File ffcall/callback/MIGRATION artifact bde8adbdc8 part of check-in 419186e44b

Migration Guide
===============

When switching from vacall+trampoline to callback, you need to make the
following changes to your code:

       vacall+trampoline                       callback
       -----------------                       --------

                              -- Include files --

#include "vacall/vacall.h"              #include "callback/callback.h"
#include "trampoline/trampoline.h"

                          -- Variable declarations --

static void* trampvar;

                           -- Callback declaration --
                                
static void                             static void
mycallback (va_alist args)              mycallback (void* data, va_alist args)
{                                       {
  void* data = trampvar;                  ...
  ...

                              -- Initialization --

vacall_function = &mycallback;

                          -- Allocating a callback --

void* trampoline =                      void* trampoline =
  alloc_trampoline(                       alloc_callback(&mycallback,data);
    (__TR_function)&vacall,
    &trampvar,
    data);

                          -- Testing for a callback --

if                                      if
(is_trampoline(address)                 (is_callback(address)
 && (trampoline_address(address)         && (callback_address(address)
     == (__TR_function)&vacall)              == &mycallback)
 && (trampoline_variable(address)       )
     == &trampvar)
)

void* data = trampoline_data(address);  void* data = callback_data(address);

                         -- Deallocating a callback --

free_trampoline(address);               free_callback(address);

                                -- Libraries --

libvacall.a libtrampoline.a             libcallback.a