1 /*
2 * Copyright (c) 1990 The Regents of the University of California.
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms are permitted
6 * provided that the above copyright notice and this paragraph are
7 * duplicated in all such forms and that any documentation,
8 * and/or other materials related to such
9 * distribution and use acknowledge that the software was developed
10 * by the University of California, Berkeley. The name of the
11 * University may not be used to endorse or promote products derived
12 * from this software without specific prior written permission.
13 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
14 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
15 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
16 */
17 /*
18 FUNCTION
19 <<ungetc>>---push data back into a stream
20
21 INDEX
22 ungetc
23 INDEX
24 _ungetc_r
25
26 SYNOPSIS
27 #include <stdio.h>
28 int ungetc(int <[c]>, FILE *<[stream]>);
29
30 int ungetc( int <[c]>, FILE *<[stream]>);
31
32 DESCRIPTION
33 <<ungetc>> is used to return bytes back to <[stream]> to be read again.
34 If <[c]> is EOF, the stream is unchanged. Otherwise, the unsigned
35 char <[c]> is put back on the stream, and subsequent reads will see
36 the bytes pushed back in reverse order. Pushed byes are lost if the
37 stream is repositioned, such as by <<fseek>>, <<fsetpos>>, or
38 <<rewind>>.
39
40 The underlying file is not changed, but it is possible to push back
41 something different than what was originally read. Ungetting a
42 character will clear the end-of-stream marker, and decrement the file
43 position indicator. Pushing back beyond the beginning of a file gives
44 unspecified behavior.
45
46 The alternate function <<_ungetc_r>> is a reentrant version. The
47 extra argument <[reent]> is a pointer to a reentrancy structure.
48
49 RETURNS
50 The character pushed back, or <<EOF>> on error.
51
52 PORTABILITY
53 ANSI C requires <<ungetc>>, but only requires a pushback buffer of one
54 byte; although this implementation can handle multiple bytes, not all
55 can. Pushing back a signed char is a common application bug.
56
57 Supporting OS subroutines required: <<sbrk>>.
58 */
59
60 #if defined(LIBC_SCCS) && !defined(lint)
61 static char sccsid[] = "%W% (Berkeley) %G%";
62 #endif /* LIBC_SCCS and not lint */
63
64 #define _DEFAULT_SOURCE
65 #include <stdio.h>
66 #include <stdlib.h>
67 #include <string.h>
68 #include "local.h"
69
70 /*
71 * Expand the ungetc buffer `in place'. That is, adjust fp->_p when
72 * the buffer moves, so that it points the same distance from the end,
73 * and move the bytes in the buffer around as necessary so that they
74 * are all at the end (stack-style).
75 */
76
77 /*static*/
78 int
__submore(register FILE * fp)79 __submore (
80 register FILE *fp)
81 {
82 register int i;
83 register unsigned char *p;
84
85 if (fp->_ub._base == fp->_ubuf)
86 {
87 /*
88 * Get a new buffer (rather than expanding the old one).
89 */
90 if ((p = (unsigned char *) malloc ((size_t) BUFSIZ)) == NULL)
91 return EOF;
92 fp->_ub._base = p;
93 fp->_ub._size = BUFSIZ;
94 p += BUFSIZ - sizeof (fp->_ubuf);
95 for (i = sizeof (fp->_ubuf); --i >= 0;)
96 p[i] = fp->_ubuf[i];
97 fp->_p = p;
98 return 0;
99 }
100 i = fp->_ub._size;
101 p = (unsigned char *) realloc ((void *) (fp->_ub._base), i << 1);
102 if (p == NULL)
103 return EOF;
104 (void) memcpy ((void *) (p + i), (void *) p, (size_t) i);
105 fp->_p = p + i;
106 fp->_ub._base = p;
107 fp->_ub._size = i << 1;
108 return 0;
109 }
110
111 int
ungetc(int c,register FILE * fp)112 ungetc (
113 int c,
114 register FILE *fp)
115 {
116 if (c == EOF)
117 return (EOF);
118
119 /* Ensure stdio has been initialized.
120 ??? Might be able to remove this as some other stdio routine should
121 have already been called to get the char we are un-getting. */
122
123 CHECK_INIT (rptr, fp);
124
125 _newlib_flockfile_start (fp);
126
127 /* After ungetc, we won't be at eof anymore */
128 fp->_flags &= ~__SEOF;
129
130 if ((fp->_flags & __SRD) == 0)
131 {
132 /*
133 * Not already reading: no good unless reading-and-writing.
134 * Otherwise, flush any current write stuff.
135 */
136 if ((fp->_flags & __SRW) == 0)
137 {
138 _newlib_flockfile_exit (fp);
139 return EOF;
140 }
141 if (fp->_flags & __SWR)
142 {
143 if (fflush ( fp))
144 {
145 _newlib_flockfile_exit (fp);
146 return EOF;
147 }
148 fp->_flags &= ~__SWR;
149 fp->_w = 0;
150 fp->_lbfsize = 0;
151 }
152 fp->_flags |= __SRD;
153 }
154 c = (unsigned char) c;
155
156 /*
157 * If we are in the middle of ungetc'ing, just continue.
158 * This may require expanding the current ungetc buffer.
159 */
160
161 if (HASUB (fp))
162 {
163 if (fp->_r >= fp->_ub._size && __submore (fp))
164 {
165 _newlib_flockfile_exit (fp);
166 return EOF;
167 }
168 *--fp->_p = c;
169 fp->_r++;
170 _newlib_flockfile_exit (fp);
171 return c;
172 }
173
174 /*
175 * If we can handle this by simply backing up, do so,
176 * but never replace the original character.
177 * (This makes sscanf() work when scanning `const' data.)
178 */
179
180 if (fp->_bf._base != NULL && fp->_p > fp->_bf._base && fp->_p[-1] == c)
181 {
182 fp->_p--;
183 fp->_r++;
184 _newlib_flockfile_exit (fp);
185 return c;
186 }
187
188 /*
189 * Create an ungetc buffer.
190 * Initially, we will use the `reserve' buffer.
191 */
192
193 fp->_ur = fp->_r;
194 fp->_up = fp->_p;
195 fp->_ub._base = fp->_ubuf;
196 fp->_ub._size = sizeof (fp->_ubuf);
197 fp->_ubuf[sizeof (fp->_ubuf) - 1] = c;
198 fp->_p = &fp->_ubuf[sizeof (fp->_ubuf) - 1];
199 fp->_r = 1;
200 _newlib_flockfile_end (fp);
201 return c;
202 }
203
