rust-rocksdb/lib

LZ4 - Library Files

The /lib directory contains many files, but depending on project's objectives, not all of them are required. Limited systems may want to reduce the nb of source files to include as a way to reduce binary size and dependencies.

Capabilities are added at the "level" granularity, detailed below.

Level 1 : Minimal LZ4 build

The minimum required is lz4.c and lz4.h, which provides the fast compression and decompression algorithms. They generate and decode data using the LZ4 block format.

Level 2 : High Compression variant

For more compression ratio at the cost of compression speed, the High Compression variant called lz4hc is available. Add files lz4hc.c and lz4hc.h. This variant also compresses data using the LZ4 block format, and depends on regular lib/lz4.* source files.

Level 3 : Frame support, for interoperability

In order to produce compressed data compatible with lz4 command line utility, it's necessary to use the official interoperable frame format. This format is generated and decoded automatically by the lz4frame library. Its public API is described in lib/lz4frame.h. In order to work properly, lz4frame needs all other modules present in /lib, including, lz4 and lz4hc, and also xxhash. So it's necessary to also include xxhash.c and xxhash.h.

Level 4 : File compression operations

As a helper around file operations, the library has been recently extended with lz4file.c and lz4file.h (still considered experimental at the time of this writing). These helpers allow opening, reading, writing, and closing files using transparent LZ4 compression / decompression. As a consequence, using lz4file adds a dependency on <stdio.h>.

lz4file relies on lz4frame in order to produce compressed data conformant to the LZ4 Frame format specification. Consequently, to enable this capability, it's necessary to include all *.c and *.h files from lib/ directory.

Advanced / Experimental API

Definitions which are not guaranteed to remain stable in future versions, are protected behind macros, such as LZ4_STATIC_LINKING_ONLY. As the name suggests, these definitions should only be invoked in the context of static linking only. Otherwise, dependent application may fail on API or ABI break in the future. The associated symbols are also not exposed by the dynamic library by default. Should they be nonetheless needed, it's possible to force their publication by using build macros LZ4_PUBLISH_STATIC_FUNCTIONS and LZ4F_PUBLISH_STATIC_FUNCTIONS.

Build macros

The following build macro can be selected to adjust source code behavior at compilation time :

• LZ4_FAST_DEC_LOOP : this triggers a speed optimized decompression loop, more powerful on modern cpus. This loop works great on x86, x64 and aarch64 cpus, and is automatically enabled for them. It's also possible to enable or disable it manually, by passing LZ4_FAST_DEC_LOOP=1 or 0 to the preprocessor. For example, with gcc : -DLZ4_FAST_DEC_LOOP=1, and with make : CPPFLAGS+=-DLZ4_FAST_DEC_LOOP=1 make lz4.

• LZ4_DISTANCE_MAX : control the maximum offset that the compressor will allow. Set to 65535 by default, which is the maximum value supported by lz4 format. Reducing maximum distance will reduce opportunities for LZ4 to find matches, hence will produce a worse compression ratio. Setting a smaller max distance could allow compatibility with specific decoders with limited memory budget. This build macro only influences the compressed output of the compressor.

• LZ4_DISABLE_DEPRECATE_WARNINGS : invoking a deprecated function will make the compiler generate a warning. This is meant to invite users to update their source code. Should this be a problem, it's generally possible to make the compiler ignore these warnings, for example with -Wno-deprecated-declarations on gcc, or _CRT_SECURE_NO_WARNINGS for Visual Studio. This build macro offers another project-specific method by defining LZ4_DISABLE_DEPRECATE_WARNINGS before including the LZ4 header files.

• LZ4_FORCE_SW_BITCOUNT : by default, the compression algorithm tries to determine lengths by using bitcount instructions, generally implemented as fast single instructions in many cpus. In case the target cpus doesn't support it, or compiler intrinsic doesn't work, or feature bad performance, it's possible to use an optimized software path instead. This is achieved by setting this build macros. In most cases, it's not expected to be necessary, but it can be legitimately considered for less common platforms.

• LZ4_ALIGN_TEST : alignment test ensures that the memory area passed as argument to become a compression state is suitably aligned. This test can be disabled if it proves flaky, by setting this value to 0.

• LZ4_USER_MEMORY_FUNCTIONS : replace calls to <stdlib,h>'s malloc(), calloc() and free() by user-defined functions, which must be named LZ4_malloc(), LZ4_calloc() and LZ4_free(). User functions must be available at link time.

• LZ4_STATIC_LINKING_ONLY_DISABLE_MEMORY_ALLOCATION : Remove support of dynamic memory allocation. For more details, see description of this macro in lib/lz4.c.

• LZ4_FREESTANDING : by setting this build macro to 1, LZ4/HC removes dependencies on the C standard library, including allocation functions and memmove(), memcpy(), and memset(). This build macro is designed to help use LZ4/HC in restricted environments (embedded, bootloader, etc). For more details, see description of this macro in lib/lz4.h.

Amalgamation

lz4 source code can be amalgamated into a single file. One can combine all source code into lz4_all.c by using following command:

cat lz4.c lz4hc.c lz4frame.c > lz4_all.c

(cat file order is important) then compile lz4_all.c. All *.h files present in /lib remain necessary to compile lz4_all.c.

Windows : using MinGW+MSYS to create DLL

DLL can be created using MinGW+MSYS with the make liblz4 command. This command creates dll\liblz4.dll and the import library dll\liblz4.lib. To override the dlltool command when cross-compiling on Linux, just set the DLLTOOL variable. Example of cross compilation on Linux with mingw-w64 64 bits:

make BUILD_STATIC=no CC=x86_64-w64-mingw32-gcc DLLTOOL=x86_64-w64-mingw32-dlltool OS=Windows_NT

The import library is only required with Visual C++. The header files lz4.h, lz4hc.h, lz4frame.h and the dynamic library dll\liblz4.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 LZ4 consists of a single test-dll.c file it should be linked with dll\liblz4.dll. For example:

    $(CC) $(CFLAGS) -Iinclude/ test-dll.c -o test-dll dll\liblz4.dll

The compiled executable will require LZ4 DLL which is available at dll\liblz4.dll.

Miscellaneous

Other files present in the directory are not source code. They are :

• LICENSE : contains the BSD license text
• Makefile : make script to compile and install lz4 library (static and dynamic)
• liblz4.pc.in : for pkg-config (used in make install)
• README.md : this file

License

All source material within lib directory are BSD 2-Clause licensed. See LICENSE for details. The license is also reminded at the top of each source file.