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You can write your own objects that you and others can use in their Pd applications. You can write them in C or (if you're smart and brave) in C++ or FORTRAN.
HOW EXTERNS ARE LOADED
Whenever you type the name of an object (into an "object" text box) that Pd doesn't yet know about, Pd looks for a relocatable object file, named, for instance, "profile.pd_irix5". Pd looks first in the directory containing the patch, then in directories in its "path." Pd will then add whatever object is defined there to its "class list," which is the set of all Pd classes you can use. If all this works, Pd then attempts again to create the object you asked for, this time perhaps successfully. There is no difference between an object defined this way and an object built into Pd.
Once you load a new object into Pd, it's there for the duration of your Pd session. If you load another Pd document which supplies a different version of some Pd object, the object won't be updated. IF you're working on a new object and decide to change it, you have to exit and re-enter Pd to get the change to take.
In the "externs" subdirectory of the documentation you can find simple examples of "externs" with their source code and test patches; there are many other on the web (see section 1.2 ).
IOhannes zmölnig has written an excellent guide to writing externs at http://pdstatic.iem.at/externals-HOWTO/ .