[asterisk-doc] How to write new application for Asterisk by using C

[Randy Resnick]

     When I write my app, how do I link it into Asterisk?
>      Why is attempting to answer the channel important in my app?
>      For an alias like datetime, why does load and unload use res |= instead
>        of res = ?  (OK, this may be my lack of C knowledge -- is it just
>        concat?)
>      Once I write it, what's the testing procedure?
>      Is there anything else to do other than write app_myapp.c?
>      What other #include files might be useful in writing apps?
>      When I test, I get (this common error).  How to fix?

And the all-important "how do I wrote those oh-so helpful 'show
application' texts