readInt64
Reads a 64-bit/8-byte value from @stream.
In order to get the correct byte order for this read operation, see g_data_input_stream_get_byte_order() and g_data_input_stream_set_byte_order().
If @cancellable is not null, then the operation can be cancelled by triggering the cancellable object from another thread. If the operation was cancelled, the error %G_IO_ERROR_CANCELLED will be returned.
Return
a signed 64-bit/8-byte value read from @stream or 0 if an error occurred.
Parameters
cancellable
optional #GCancellable object, null to ignore.