160 lines
7.6 KiB
Markdown
160 lines
7.6 KiB
Markdown
Zstandard library files
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================================
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The __lib__ directory is split into several sub-directories,
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in order to make it easier to select or exclude features.
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#### Building
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`Makefile` script is provided, supporting [Makefile conventions](https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html#Makefile-Conventions),
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including commands variables, staged install, directory variables and standard targets.
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- `make` : generates both static and dynamic libraries
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- `make install` : install libraries and headers in target system directories
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`libzstd` default scope is pretty large, including compression, decompression, dictionary builder,
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and support for decoding legacy formats >= v0.5.0.
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The scope can be reduced on demand (see paragraph _modular build_).
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#### Multithreading support
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Multithreading is disabled by default when building with `make`.
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Enabling multithreading requires 2 conditions :
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- set build macro `ZSTD_MULTITHREAD` (`-DZSTD_MULTITHREAD` for `gcc`)
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- for POSIX systems : compile with pthread (`-pthread` compilation flag for `gcc`)
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Both conditions are automatically applied when invoking `make lib-mt` target.
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When linking a POSIX program with a multithreaded version of `libzstd`,
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note that it's necessary to invoke the `-pthread` flag during link stage.
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Multithreading capabilities are exposed
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via the [advanced API defined in `lib/zstd.h`](https://github.com/facebook/zstd/blob/v1.4.3/lib/zstd.h#L351).
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#### API
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Zstandard's stable API is exposed within [lib/zstd.h](zstd.h).
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#### Advanced API
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Optional advanced features are exposed via :
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- `lib/common/zstd_errors.h` : translates `size_t` function results
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into a `ZSTD_ErrorCode`, for accurate error handling.
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- `ZSTD_STATIC_LINKING_ONLY` : if this macro is defined _before_ including `zstd.h`,
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it unlocks access to the experimental API,
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exposed in the second part of `zstd.h`.
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All definitions in the experimental APIs are unstable,
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they may still change in the future, or even be removed.
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As a consequence, experimental definitions shall ___never be used with dynamic library___ !
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Only static linking is allowed.
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#### Modular build
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It's possible to compile only a limited set of features within `libzstd`.
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The file structure is designed to make this selection manually achievable for any build system :
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- Directory `lib/common` is always required, for all variants.
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- Compression source code lies in `lib/compress`
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- Decompression source code lies in `lib/decompress`
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- It's possible to include only `compress` or only `decompress`, they don't depend on each other.
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- `lib/dictBuilder` : makes it possible to generate dictionaries from a set of samples.
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The API is exposed in `lib/dictBuilder/zdict.h`.
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This module depends on both `lib/common` and `lib/compress` .
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- `lib/legacy` : makes it possible to decompress legacy zstd formats, starting from `v0.1.0`.
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This module depends on `lib/common` and `lib/decompress`.
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To enable this feature, define `ZSTD_LEGACY_SUPPORT` during compilation.
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Specifying a number limits versions supported to that version onward.
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For example, `ZSTD_LEGACY_SUPPORT=2` means : "support legacy formats >= v0.2.0".
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Conversely, `ZSTD_LEGACY_SUPPORT=0` means "do __not__ support legacy formats".
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By default, this build macro is set as `ZSTD_LEGACY_SUPPORT=5`.
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Decoding supported legacy format is a transparent capability triggered within decompression functions.
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It's also allowed to invoke legacy API directly, exposed in `lib/legacy/zstd_legacy.h`.
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Each version does also provide its own set of advanced API.
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For example, advanced API for version `v0.4` is exposed in `lib/legacy/zstd_v04.h` .
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- While invoking `make libzstd`, it's possible to define build macros
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`ZSTD_LIB_COMPRESSION, ZSTD_LIB_DECOMPRESSION`, `ZSTD_LIB_DICTBUILDER`,
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and `ZSTD_LIB_DEPRECATED` as `0` to forgo compilation of the corresponding features.
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This will also disable compilation of all dependencies
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(eg. `ZSTD_LIB_COMPRESSION=0` will also disable dictBuilder).
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- There are some additional build macros that can be used to minify the decoder.
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Zstandard often has more than one implementation of a piece of functionality,
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where each implementation optimizes for different scenarios. For example, the
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Huffman decoder has complementary implementations that decode the stream one
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symbol at a time or two symbols at a time. Zstd normally includes both (and
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dispatches between them at runtime), but by defining `HUF_FORCE_DECOMPRESS_X1`
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or `HUF_FORCE_DECOMPRESS_X2`, you can force the use of one or the other, avoiding
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compilation of the other. Similarly, `ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT`
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and `ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG` force the compilation and use of
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only one or the other of two decompression implementations. The smallest
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binary is achieved by using `HUF_FORCE_DECOMPRESS_X1` and
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`ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT`.
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For squeezing the last ounce of size out, you can also define
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`ZSTD_NO_INLINE`, which disables inlining, and `ZSTD_STRIP_ERROR_STRINGS`,
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which removes the error messages that are otherwise returned by
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`ZSTD_getErrorName`.
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- While invoking `make libzstd`, the build macro `ZSTD_LEGACY_MULTITHREADED_API=1`
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will expose the deprecated `ZSTDMT` API exposed by `zstdmt_compress.h` in
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the shared library, which is now hidden by default.
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- The build macro `DYNAMIC_BMI2` can be set to 1 or 0 in order to generate binaries
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which can detect at runtime the presence of BMI2 instructions, and use them only if present.
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These instructions contribute to better performance, notably on the decoder side.
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By default, this feature is automatically enabled on detecting
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the right instruction set (x64) and compiler (clang or gcc >= 5).
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It's obviously disabled for different cpus,
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or when BMI2 instruction set is _required_ by the compiler command line
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(in this case, only the BMI2 code path is generated).
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Setting this macro will either force to generate the BMI2 dispatcher (1)
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or prevent it (0). It overrides automatic detection.
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#### Windows : using MinGW+MSYS to create DLL
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DLL can be created using MinGW+MSYS with the `make libzstd` command.
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This command creates `dll\libzstd.dll` and the import library `dll\libzstd.lib`.
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The import library is only required with Visual C++.
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The header file `zstd.h` and the dynamic library `dll\libzstd.dll` are required to
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compile a project using gcc/MinGW.
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The dynamic library has to be added to linking options.
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It means that if a project that uses ZSTD consists of a single `test-dll.c`
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file it should be linked with `dll\libzstd.dll`. For example:
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```
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gcc $(CFLAGS) -Iinclude/ test-dll.c -o test-dll dll\libzstd.dll
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```
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The compiled executable will require ZSTD DLL which is available at `dll\libzstd.dll`.
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#### Deprecated API
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Obsolete API on their way out are stored in directory `lib/deprecated`.
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At this stage, it contains older streaming prototypes, in `lib/deprecated/zbuff.h`.
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These prototypes will be removed in some future version.
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Consider migrating code towards supported streaming API exposed in `zstd.h`.
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#### Miscellaneous
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The other files are not source code. There are :
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- `BUCK` : support for `buck` build system (https://buckbuild.com/)
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- `Makefile` : `make` script to build and install zstd library (static and dynamic)
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- `README.md` : this file
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- `dll/` : resources directory for Windows compilation
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- `libzstd.pc.in` : script for `pkg-config` (used in `make install`)
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