rem: struct file_descriptor *f;
add: struct file_descriptor *descriptor = NULL;
context: get_fd(struct io_context *ioctx, int fd){	struct file_descriptor *f;	mutex_lock(&ioctx->io_mutex);	if (fd >= 0 && fd < ioctx->table_size && ioctx->fds[fd]) {		// valid fd		f = ioctx->fds[fd];		atomic_add(&f->ref_count, 1); 	} else {		f = NULL;	}	mutex_unlock(&ioctx->io_mutex);	return f;}