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diff --git a/mfbt/lz4/README.md b/mfbt/lz4/README.md new file mode 100644 index 0000000000..08d1cef2bf --- /dev/null +++ b/mfbt/lz4/README.md @@ -0,0 +1,169 @@ +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 + +[official interoperable frame format]: ../doc/lz4_Frame_format.md +[LZ4 Frame format]: ../doc/lz4_Frame_format.md +[LZ4 block format]: ../doc/lz4_Block_format.md + + +#### License + +All source material within __lib__ directory are BSD 2-Clause licensed. +See [LICENSE](LICENSE) for details. +The license is also reminded at the top of each source file. |