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<h1>zstd 1.1.2 Manual</h1>
<hr>
<a name="Contents"></a><h2>Contents</h2>
<ol>
<li><a href="#Chapter1">Introduction</a></li>
<li><a href="#Chapter2">Version</a></li>
<li><a href="#Chapter3">Simple API</a></li>
<li><a href="#Chapter4">Explicit memory management</a></li>
<li><a href="#Chapter5">Simple dictionary API</a></li>
<li><a href="#Chapter6">Fast dictionary API</a></li>
<li><a href="#Chapter7">Streaming</a></li>
<li><a href="#Chapter8">Streaming compression - HowTo</a></li>
<li><a href="#Chapter9">Streaming decompression - HowTo</a></li>
<li><a href="#Chapter10">START OF ADVANCED AND EXPERIMENTAL FUNCTIONS</a></li>
<li><a href="#Chapter11">Advanced types</a></li>
<li><a href="#Chapter12">Advanced compression functions</a></li>
<li><a href="#Chapter13">Advanced decompression functions</a></li>
<li><a href="#Chapter14">Advanced streaming functions</a></li>
<li><a href="#Chapter15">Buffer-less and synchronous inner streaming functions</a></li>
<li><a href="#Chapter16">Buffer-less streaming compression (synchronous mode)</a></li>
<li><a href="#Chapter17">Buffer-less streaming decompression (synchronous mode)</a></li>
<li><a href="#Chapter18">Block functions</a></li>
</ol>
<hr>
<a name="Chapter1"></a><h2>Introduction</h2><pre>
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.
<BR></pre>
<a name="Chapter2"></a><h2>Version</h2><pre></pre>
<pre><b>unsigned ZSTD_versionNumber(void); </b>/**< library version number; to be used when checking dll version */<b>
</b></pre><BR>
<a name="Chapter3"></a><h2>Simple API</h2><pre></pre>
<pre><b>size_t ZSTD_compress( void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
int compressionLevel);
</b><p> 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()).
</p></pre><BR>
<pre><b>size_t ZSTD_decompress( void* dst, size_t dstCapacity,
const void* src, size_t compressedSize);
</b><p> `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()).
</p></pre><BR>
<pre><b>unsigned long long ZSTD_getDecompressedSize(const void* src, size_t srcSize);
</b><p> '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.
</p></pre><BR>
<h3>Helper functions</h3><pre><b>int ZSTD_maxCLevel(void); </b>/*!< maximum compression level available */<b>
size_t ZSTD_compressBound(size_t srcSize); </b>/*!< maximum compressed size in worst case scenario */<b>
unsigned ZSTD_isError(size_t code); </b>/*!< tells if a `size_t` function result is an error code */<b>
const char* ZSTD_getErrorName(size_t code); </b>/*!< provides readable string from an error code */<b>
</b></pre><BR>
<a name="Chapter4"></a><h2>Explicit memory management</h2><pre></pre>
<pre><b>size_t ZSTD_compressCCtx(ZSTD_CCtx* ctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize, int compressionLevel);
</b><p> Same as ZSTD_compress(), requires an allocated ZSTD_CCtx (see ZSTD_createCCtx()).
</p></pre><BR>
<h3>Decompression context</h3><pre><b>typedef struct ZSTD_DCtx_s ZSTD_DCtx;
ZSTD_DCtx* ZSTD_createDCtx(void);
size_t ZSTD_freeDCtx(ZSTD_DCtx* dctx);
</b></pre><BR>
<pre><b>size_t ZSTD_decompressDCtx(ZSTD_DCtx* ctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize);
</b><p> Same as ZSTD_decompress(), requires an allocated ZSTD_DCtx (see ZSTD_createDCtx()).
</p></pre><BR>
<a name="Chapter5"></a><h2>Simple dictionary API</h2><pre></pre>
<pre><b>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);
</b><p> 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.
</p></pre><BR>
<pre><b>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);
</b><p> 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.
</p></pre><BR>
<a name="Chapter6"></a><h2>Fast dictionary API</h2><pre></pre>
<pre><b>ZSTD_CDict* ZSTD_createCDict(const void* dict, size_t dictSize, int compressionLevel);
</b><p> 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.
</p></pre><BR>
<pre><b>size_t ZSTD_freeCDict(ZSTD_CDict* CDict);
</b><p> Function frees memory allocated by ZSTD_createCDict().
</p></pre><BR>
<pre><b>size_t ZSTD_compress_usingCDict(ZSTD_CCtx* cctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const ZSTD_CDict* cdict);
</b><p> 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.
</p></pre><BR>
<pre><b>ZSTD_DDict* ZSTD_createDDict(const void* dict, size_t dictSize);
</b><p> Create a digested dictionary, ready to start decompression operation without startup delay.
`dict` can be released after creation.
</p></pre><BR>
<pre><b>size_t ZSTD_freeDDict(ZSTD_DDict* ddict);
</b><p> Function frees memory allocated with ZSTD_createDDict()
</p></pre><BR>
<pre><b>size_t ZSTD_decompress_usingDDict(ZSTD_DCtx* dctx,
void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
const ZSTD_DDict* ddict);
</b><p> Decompression using a digested Dictionary.
Faster startup than ZSTD_decompress_usingDict(), recommended when same dictionary is used multiple times.
</p></pre><BR>
<a name="Chapter7"></a><h2>Streaming</h2><pre></pre>
<pre><b>typedef struct ZSTD_inBuffer_s {
const void* src; </b>/**< start of input buffer */<b>
size_t size; </b>/**< size of input buffer */<b>
size_t pos; </b>/**< position where reading stopped. Will be updated. Necessarily 0 <= pos <= size */<b>
} ZSTD_inBuffer;
</b></pre><BR>
<pre><b>typedef struct ZSTD_outBuffer_s {
void* dst; </b>/**< start of output buffer */<b>
size_t size; </b>/**< size of output buffer */<b>
size_t pos; </b>/**< position where writing stopped. Will be updated. Necessarily 0 <= pos <= size */<b>
} ZSTD_outBuffer;
</b></pre><BR>
<a name="Chapter8"></a><h2>Streaming compression - HowTo</h2><pre>
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() or ZSTD_initCStream_usingCDict() for a compression which requires a dictionary (experimental section)
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
or an error code, which can be tested using ZSTD_isError().
Note 1 : it's just a hint, to help latency a little, any other value will work fine.
Note 2 : size hint is guaranteed to be <= ZSTD_CStreamInSize()
At any moment, it's possible to flush whatever data remains within internal buffer, using ZSTD_flushStream().
`output->pos` will be updated.
Note that 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, hence compression completed)
or an error code, which can be tested using ZSTD_isError().
<BR></pre>
<pre><b>size_t ZSTD_CStreamInSize(void); </b>/**< recommended size for input buffer */<b>
</b></pre><BR>
<pre><b>size_t ZSTD_CStreamOutSize(void); </b>/**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block in all circumstances. */<b>
</b></pre><BR>
<a name="Chapter9"></a><h2>Streaming decompression - HowTo</h2><pre>
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.
<BR></pre>
<pre><b>size_t ZSTD_DStreamInSize(void); </b>/*!< recommended size for input buffer */<b>
</b></pre><BR>
<pre><b>size_t ZSTD_DStreamOutSize(void); </b>/*!< recommended size for output buffer. Guarantee to successfully flush at least one complete block in all circumstances. */<b>
</b></pre><BR>
<a name="Chapter10"></a><h2>START OF ADVANCED AND EXPERIMENTAL FUNCTIONS</h2><pre> 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.
<BR></pre>
<a name="Chapter11"></a><h2>Advanced types</h2><pre></pre>
<pre><b>typedef enum { ZSTD_fast, ZSTD_dfast, ZSTD_greedy, ZSTD_lazy, ZSTD_lazy2, ZSTD_btlazy2, ZSTD_btopt, ZSTD_btopt2 } ZSTD_strategy; </b>/* from faster to stronger */<b>
</b></pre><BR>
<pre><b>typedef struct {
unsigned windowLog; </b>/**< largest match distance : larger == more compression, more memory needed during decompression */<b>
unsigned chainLog; </b>/**< fully searched segment : larger == more compression, slower, more memory (useless for fast) */<b>
unsigned hashLog; </b>/**< dispatch table : larger == faster, more memory */<b>
unsigned searchLog; </b>/**< nb of searches : larger == more compression, slower */<b>
unsigned searchLength; </b>/**< match length searched : larger == faster decompression, sometimes less compression */<b>
unsigned targetLength; </b>/**< acceptable match size for optimal parser (only) : larger == more compression, slower */<b>
ZSTD_strategy strategy;
} ZSTD_compressionParameters;
</b></pre><BR>
<pre><b>typedef struct {
unsigned contentSizeFlag; </b>/**< 1: content size will be in frame header (if known). */<b>
unsigned checksumFlag; </b>/**< 1: will generate a 22-bits checksum at end of frame, to be used for error detection by decompressor */<b>
unsigned noDictIDFlag; </b>/**< 1: no dict ID will be saved into frame header (if dictionary compression) */<b>
} ZSTD_frameParameters;
</b></pre><BR>
<pre><b>typedef struct {
ZSTD_compressionParameters cParams;
ZSTD_frameParameters fParams;
} ZSTD_parameters;
</b></pre><BR>
<h3>Custom memory allocation functions</h3><pre><b>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;
</b></pre><BR>
<a name="Chapter12"></a><h2>Advanced compression functions</h2><pre></pre>
<pre><b>size_t ZSTD_estimateCCtxSize(ZSTD_compressionParameters cParams);
</b><p> 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
</p></pre><BR>
<pre><b>ZSTD_CCtx* ZSTD_createCCtx_advanced(ZSTD_customMem customMem);
</b><p> Create a ZSTD compression context using external alloc and free functions
</p></pre><BR>
<pre><b>size_t ZSTD_sizeof_CCtx(const ZSTD_CCtx* cctx);
</b><p> Gives the amount of memory used by a given ZSTD_CCtx
</p></pre><BR>
<pre><b>ZSTD_CDict* ZSTD_createCDict_advanced(const void* dict, size_t dictSize,
ZSTD_parameters params, ZSTD_customMem customMem);
</b><p> Create a ZSTD_CDict using external alloc and free, and customized compression parameters
</p></pre><BR>
<pre><b>size_t ZSTD_sizeof_CDict(const ZSTD_CDict* cdict);
</b><p> Gives the amount of memory used by a given ZSTD_sizeof_CDict
</p></pre><BR>
<pre><b>ZSTD_compressionParameters ZSTD_getCParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
</b><p> @return ZSTD_compressionParameters structure for a selected compression level and estimated srcSize.
`estimatedSrcSize` value is optional, select 0 if not known
</p></pre><BR>
<pre><b>ZSTD_parameters ZSTD_getParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
</b><p> same as ZSTD_getCParams(), but @return a full `ZSTD_parameters` object instead of sub-component `ZSTD_compressionParameters`.
All fields of `ZSTD_frameParameters` are set to default (0)
</p></pre><BR>
<pre><b>size_t ZSTD_checkCParams(ZSTD_compressionParameters params);
</b><p> Ensure param values remain within authorized range
</p></pre><BR>
<pre><b>ZSTD_compressionParameters ZSTD_adjustCParams(ZSTD_compressionParameters cPar, unsigned long long srcSize, size_t dictSize);
</b><p> optimize params for a given `srcSize` and `dictSize`.
both values are optional, select `0` if unknown.
</p></pre><BR>
<pre><b>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);
</b><p> Same as ZSTD_compress_usingDict(), with fine-tune control of each compression parameter
</p></pre><BR>
<a name="Chapter13"></a><h2>Advanced decompression functions</h2><pre></pre>
<pre><b>unsigned ZSTD_isFrame(const void* buffer, size_t size);
</b><p> 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.
</p></pre><BR>
<pre><b>size_t ZSTD_estimateDCtxSize(void);
</b><p> Gives the potential amount of memory allocated to create a ZSTD_DCtx
</p></pre><BR>
<pre><b>ZSTD_DCtx* ZSTD_createDCtx_advanced(ZSTD_customMem customMem);
</b><p> Create a ZSTD decompression context using external alloc and free functions
</p></pre><BR>
<pre><b>size_t ZSTD_sizeof_DCtx(const ZSTD_DCtx* dctx);
</b><p> Gives the amount of memory used by a given ZSTD_DCtx
</p></pre><BR>
<pre><b>size_t ZSTD_sizeof_DDict(const ZSTD_DDict* ddict);
</b><p> Gives the amount of memory used by a given ZSTD_DDict
</p></pre><BR>
<pre><b>unsigned ZSTD_getDictID_fromDict(const void* dict, size_t dictSize);
</b><p> 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.
</p></pre><BR>
<pre><b>unsigned ZSTD_getDictID_fromDDict(const ZSTD_DDict* ddict);
</b><p> 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.
</p></pre><BR>
<pre><b>unsigned ZSTD_getDictID_fromFrame(const void* src, size_t srcSize);
</b><p> 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.
</p></pre><BR>
<a name="Chapter14"></a><h2>Advanced streaming functions</h2><pre></pre>
<h3>Advanced Streaming compression functions</h3><pre><b>ZSTD_CStream* ZSTD_createCStream_advanced(ZSTD_customMem customMem);
size_t ZSTD_initCStream_srcSize(ZSTD_CStream* zcs, int compressionLevel, unsigned long long pledgedSrcSize); </b>/**< pledgedSrcSize must be correct */<b>
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); </b>/**< pledgedSrcSize is optional and can be zero == unknown */<b>
size_t ZSTD_initCStream_usingCDict(ZSTD_CStream* zcs, const ZSTD_CDict* cdict); </b>/**< note : cdict will just be referenced, and must outlive compression session */<b>
size_t ZSTD_resetCStream(ZSTD_CStream* zcs, unsigned long long pledgedSrcSize); </b>/**< re-use compression parameters from previous init; skip dictionary loading stage; zcs must be init at least once before */<b>
size_t ZSTD_sizeof_CStream(const ZSTD_CStream* zcs);
</b></pre><BR>
<h3>Advanced Streaming decompression functions</h3><pre><b>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); </b>/**< note : ddict will just be referenced, and must outlive decompression session */<b>
size_t ZSTD_resetDStream(ZSTD_DStream* zds); </b>/**< re-use decompression parameters from previous init; saves dictionary loading */<b>
size_t ZSTD_sizeof_DStream(const ZSTD_DStream* zds);
</b></pre><BR>
<a name="Chapter15"></a><h2>Buffer-less and synchronous inner streaming functions</h2><pre>
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
<BR></pre>
<a name="Chapter16"></a><h2>Buffer-less streaming compression (synchronous mode)</h2><pre>
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.
<BR></pre>
<h3>Buffer-less streaming compression functions</h3><pre><b>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);
</b></pre><BR>
<a name="Chapter17"></a><h2>Buffer-less streaming decompression (synchronous mode)</h2><pre>
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.
<BR></pre>
<pre><b>typedef struct {
unsigned long long frameContentSize;
unsigned windowSize;
unsigned dictID;
unsigned checksumFlag;
} ZSTD_frameParams;
</b></pre><BR>
<h3>Buffer-less streaming decompression functions</h3><pre><b>size_t ZSTD_getFrameParams(ZSTD_frameParams* fparamsPtr, const void* src, size_t srcSize); </b>/**< doesn't consume input, see details below */<b>
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);
</b></pre><BR>
<a name="Chapter18"></a><h2>Block functions</h2><pre>
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.
<BR></pre>
<h3>Raw zstd block functions</h3><pre><b>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); </b>/**< insert block into `dctx` history. Useful for uncompressed blocks */<b>
</b></pre><BR>
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