Zstandard wrapper for zlib

The main objective of creating a zstd wrapper for zlib is to allow a quick and smooth transition to zstd for projects already using zlib.

Required files

To build the zstd wrapper for zlib the following files are required:

zlib.h
a static or dynamic zlib library
zlibWrapper/zstd_zlibwrapper.h
zlibWrapper/zstd_zlibwrapper.c
a static or dynamic zstd library

The first two files are required by all projects using zlib and they are not included with the zstd distribution. The further files are supplied with the zstd distribution.

Embedding the zstd wrapper within your project

Let's assume that your project that uses zlib is compiled with: gcc project.o -lz

To compile the zstd wrapper with your project you have to do the following:

change all references with #include "zlib.h" to #include "zstd_zlibwrapper.h"
compile your project with zlib_wrapper.c and a static or dynamic zstd library

The linking should be changed to: gcc project.o zlib_wrapper.o -lz -lzstd

Enabling zstd compression within your project

After embedding the zstd wrapper within your project the zstd library is turned off by default. Your project should work as before with zlib. There are two options to enable zstd compression:

compilation with -DZWRAP_USE_ZSTD=1 (or using #define ZWRAP_USE_ZSTD 1 before #include "zstd_zlibwrapper.h")
using the void useZSTD(int turn_on) function (declared in #include "zstd_zlibwrapper.h") There is no switch for zstd decompression because zlib and zstd streams are automatically detected and decompressed using a proper library.

Example

We have take the file test/example.c from the zlib library distribution and copied it to zlibWrapper/examples/example.c. After compilation and execution it shows the following results:

zlib version 1.2.8 = 0x1280, compile flags = 0x65
uncompress(): hello, hello!
gzread(): hello, hello!
gzgets() after gzseek:  hello!
inflate(): hello, hello!
large_inflate(): OK
after inflateSync(): hello, hello!
inflate with dictionary: hello, hello!

Then we have changed #include "zlib.h" to #include "zstd_zlibwrapper.h", compiled the example.c file with -DZWRAP_USE_ZSTD=1 and linked with additional zlib_wrapper.o -lzstd. We were forced to turn off the following functions: test_gzio, test_flush, test_sync which use currently unsupported features. After running it shows the following results:

zlib version 1.2.8 = 0x1280, compile flags = 0x65
uncompress(): hello, hello!
inflate(): hello, hello!
large_inflate(): OK
inflate with dictionary: hello, hello!

The script used for compilation can be found at zlibWrapper/Makefile.

Compatibility issues

After enabling zstd compression not all native zlib functions are supported. When calling unsupported methods they print error message and return an error value.

Supported methods:

deflateInit
deflate (with exception of Z_FULL_FLUSH)
deflateSetDictionary
deflateEnd
deflateReset
deflateBound
inflateInit
inflate
inflateSetDictionary
inflateReset
compress
compress2
compressBound
uncompress

Ignored methods (they do nothing):

deflateParams

Unsupported methods:

gzip file access functions
deflateCopy
deflateTune
deflatePending
deflatePrime
deflateSetHeader
inflateGetDictionary
inflateCopy
inflateSync
inflateReset2
inflatePrime
inflateMark
inflateGetHeader
inflateBackInit
inflateBack
inflateBackEnd

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