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+/*
+    Copyright (C) 2010 Paul Davis
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation; either version 2.1 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+
+*/
+
+#ifndef __weakjack_h__
+#define __weakjack_h__
+
+/**
+ * @defgroup WeakLinkage 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 absence 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 it's not null, then Jill can use it
+ *    just like any other function in the API. For example:
+ *
+ * \code
+ * if (jack_set_latency_callback) {
+ *       jack_set_latency_callback (jill_client, jill_latency_callback, arg);
+ * }
+ * \endcode
+ *
+ *    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 \b 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.
+ *
+ *
+ */
+
+#ifdef __APPLE__
+#define WEAK_ATTRIBUTE weak_import
+#else
+#define WEAK_ATTRIBUTE __weak__
+#endif
+
+#ifndef JACK_OPTIONAL_WEAK_EXPORT
+/* JACK_OPTIONAL_WEAK_EXPORT needs to be a macro which
+   expands into a compiler directive. If non-null, the directive
+   must tell the compiler to arrange for weak linkage of
+   the symbol it used with. For this to work fully may
+   require linker arguments for the client as well.
+*/
+#ifdef __GNUC__
+#define JACK_OPTIONAL_WEAK_EXPORT __attribute__((WEAK_ATTRIBUTE))
+#else
+/* Add other things here for non-gcc platforms */
+#endif
+#endif
+
+#ifndef JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT
+/* JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT needs to be a macro
+   which expands into a compiler directive. If non-null, the directive
+   must tell the compiler to arrange for weak linkage of the
+   symbol it is used with AND optionally to mark the symbol
+   as deprecated. For this to work fully may require
+   linker arguments for the client as well.
+*/
+#ifdef __GNUC__
+#define JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT __attribute__((WEAK_ATTRIBUTE,__deprecated__))
+#else
+/* Add other things here for non-gcc platforms */
+#endif
+#endif
+
+/*@}*/
+
+#endif /* weakjack */