fn malloc_usable_size(opt_old_base: ?[*]align(alignment_bytes) u8) callconv(.c) usize
fn malloc_usable_size(opt_old_base: ?[*]align(alignment_bytes) u8) callconv(.c) usize { const old_base = opt_old_base orelse return 0; const old_header: Header = .get(old_base); const old_size = old_header.size; return old_size; }