Weak-JACK

This small library abstracts the JACK Application Binary Interface.

Background and Motivation

The jack shared library needs to be installed system-wide (for all jack applications to share), it can not be part of an application itself.

JACK developers take great care to not break binary compatibility of libjack. An application compiled with one version of jack will work with all future versions of jack. However, this only works well on GNU/Linux, BSD and to some extend on OSX.

weak-jack linking is useful (at least) in the following cases:

the resulting application should not be directly linked to libjack.[so|dll|dylib] IOW: the application should start even if libjack is not installed.
the ABI of libjack is not stable. Note, this is only relevant for the Windows .dll (e.g. applications compiled and linked with jack-1.9.9 will crash with jack-1.9.10 on windows. -- MSVC has a workaround: link by function-name, not ordinal, mingw does not offer this)
Reference new API functions (e.g. meta-data API or new latency compensation) in the application, which is not available on older versions of jack.

Usage

Copy the source files into your application's source (or use a git submodule)
replace all #include<jack/*> in your sources with #include "weak-jack.h"
add weak_libjack.c to the build-source of your project (in case your build-system does not detect #include dependencies automatically, also reference the header and .def file).
Define USE_WEAK_JACK for all platforms where you want to use weak-linking. Usually CFLAGS+=-DUSE_WEAK_JACK CXXFLAGS+=-DUSE_WEAK_JACK
Do not link your application to libjack (-ljack) when USE_WEAK_JACK is defined.

Note the jack-headers still need to be present when compiling the application.

The application code itself does not need to be changed.

The first call to jack_client_open() will try to find and load libjack, if it cannot be found, it will fail (return NULL and set jack_status_t if provided to JackFailure.)

It is possible to explicitly initialize and query availability of libjack using have_libjack(); it returns 0 if libjack is available and can be used. (see the header file for non-zero error codes).

Caveats

If libjack is not available, all jack_* API calls are turned into no-operation functions. This is not a problem in general, as jack-applications will not use any part of the jack API if jack_client_open fails. The only exception here may be jack_ringbuffer. Note that the ringbuffer implementation is also part of libjack and will not be available.

The dummy implementation for the ringbuffer API is safe (read, writes are ignored and return failure or zero-bytes length), but if your application depends on it to work, you're out of luck :)

The function wrappers in weak_libjack.def were collected pragmatically it's quite possible that some JACK API calls have been missed. If you application fails to link (without -ljack), please report at https://github.com/x42/weakjack/issues

License

GNU General Public License version 2 (or later).

Alternatives

An alternative, more liberally licensed, implementation that abstracts and wraps jack completely (incl headers) can be found at https://github.com/falkTX/Carla/tree/master/source/jackbridge (C++ only), and a jack2 specific version at https://github.com/sletz/jack2/blob/master/common/JackWeakAPI.c

A variant for python bindings is also provided by falkTX: https://github.com/falkTX/Cadence/blob/master/src/jacklib.py

kmatheussen / weakjack