208 lines
9.9 KiB
Markdown
208 lines
9.9 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
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corresponding features. This will also disable compilation of all
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dependencies (eg. `ZSTD_LIB_COMPRESSION=0` will also disable
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dictBuilder).
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- There are a number of options that can help minimize the binary size of
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`libzstd`.
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The first step is to select the components needed (using the above-described
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`ZSTD_LIB_COMPRESSION` etc.).
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The next step is to set `ZSTD_LIB_MINIFY` to `1` when invoking `make`. This
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disables various optional components and changes the compilation flags to
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prioritize space-saving.
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Detailed options: Zstandard's code and build environment is set up by default
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to optimize above all else for performance. In pursuit of this goal, Zstandard
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makes significant trade-offs in code size. For example, Zstandard often has
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more than one implementation of a particular component, with each
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implementation optimized for different scenarios. For example, the Huffman
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decoder has complementary implementations that decode the stream one symbol at
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a time or two symbols at a time. Zstd normally includes both (and dispatches
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between them at runtime), but by defining `HUF_FORCE_DECOMPRESS_X1` or
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`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` (implied by `ZSTD_LIB_MINIFY`).
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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` (implied by `ZSTD_LIB_MINIFY`).
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Finally, when integrating into your application, make sure you're doing link-
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time optimation and unused symbol garbage collection (via some combination of,
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e.g., `-flto`, `-ffat-lto-objects`, `-fuse-linker-plugin`,
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`-ffunction-sections`, `-fdata-sections`, `-fmerge-all-constants`,
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`-Wl,--gc-sections`, `-Wl,-z,norelro`, and an archiver that understands
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the compiler's intermediate representation, e.g., `AR=gcc-ar`). Consult your
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compiler's documentation.
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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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- The build macro `ZSTD_NO_UNUSED_FUNCTIONS` can be defined to hide the definitions of functions
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that zstd does not use. Not all unused functions are hidden, but they can be if needed.
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Currently, this macro will hide function definitions in FSE and HUF that use an excessive
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amount of stack space.
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- The build macro `ZSTD_NO_INTRINSICS` can be defined to disable all explicit intrinsics.
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Compiler builtins are still used.
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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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#### Advanced Build options
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The build system requires a hash function in order to
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separate object files created with different compilation flags.
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By default, it tries to use `md5sum` or equivalent.
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The hash function can be manually switched by setting the `HASH` variable.
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For example : `make HASH=xxhsum`
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The hash function needs to generate at least 64-bit using hexadecimal format.
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When no hash function is found,
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the Makefile just generates all object files into the same default directory,
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irrespective of compilation flags.
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This functionality only matters if `libzstd` is compiled multiple times
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with different build flags.
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The build directory, where object files are stored
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can also be manually controlled using variable `BUILD_DIR`,
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for example `make BUILD_DIR=objectDir/v1`.
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In which case, the hash function doesn't matter.
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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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