+/*
+ * rename newfile onto oldfile unless
+ * they are identical
+ */
+static void cond_rename(char *newfile, char *oldfile)
+{
+ int nfd, ofd;
+ char nbuf[4096], obuf[4096];
+ int ncnt, ocnt;
+
+ nfd = open(newfile, 0);
+ if (nfd < 0)
+ return;
+ ofd = open(oldfile, 0);
+ if (ofd < 0) {
+ close(nfd);
+ rename(newfile, oldfile);
+ return;
+ }
+
+ do {
+ ncnt = read(nfd, nbuf, sizeof(nbuf));
+ if (ncnt < 0)
+ break;
+ ocnt = read(ofd, obuf, sizeof(obuf));
+ if (ocnt < 0)
+ break;
+ if (ncnt != ocnt)
+ break;
+ if (ncnt == 0) {
+ close(nfd);
+ close(ofd);
+ unlink(newfile);
+ return;
+ }
+ } while (memcmp(obuf, nbuf, ncnt) == 0);
+
+ /* some mis-match */
+ close(nfd);
+ close(ofd);
+ rename(newfile, oldfile);
+ return;
+}