Sets the length of the stream to offset. If the stream was previously larger than offset, the extra data is discarded. If the stream was previouly shorter than offset, it is extended with NUL ('\0') bytes.
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. If an operation was partially finished when the operation was cancelled the partial result will be returned, without an error.