The __callback type qualifier

The keyword __callback is a qualifier that can be applied only to a function pointer type. The qualifier instructs the compiler to generate extra code in the call sites to assist the call, and thus allows the function pointer to point to either XPLINK or non-XPLINK functions. Under normal circumstances, a non-XPLINK function pointer is incompatible with XPLINK compilation units.

The keyword can appear in the declarator part of a function pointer declaration, wherever a cv-qualifier can appear. For example,

      int (*__callback foo)(int);

declares foo to be a function pointer that might point to non-XPLINK functions. foo will then have fewer restrictions on what it can reference and can thus be used with XPLINK compilation units.

XPLINK and non-XPLINK compilation units cannot be statically bound; the two linkages can be mixed only across DLL boundaries. Moreover, a function pointer that points to a non-XPLINK function cannot be used in XPLINK DLLs unless the pointer is passed across the boundary explicitly as a function argument. The __callback qualifier relaxes the latter restriction, at the expense of extra code sequences in the call site.

Semantically, the __callback keyword is a language extension that has a single effect: to instruct the compiler to generate assistance code. It does not take part in type definition. The keyword also has no effect on the following:

Type (such as in overload resolution).
Name mangling.
Allocation of the pointer object in memory.

It is the responsibility of the programmer to make sure that the function pointer is appropriately __callback-qualified for all call sites that require it.