managing support for newer/older versions of JACK

One challenge faced by developers is that of taking advantage of new features introduced in new versions of [ JACK ] while still supporting older versions of the system. Normally, if an application uses a new feature in a library/API, it is unable to run on earlier versions of the library/API that do not support that feature. Such applications would either fail to launch or crash when an attempt to use the feature was made. This problem cane be solved using weakly-linked symbols.

When a symbol in a framework is defined as weakly linked, the symbol does not have to be present at runtime for a process to continue running. The static linker identifies a weakly linked symbol as such in any code module that references the symbol. The dynamic linker uses this same information at runtime to determine whether a process can continue running. If a weakly linked symbol is not present in the framework, the code module can continue to run as long as it does not reference the symbol. However, if the symbol is present, the code can use it normally.

    (adapted from: http://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html)

A concrete example will help. Suppose that someone uses a version of a JACK client we'll call "Jill". Jill was linked against a version of JACK that contains a newer part of the API (say, jack_set_latency_callback()) and would like to use it if it is available.

When Jill is run on a system that has a suitably "new" version of JACK, this function will be available entirely normally. But if Jill is run on a system with an old version of JACK, the function isn't available.

With normal symbol linkage, this would create a startup error whenever someone tries to run Jill with the "old" version of JACK. However, functions added to JACK after version 0.116.2 are all declared to have "weak" linkage which means that their abscence doesn't cause an error during program startup. Instead, Jill can test whether or not the symbol jack_set_latency_callback is null or not. If its null, it means that the JACK installed on this machine is too old to support this function. If its not null, then Jill can use it just like any other function in the API. For example:

if (jack_set_latency_callback) {
jack_set_latency_callback (jill_client, jill_latency_callback, arg);
}

However, there are clients that may want to use this approach to parts of the the JACK API that predate 0.116.2. For example, they might want to see if even really old basic parts of the API like jack_client_open() exist at runtime.

Such clients should include <jack/weakjack.h> before any other JACK header. This will make the entire JACK API be subject to weak linkage, so that any and all functions can be checked for existence at runtime. It is important to understand that very few clients need to do this - if you use this feature you should have a clear reason to do so.