ungetc.c revision 1.11 
1/*	$OpenBSD: ungetc.c,v 1.11 2009/10/22 01:23:16 guenther Exp $ */
2/*-
3 * Copyright (c) 1990, 1993
4 *	The Regents of the University of California.  All rights reserved.
5 *
6 * This code is derived from software contributed to Berkeley by
7 * Chris Torek.
8 *
9 * Redistribution and use in source and binary forms, with or without
10 * modification, are permitted provided that the following conditions
11 * are met:
12 * 1. Redistributions of source code must retain the above copyright
13 *    notice, this list of conditions and the following disclaimer.
14 * 2. Redistributions in binary form must reproduce the above copyright
15 *    notice, this list of conditions and the following disclaimer in the
16 *    documentation and/or other materials provided with the distribution.
17 * 3. Neither the name of the University nor the names of its contributors
18 *    may be used to endorse or promote products derived from this software
19 *    without specific prior written permission.
20 *
21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
24 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
31 * SUCH DAMAGE.
32 */
33
34#include <stdio.h>
35#include <stdlib.h>
36#include <string.h>
37#include "local.h"
38
39static int __submore(FILE *);
40/*
41 * Expand the ungetc buffer `in place'.  That is, adjust fp->_p when
42 * the buffer moves, so that it points the same distance from the end,
43 * and move the bytes in the buffer around as necessary so that they
44 * are all at the end (stack-style).
45 */
46static int
47__submore(FILE *fp)
48{
49	int i;
50	unsigned char *p;
51
52	if (_UB(fp)._base == fp->_ubuf) {
53		/*
54		 * Get a new buffer (rather than expanding the old one).
55		 */
56		if ((p = malloc((size_t)BUFSIZ)) == NULL)
57			return (EOF);
58		_UB(fp)._base = p;
59		_UB(fp)._size = BUFSIZ;
60		p += BUFSIZ - sizeof(fp->_ubuf);
61		for (i = sizeof(fp->_ubuf); --i >= 0;)
62			p[i] = fp->_ubuf[i];
63		fp->_p = p;
64		return (0);
65	}
66	i = _UB(fp)._size;
67	p = realloc(_UB(fp)._base, i << 1);
68	if (p == NULL)
69		return (EOF);
70	/* no overlap (hence can use memcpy) because we doubled the size */
71	(void)memcpy((void *)(p + i), (void *)p, (size_t)i);
72	fp->_p = p + i;
73	_UB(fp)._base = p;
74	_UB(fp)._size = i << 1;
75	return (0);
76}
77
78int
79ungetc(int c, FILE *fp)
80{
81	if (c == EOF)
82		return (EOF);
83	if (!__sdidinit)
84		__sinit();
85	_SET_ORIENTATION(fp, -1);
86	if ((fp->_flags & __SRD) == 0) {
87		/*
88		 * Not already reading: no good unless reading-and-writing.
89		 * Otherwise, flush any current write stuff.
90		 */
91		if ((fp->_flags & __SRW) == 0)
92			return (EOF);
93		if (fp->_flags & __SWR) {
94			if (__sflush(fp))
95				return (EOF);
96			fp->_flags &= ~__SWR;
97			fp->_w = 0;
98			fp->_lbfsize = 0;
99		}
100		fp->_flags |= __SRD;
101	}
102	c = (unsigned char)c;
103
104	/*
105	 * If we are in the middle of ungetc'ing, just continue.
106	 * This may require expanding the current ungetc buffer.
107	 */
108	if (HASUB(fp)) {
109		if (fp->_r >= _UB(fp)._size && __submore(fp))
110			return (EOF);
111		*--fp->_p = c;
112		fp->_r++;
113		return (c);
114	}
115	fp->_flags &= ~__SEOF;
116
117	/*
118	 * If we can handle this by simply backing up, do so,
119	 * but never replace the original character.
120	 * (This makes sscanf() work when scanning `const' data.)
121	 */
122	if (fp->_bf._base != NULL && fp->_p > fp->_bf._base &&
123	    fp->_p[-1] == c) {
124		fp->_p--;
125		fp->_r++;
126		return (c);
127	}
128
129	/*
130	 * Create an ungetc buffer.
131	 * Initially, we will use the `reserve' buffer.
132	 */
133	fp->_ur = fp->_r;
134	fp->_up = fp->_p;
135	_UB(fp)._base = fp->_ubuf;
136	_UB(fp)._size = sizeof(fp->_ubuf);
137	fp->_ubuf[sizeof(fp->_ubuf) - 1] = c;
138	fp->_p = &fp->_ubuf[sizeof(fp->_ubuf) - 1];
139	fp->_r = 1;
140	return (c);
141}
142