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zstd/lib/README.md
Yann Collet ce6cd07c33 updated build documentation
2020-10-22 12:31:23 -07:00

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Zstandard library files

The lib directory is split into several sub-directories, in order to make it easier to select or exclude features.

Building

Makefile script is provided, supporting Makefile conventions, including commands variables, staged install, directory variables and standard targets.

  • make : generates both static and dynamic libraries
  • make install : install libraries and headers in target system directories

libzstd default scope is pretty large, including compression, decompression, dictionary builder, and support for decoding legacy formats >= v0.5.0. The scope can be reduced on demand (see paragraph modular build).

Multithreading support

Multithreading is disabled by default when building with make. Enabling multithreading requires 2 conditions :

  • set build macro ZSTD_MULTITHREAD (-DZSTD_MULTITHREAD for gcc)
  • for POSIX systems : compile with pthread (-pthread compilation flag for gcc)

Both conditions are automatically applied when invoking make lib-mt target.

When linking a POSIX program with a multithreaded version of libzstd, note that it's necessary to invoke the -pthread flag during link stage.

Multithreading capabilities are exposed via the advanced API defined in lib/zstd.h.

API

Zstandard's stable API is exposed within lib/zstd.h.

Advanced API

Optional advanced features are exposed via :

  • lib/common/zstd_errors.h : translates size_t function results into a ZSTD_ErrorCode, for accurate error handling.

  • ZSTD_STATIC_LINKING_ONLY : if this macro is defined before including zstd.h, it unlocks access to the experimental API, exposed in the second part of zstd.h. All definitions in the experimental APIs are unstable, they may still change in the future, or even be removed. As a consequence, experimental definitions shall never be used with dynamic library ! Only static linking is allowed.

Modular build

It's possible to compile only a limited set of features within libzstd. The file structure is designed to make this selection manually achievable for any build system :

  • Directory lib/common is always required, for all variants.

  • Compression source code lies in lib/compress

  • Decompression source code lies in lib/decompress

  • It's possible to include only compress or only decompress, they don't depend on each other.

  • lib/dictBuilder : makes it possible to generate dictionaries from a set of samples. The API is exposed in lib/dictBuilder/zdict.h. This module depends on both lib/common and lib/compress .

  • lib/legacy : makes it possible to decompress legacy zstd formats, starting from v0.1.0. This module depends on lib/common and lib/decompress. To enable this feature, define ZSTD_LEGACY_SUPPORT during compilation. Specifying a number limits versions supported to that version onward. For example, ZSTD_LEGACY_SUPPORT=2 means : "support legacy formats >= v0.2.0". Conversely, ZSTD_LEGACY_SUPPORT=0 means "do not support legacy formats". By default, this build macro is set as ZSTD_LEGACY_SUPPORT=5. Decoding supported legacy format is a transparent capability triggered within decompression functions. It's also allowed to invoke legacy API directly, exposed in lib/legacy/zstd_legacy.h. Each version does also provide its own set of advanced API. For example, advanced API for version v0.4 is exposed in lib/legacy/zstd_v04.h .

  • While invoking make libzstd, it's possible to define build macros ZSTD_LIB_COMPRESSION, ZSTD_LIB_DECOMPRESSION, ZSTD_LIB_DICTBUILDER, and ZSTD_LIB_DEPRECATED as 0 to forgo compilation of the corresponding features. This will also disable compilation of all dependencies (eg. ZSTD_LIB_COMPRESSION=0 will also disable dictBuilder).

  • There are a number of options that can help minimize the binary size of libzstd.

    The first step is to select the components needed (using the above-described ZSTD_LIB_COMPRESSION etc.).

    The next step is to set ZSTD_LIB_MINIFY to 1 when invoking make. This disables various optional components and changes the compilation flags to prioritize space-saving.

    Detailed options: Zstandard's code and build environment is set up by default to optimize above all else for performance. In pursuit of this goal, Zstandard makes significant trade-offs in code size. For example, Zstandard often has more than one implementation of a particular component, with each implementation optimized for different scenarios. For example, the Huffman decoder has complementary implementations that decode the stream one symbol at a time or two symbols at a time. Zstd normally includes both (and dispatches between them at runtime), but by defining HUF_FORCE_DECOMPRESS_X1 or HUF_FORCE_DECOMPRESS_X2, you can force the use of one or the other, avoiding compilation of the other. Similarly, ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT and ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG force the compilation and use of only one or the other of two decompression implementations. The smallest binary is achieved by using HUF_FORCE_DECOMPRESS_X1 and ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT (implied by ZSTD_LIB_MINIFY).

    For squeezing the last ounce of size out, you can also define ZSTD_NO_INLINE, which disables inlining, and ZSTD_STRIP_ERROR_STRINGS, which removes the error messages that are otherwise returned by ZSTD_getErrorName (implied by ZSTD_LIB_MINIFY).

    Finally, when integrating into your application, make sure you're doing link- time optimation and unused symbol garbage collection (via some combination of, e.g., -flto, -ffat-lto-objects, -fuse-linker-plugin, -ffunction-sections, -fdata-sections, -fmerge-all-constants, -Wl,--gc-sections, -Wl,-z,norelro, and an archiver that understands the compiler's intermediate representation, e.g., AR=gcc-ar). Consult your compiler's documentation.

  • While invoking make libzstd, the build macro ZSTD_LEGACY_MULTITHREADED_API=1 will expose the deprecated ZSTDMT API exposed by zstdmt_compress.h in the shared library, which is now hidden by default.

  • The build macro DYNAMIC_BMI2 can be set to 1 or 0 in order to generate binaries which can detect at runtime the presence of BMI2 instructions, and use them only if present. These instructions contribute to better performance, notably on the decoder side. By default, this feature is automatically enabled on detecting the right instruction set (x64) and compiler (clang or gcc >= 5). It's obviously disabled for different cpus, or when BMI2 instruction set is required by the compiler command line (in this case, only the BMI2 code path is generated). Setting this macro will either force to generate the BMI2 dispatcher (1) or prevent it (0). It overrides automatic detection.

  • The build macro ZSTD_NO_UNUSED_FUNCTIONS can be defined to hide the definitions of functions that zstd does not use. Not all unused functions are hidden, but they can be if needed. Currently, this macro will hide function definitions in FSE and HUF that use an excessive amount of stack space.

  • The build macro ZSTD_NO_INTRINSICS can be defined to disable all explicit intrinsics. Compiler builtins are still used.

Windows : using MinGW+MSYS to create DLL

DLL can be created using MinGW+MSYS with the make libzstd command. This command creates dll\libzstd.dll and the import library dll\libzstd.lib. The import library is only required with Visual C++. The header file zstd.h and the dynamic library dll\libzstd.dll are required to compile a project using gcc/MinGW. The dynamic library has to be added to linking options. It means that if a project that uses ZSTD consists of a single test-dll.c file it should be linked with dll\libzstd.dll. For example:

    gcc $(CFLAGS) -Iinclude/ test-dll.c -o test-dll dll\libzstd.dll

The compiled executable will require ZSTD DLL which is available at dll\libzstd.dll.

Advanced Build options

The build system requires a hash function in order to separate object files created with different compilation flags. By default, it tries to use md5sum or equivalent. The hash function can be manually switched by setting the HASH variable. For example : make HASH=xxhsum The hash function needs to generate at least 64-bit using hexadecimal format. When no hash function is found, the Makefile just generates all object files into the same default directory, irrespective of compilation flags. This functionality only matters if libzstd is compiled multiple times with different build flags.

The build directory, where object files are stored can also be manually controlled using variable BUILD_DIR, for example make BUILD_DIR=objectDir/v1. In which case, the hash function doesn't matter.

Deprecated API

Obsolete API on their way out are stored in directory lib/deprecated. At this stage, it contains older streaming prototypes, in lib/deprecated/zbuff.h. These prototypes will be removed in some future version. Consider migrating code towards supported streaming API exposed in zstd.h.

Miscellaneous

The other files are not source code. There are :

  • BUCK : support for buck build system (https://buckbuild.com/)
  • Makefile : make script to build and install zstd library (static and dynamic)
  • README.md : this file
  • dll/ : resources directory for Windows compilation
  • libzstd.pc.in : script for pkg-config (used in make install)