Back out disabling ungetc() at 0, use different solution:

keep negative offset internally, but return 0 externally in ftell*()
I.e. use 0 now as 'unspecified value' per POSIX ungetc() description.

lib/libc/stdio/fseek.c

@@ -129,9 +129,10 @@ _fseeko(fp, offset, whence, ltest)
 		 * we have to first find the current stream offset via
 		 * ftell (see ftell for details).
 		 */
-		if ((curoff = _ftello(fp)) == -1)
+		if (_ftello(fp, &curoff))
 			return (-1);
-		if (offset > 0 && curoff > OFF_MAX - offset) {
+		if ((offset > 0 && curoff > OFF_MAX - offset) ||
+		    (offset < 0 && curoff < OFF_MIN - offset)) {
 			errno = EOVERFLOW;
 			return (-1);
 		}
@@ -210,9 +211,16 @@ _fseeko(fp, offset, whence, ltest)
 		}
 	}
 
-	if (!havepos && (curoff = _ftello(fp)) == -1)
+	if (!havepos && _ftello(fp, &curoff))
 		goto dumb;
 
+	/*
+	 * (If the buffer was modified, we have to
+	 * skip this; see fgetln.c.)
+	 */
+	if (fp->_flags & __SMOD)
+		goto abspos;
+
 	/*
 	 * Compute the number of bytes in the input buffer (pretending
 	 * that any ungetc() input has been discarded).  Adjust current
@@ -220,28 +228,29 @@ _fseeko(fp, offset, whence, ltest)
 	 * file offset for the first byte in the current input buffer.
 	 */
 	if (HASUB(fp)) {
-		if (curoff > OFF_MAX - fp->_r)
+		if (curoff > 0 && fp->_r > OFF_MAX - curoff)
 			goto abspos;
 		curoff += fp->_r;	/* kill off ungetc */
 		n = fp->_extra->_up - fp->_bf._base;
+		if (curoff < 0 && -((off_t)n) < OFF_MIN - curoff)
+			goto abspos;
 		curoff -= n;
 		n += fp->_ur;
 	} else {
 		n = fp->_p - fp->_bf._base;
+		if (curoff < 0 && -((off_t)n) < OFF_MIN - curoff)
+			goto abspos;
 		curoff -= n;
 		n += fp->_r;
 	}
-	/* curoff can be negative at this point. */
 
 	/*
 	 * If the target offset is within the current buffer,
 	 * simply adjust the pointers, clear EOF, undo ungetc(),
-	 * and return.  (If the buffer was modified, we have to
-	 * skip this; see fgetln.c.)
+	 * and return.
 	 */
-	if ((fp->_flags & __SMOD) == 0 &&
-	    target >= curoff &&
-	    (curoff <= 0 || curoff <= OFF_MAX - n) &&
+	if (target >= curoff &&
+	    (curoff <= 0 || n <= OFF_MAX - curoff) &&
 	    target < curoff + n) {
 		size_t o = target - curoff;
 

lib/libc/stdio/ftell.c

@@ -75,28 +75,34 @@ off_t
 ftello(fp)
 	register FILE *fp;
 {
-	register off_t rv;
+	fpos_t rv;
+	int ret;
 
 	/* make sure stdio is set up */
 	if (!__sdidinit)
 		__sinit();
 
 	FLOCKFILE(fp);
-	rv = _ftello(fp);
+	ret = _ftello(fp, &rv);
 	FUNLOCKFILE(fp);
+	if (ret)
+		return (-1);
+	if (rv < 0)     /* Unspecified value because of ungetc() at 0 */
+		rv = 0;
 	return (rv);
 }
 
-off_t
-_ftello(fp)
+int
+_ftello(fp, offset)
 	register FILE *fp;
+	fpos_t *offset;
 {
 	register fpos_t pos, spos;
 	size_t n;
 
 	if (fp->_seek == NULL) {
 		errno = ESPIPE;			/* historic practice */
-		return (-1);
+		return (1);
 	}
 
 	/*
@@ -110,7 +116,7 @@ _ftello(fp)
 get_real_pos:
 		spos = pos = (*fp->_seek)(fp->_cookie, (fpos_t)0, SEEK_CUR);
 		if (pos == -1)
-			return (-1);
+			return (1);
 	}
 	if (fp->_flags & __SRD) {
 		/*
@@ -118,16 +124,21 @@ _ftello(fp)
 		 * those from ungetc) cause the position to be
 		 * smaller than that in the underlying object.
 		 */
-		if ((pos -= fp->_r) < 0 ||
-		    (HASUB(fp) && (pos -= fp->_ur) < 0)) {
-			fp->_p = fp->_bf._base;
-			fp->_r = 0;
-			if (HASUB(fp))
-				FREEUB(fp);
+		if ((pos -= (HASUB(fp) ? fp->_ur : fp->_r)) < 0) {
+			/* Lost position, resync. */
+			if (HASUB(fp)) {
+				fp->_extra->_up = fp->_bf._base;
+				fp->_ur = 0;
+			} else {
+				fp->_p = fp->_bf._base;
+				fp->_r = 0;
+			}
 			if (spos == -1)
 				goto get_real_pos;
 			pos = spos;
 		}
+		if (HASUB(fp))
+			pos -= fp->_r;  /* Can be negative at this point. */
 	} else if ((fp->_flags & __SWR) && fp->_p != NULL) {
 		/*
 		 * Writing.  Any buffered characters cause the
@@ -137,9 +148,10 @@ _ftello(fp)
 		n = fp->_p - fp->_bf._base;
 		if (pos > OFF_MAX - n) {
 			errno = EOVERFLOW;
-			return (-1);
+			return (1);
 		}
 		pos += n;
 	}
-	return (pos);
+	*offset = pos;
+	return (0);
 }

lib/libc/stdio/local.h

@@ -46,8 +46,8 @@
  * in particular, macros and private variables.
  */
 
-extern off_t	_ftello __P((FILE *));
-extern int      _fseeko __P((FILE *, off_t, int, int));
+extern int	_ftello __P((FILE *, fpos_t *));
+extern int	_fseeko __P((FILE *, off_t, int, int));
 extern int	__fflush __P((FILE *fp));
 extern int	__sflush __P((FILE *));
 extern FILE	*__sfp __P((void));

lib/libc/stdio/ungetc.3

@@ -83,10 +83,7 @@ if the operation fails.
 If the value of the argument
 .Fa c
 character equals
-.Dv EOF
-or current
-.Fa stream
-offset equals 0,
+.Dv EOF ,
 the operation will fail and the stream will remain unchanged.
 .Sh SEE ALSO
 .Xr fseek 3 ,

lib/libc/stdio/ungetc.c

@@ -132,10 +132,6 @@ __ungetc(int c, FILE *fp)
 		}
 		fp->_flags |= __SRD;
 	}
-
-	if (!(fp->_flags & __SSTR) && _ftello(fp) == 0)
-		return (EOF);
-
 	c = (unsigned char)c;
 
 	/*