in perl
sub add_idle_callback($$)
{
    printf("\nadd_idle_callback x%x , x%x  \n", @_);
    &idle_callback_run(@_); # calls 
}

##</code><code>##

  in .swg
%typemap(in) void (* cb)(void *) {
   $1 = INT2PTR($1_ltype, SvUV($input));
   printf(":: x%x to x%x\n", (unsigned long long)SvUV($input), $1);

 if(SvGMAGICAL($input)) mg_get($input);
 $1 =  DPTR2FPTR($1_ltype, SvUV($input));
 printf("::DPTR2FPTR x%x to x%x\n", SvIV($input), $1);

}

##</code><code>##

  log
...
add_idle_callback x7ffb339f8040 , x0   <==== here in perl  the pointer to function is 64bits
... 
<=========== but then swig- SvUV() or SvIV() cast it to 32 bits
:: x339f8040 to x339f8040  
::DPTR2FPTR x339f8040 to x339f8040 
....