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PORTING.md

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Porting applications to use zlib-ng

Zlib-ng can be used/compiled in two different modes, that require some consideration by the application developer.

Changes from zlib affecting native and compat modes

Zlib-ng is not as conservative with memory allocation as Zlib is.

Where Zlib's inflate will allocate a lower amount of memory depending on compression level and window size, zlib-ng will always allocate the maximum amount of memory and possibly leave parts of it unused. Zlib-ng's deflate will however allocate a lower amount of memory depending on compression level and window size.

Zlib-ng also allocates one "big" buffer instead of doing multiple smaller allocations. This is faster, can lead to better cache locality and reduces space lost to alignment padding.

At the time of writing, by default zlib-ng allocates the following amounts of memory on a 64-bit system (except on S390x that requires ~4KiB more):

  • Deflate: 350.272 Bytes
  • Inflate: 42.112 Bytes

Advantages:

  • All memory is allocated during DeflateInit or InflateInit functions, leaving the actual deflate/inflate functions free from allocations.
  • Zlib-ng can only fail from memory allocation errors during init.
  • Time spent doing memory allocation systemcalls is all done during init, allowing applications to do prepare this before doing latency-sensitive deflate/inflate later.
  • Can reduce wasted memory due to buffer alignment padding both by OS and zlib-ng.
  • Potentially improved memory locality.

Disadvantages:

  • Zlib-ng allocates a little more memory than zlib does.

zlib-compat mode

Zlib-ng can be compiled in zlib-compat mode, suitable for zlib-replacement in a single application or system-wide.

Please note that zlib-ng in zlib-compat mode tries to maintain both API and ABI compatibility with the original zlib. Any issues regarding compatibility can be reported as bugs.

In certain instances you may not be able to simply replace the zlib library/dll files and expect the application to work. The application may need to be recompiled against the zlib-ng headers and libs to ensure full compatibility.

It is also possible for the deflate output stream to differ from the original zlib due to algorithmic differences between the two libraries. Any tests or applications that depend on the exact length of the deflate stream being a certain value will need to be updated.

Advantages:

  • Easy to port to, since it only requires a recompile of the application and no changes to the application code.

Disadvantages:

  • Can conflict with a system-installed zlib, as that can often be linked in by another library you are linking into your application. This can cause crashes or incorrect output.
  • If your application is pre-allocating a memory buffer and you are providing deflate/inflate init with your own allocator that allocates from that buffer (looking at you nginx), you should be aware that zlib-ng needs to allocate more memory than stock zlib needs. The same problem exists with Intel’s and Cloudflare’s zlib forks. Doing this is not recommended since it makes it very hard to maintain compatibility over time.

Build Considerations:

  • Compile against the zlib.h provided by zlib-ng
  • Configuration header is named zconf.h
  • Static library is libz.a on Unix and macOS, or zlib.lib on Windows
  • Shared library is libz.so on Unix, libz.dylib on macOS, or zlib1.dll on Windows
  • Type z_size_t is unsigned __int64 on 64-bit Windows, and unsigned long on 32-bit Windows, Unix and macOS
  • Type z_uintmax_t is unsigned long in zlib-compat mode, and size_t with zlib-ng API

zlib-ng native mode

Zlib-ng in native mode is suitable for co-existing with the standard zlib library, allowing applications to implement support and testing separately.

The zlib-ng native has implemented some modernization and simplifications in its API, intended to make life easier for application developers.

Advantages:

  • Does not conflict with other zlib implementations, and can co-exist as a system library along with zlib.
  • In certain places zlib-ng native uses more appropriate data types, removing the need for some workarounds in the API compared to zlib.

Disadvantages:

  • Requires minor changes to applications to use the prefixed zlib-ng function calls and structs. Usually this means a small prefix zng_ has to be added.

Build Considerations:

  • Compile against zlib-ng.h
  • Configuration header is named zconf-ng.h
  • Static library is libz-ng.a on Unix and macOS, or zlib-ng.lib on Windows
  • Shared library is libz-ng.so on Unix, libz-ng.dylib on macOS, or zlib-ng2.dll on Windows
  • Type z_size_t is size_t

zlib-ng compile-time detection

To distinguish zlib-ng from other zlib implementations at compile-time check for the existence of ZLIBNG_VERSION defined in the zlib header.