# zstd 1.1.2 Manual

## Contents

1. Introduction
2. Version
3. Simple API
4. Explicit memory management
5. Simple dictionary API
6. Fast dictionary API
7. Streaming
8. Streaming compression - HowTo
9. Streaming decompression - HowTo
10. START OF ADVANCED AND EXPERIMENTAL FUNCTIONS
11. Advanced types
12. Advanced compression functions
13. Advanced decompression functions
14. Advanced streaming functions
15. Buffer-less and synchronous inner streaming functions
16. Buffer-less streaming compression (synchronous mode)
17. Buffer-less streaming decompression (synchronous mode)
18. Block functions

## Introduction

      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.

## Version

    unsigned ZSTD_versionNumber(void);   /**< library version number; to be used when checking dll version */

## Simple API

    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.

### Helper functions

    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 */

## Explicit memory management

    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()).

### Decompression context

    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()).

## Simple dictionary API

    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.

## Fast dictionary API

    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.

## Streaming

    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;

## Streaming compression - HowTo

      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().

    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. */

## Streaming decompression - HowTo

      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.

    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. */

## START OF ADVANCED AND EXPERIMENTAL FUNCTIONS

     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.

## Advanced types

    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;

### Custom memory allocation functions

    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;

## Advanced compression functions

    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_compressionParameters ZSTD_getCParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
       @return ZSTD_compressionParameters structure for a selected compression level and estimated srcSize.
       `estimatedSrcSize` value is optional, select 0 if not known

    ZSTD_parameters ZSTD_getParams(int compressionLevel, unsigned long long estimatedSrcSize, size_t dictSize);
       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)

    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

## Advanced decompression functions

    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.

## Advanced streaming functions

### Advanced Streaming compression functions

    ZSTD_CStream* ZSTD_createCStream_advanced(ZSTD_customMem customMem);
    size_t ZSTD_initCStream_srcSize(ZSTD_CStream* zcs, int compressionLevel, unsigned long long pledgedSrcSize);   /**< pledgedSrcSize must be correct */
    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);

### Advanced Streaming decompression functions

    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);

## Buffer-less and synchronous inner streaming functions

      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

## Buffer-less streaming compression (synchronous mode)

      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.

### Buffer-less streaming compression functions

    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);

## Buffer-less streaming decompression (synchronous mode)

      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;

### Buffer-less streaming decompression functions

    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

        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.

### Raw zstd block functions

    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 */