void (*func)(void *closure, void *ptr));
Defines a callback function to be used for deallocating local data storage
to be associated with signatures.
When destroying a DKIM_SIGINFO
handle, any user space allocated by a call to the callback set by
must also be deallocated. If this callback function is defined, it will be
called once for each such handle to deallocate any caller-side storage that
was previously allocated.