+/* Return true iff the buffer is all zero bytes.
+ *
+ * Note that gcc is smart enough to optimize this properly:
+ * http://stackoverflow.com/questions/1493936/faster-means-of-checking-for-an-empty-buffer-in-c/1493989#1493989
+ */
+static int
+is_zero (const char *buffer, size_t size)
+{
+ size_t i;
+
+ for (i = 0; i < size; ++i) {
+ if (buffer[i] != 0)
+ return 0;
+ }
+
+ return 1;
+}
+
+static const char zero_buf[4096];
+
+int
+do_zero (const char *device)
+{
+ char buf[sizeof zero_buf];
+ int fd;
+ size_t i, offset;
+
+ fd = open (device, O_RDWR);
+ if (fd == -1) {
+ reply_with_perror ("%s", device);
+ return -1;
+ }
+
+ for (i = 0; i < 32; ++i) {
+ offset = i * sizeof zero_buf;
+
+ /* Check if the block is already zero before overwriting it. */
+ if (pread (fd, buf, sizeof buf, offset) != sizeof buf) {
+ reply_with_perror ("pread: %s", device);
+ close (fd);
+ return -1;
+ }
+
+ if (!is_zero (buf, sizeof buf)) {
+ if (pwrite (fd, zero_buf, sizeof zero_buf, offset) != sizeof zero_buf) {
+ reply_with_perror ("pwrite: %s", device);
+ close (fd);
+ return -1;
+ }
+ }
+
+ notify_progress ((uint64_t) i, 32);
+ }
+
+ if (close (fd) == -1) {
+ reply_with_perror ("close: %s", device);
+ return -1;
+ }
+
+ return 0;
+}
+