity at the same place,
or that previous contiguous segment is large enough to properly handle maximum back-reference.
A frame is fully decoded when ZSTD_nextSrcSizeToDecompress() returns zero.
Context can then be reset to start a new decompression.
Note : it's possible to know if next input to present is a header or a block, using ZSTD_nextInputType().
This information is not required to properly decode a frame.
== Special case : skippable frames ==
Skippable frames allow integration of user-defined data into a flow of concatenated frames.
Skippable frames will be ignored (skipped) by a decompressor. The format of skippable frames is as follows :
a) Skippable frame ID - 4 Bytes, Little endian format, any value from 0x184D2A50 to 0x184D2A5F
b) Frame Size - 4 Bytes, Little endian format, unsigned 32-bits
c) Frame Content - any content (User Data) of length equal to Frame Size
For skippable frames ZSTD_decompressContinue() always returns 0.
For skippable frames ZSTD_getFrameParams() returns fparamsPtr->windowLog==0 what means that a frame is skippable.
It also returns Frame Size as fparamsPtr->frameContentSize.
typedef struct {
unsigned long long frameContentSize;
unsigned windowSize;
unsigned dictID;
unsigned checksumFlag;
} ZSTD_frameParams;
size_t ZSTD_getFrameParams(ZSTD_frameParams* fparamsPtr, const void* src, size_t srcSize);/**< doesn't consume input, see details below */ size_t ZSTD_decompressBegin(ZSTD_DCtx* dctx); size_t ZSTD_decompressBegin_usingDict(ZSTD_DCtx* dctx, const void* dict, size_t dictSize); void ZSTD_copyDCtx(ZSTD_DCtx* dctx, const ZSTD_DCtx* preparedDCtx); size_t ZSTD_nextSrcSizeToDecompress(ZSTD_DCtx* dctx); size_t ZSTD_decompressContinue(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize); typedef enum { ZSTDnit_frameHeader, ZSTDnit_blockHeader, ZSTDnit_block, ZSTDnit_lastBlock, ZSTDnit_checksum, ZSTDnit_skippableFrame } ZSTD_nextInputType_e; ZSTD_nextInputType_e ZSTD_nextInputType(ZSTD_DCtx* dctx);
Block functions produce and decode raw zstd blocks, without frame metadata.
Frame metadata cost is typically ~18 bytes, which can be non-negligible for very small blocks (< 100 bytes).
User will have to take in charge required information to regenerate data, such as compressed and content sizes.
A few rules to respect :
- Compressing and decompressing require a context structure
+ Use ZSTD_createCCtx() and ZSTD_createDCtx()
- It is necessary to init context before starting
+ compression : ZSTD_compressBegin()
+ decompression : ZSTD_decompressBegin()
+ variants _usingDict() are also allowed
+ copyCCtx() and copyDCtx() work too
- Block size is limited, it must be <= ZSTD_getBlockSizeMax()
+ If you need to compress more, cut data into multiple blocks
+ Consider using the regular ZSTD_compress() instead, as frame metadata costs become negligible when source size is large.
- When a block is considered not compressible enough, ZSTD_compressBlock() result will be zero.
In which case, nothing is produced into `dst`.
+ User must test for such outcome and deal directly with uncompressed data
+ ZSTD_decompressBlock() doesn't accept uncompressed data as input !!!
+ In case of multiple successive blocks, decoder must be informed of uncompressed block existence to follow proper history.
Use ZSTD_insertBlock() in such a case.
size_t ZSTD_getBlockSizeMax(ZSTD_CCtx* cctx); size_t ZSTD_compressBlock (ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize); size_t ZSTD_decompressBlock(ZSTD_DCtx* dctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize); size_t ZSTD_insertBlock(ZSTD_DCtx* dctx, const void* blockStart, size_t blockSize);/**< insert block into `dctx` history. Useful for uncompressed blocks */