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125 lines
5.2 KiB
C
125 lines
5.2 KiB
C
/*
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Copyright (C) 2010 Paul Davis
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
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*/
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#ifndef __weakjack_h__
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#define __weakjack_h__
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/**
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* @defgroup WeakLinkage Managing support for newer/older versions of JACK
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* @{ One challenge faced by developers is that of taking
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* advantage of new features introduced in new versions
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* of [ JACK ] while still supporting older versions of
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* the system. Normally, if an application uses a new
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* feature in a library/API, it is unable to run on
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* earlier versions of the library/API that do not
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* support that feature. Such applications would either
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* fail to launch or crash when an attempt to use the
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* feature was made. This problem cane be solved using
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* weakly-linked symbols.
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*
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* When a symbol in a framework is defined as weakly
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* linked, the symbol does not have to be present at
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* runtime for a process to continue running. The static
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* linker identifies a weakly linked symbol as such in
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* any code module that references the symbol. The
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* dynamic linker uses this same information at runtime
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* to determine whether a process can continue
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* running. If a weakly linked symbol is not present in
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* the framework, the code module can continue to run as
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* long as it does not reference the symbol. However, if
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* the symbol is present, the code can use it normally.
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*
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* (adapted from: http://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html)
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*
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* A concrete example will help. Suppose that someone uses a version
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* of a JACK client we'll call "Jill". Jill was linked against a version
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* of JACK that contains a newer part of the API (say, jack_set_latency_callback())
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* and would like to use it if it is available.
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*
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* When Jill is run on a system that has a suitably "new" version of
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* JACK, this function will be available entirely normally. But if Jill
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* is run on a system with an old version of JACK, the function isn't
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* available.
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*
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* With normal symbol linkage, this would create a startup error whenever
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* someone tries to run Jill with the "old" version of JACK. However, functions
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* added to JACK after version 0.116.2 are all declared to have "weak" linkage
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* which means that their absence doesn't cause an error during program
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* startup. Instead, Jill can test whether or not the symbol jack_set_latency_callback
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* is null or not. If its null, it means that the JACK installed on this machine
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* is too old to support this function. If it's not null, then Jill can use it
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* just like any other function in the API. For example:
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*
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* \code
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* if (jack_set_latency_callback) {
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* jack_set_latency_callback (jill_client, jill_latency_callback, arg);
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* }
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* \endcode
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*
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* However, there are clients that may want to use this approach to parts of the
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* the JACK API that predate 0.116.2. For example, they might want to see if even
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* really old basic parts of the API like jack_client_open() exist at runtime.
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*
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* Such clients should include <jack/weakjack.h> before any other JACK header.
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* This will make the \b entire JACK API be subject to weak linkage, so that any
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* and all functions can be checked for existence at runtime. It is important
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* to understand that very few clients need to do this - if you use this
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* feature you should have a clear reason to do so.
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*
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*
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*/
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#ifdef __APPLE__
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#define WEAK_ATTRIBUTE weak_import
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#else
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#define WEAK_ATTRIBUTE __weak__
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#endif
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#ifndef JACK_OPTIONAL_WEAK_EXPORT
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/* JACK_OPTIONAL_WEAK_EXPORT needs to be a macro which
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expands into a compiler directive. If non-null, the directive
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must tell the compiler to arrange for weak linkage of
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the symbol it used with. For this to work fully may
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require linker arguments for the client as well.
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*/
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#ifdef __GNUC__
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#define JACK_OPTIONAL_WEAK_EXPORT __attribute__((WEAK_ATTRIBUTE))
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#else
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/* Add other things here for non-gcc platforms */
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#endif
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#endif
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#ifndef JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT
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/* JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT needs to be a macro
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which expands into a compiler directive. If non-null, the directive
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must tell the compiler to arrange for weak linkage of the
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symbol it is used with AND optionally to mark the symbol
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as deprecated. For this to work fully may require
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linker arguments for the client as well.
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*/
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#ifdef __GNUC__
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#define JACK_OPTIONAL_WEAK_DEPRECATED_EXPORT __attribute__((WEAK_ATTRIBUTE,__deprecated__))
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#else
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/* Add other things here for non-gcc platforms */
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#endif
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#endif
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/**@}*/
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#endif /* weakjack */
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