1 /* fflush.c -- allow flushing input streams
2 Copyright (C) 2007-2021 Free Software Foundation, Inc.
3
4 This program is free software: you can redistribute it and/or modify
5 it under the terms of the GNU General Public License as published by
6 the Free Software Foundation; either version 3 of the License, or
7 (at your option) any later version.
8
9 This program is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 GNU General Public License for more details.
13
14 You should have received a copy of the GNU General Public License
15 along with this program. If not, see <https://www.gnu.org/licenses/>. */
16
17 /* Written by Eric Blake. */
18
19 #include <config.h>
20
21 /* Specification. */
22 #include <stdio.h>
23
24 #include <errno.h>
25 #include <unistd.h>
26
27 #include "freading.h"
28
29 #include "stdio-impl.h"
30
31 #include "unused-parameter.h"
32
33 #undef fflush
34
35
36 #if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
37 /* GNU libc, BeOS, Haiku, Linux libc5 */
38
39 /* Clear the stream's ungetc buffer, preserving the value of ftello (fp). */
40 static void
41 clear_ungetc_buffer_preserving_position (FILE *fp)
42 {
43 if (fp->_flags & _IO_IN_BACKUP)
44 /* _IO_free_backup_area is a bit complicated. Simply call fseek. */
45 fseeko (fp, 0, SEEK_CUR);
46 }
47
48 #else
49
50 /* Clear the stream's ungetc buffer. May modify the value of ftello (fp). */
51 static void
52 clear_ungetc_buffer (FILE *fp)
53 {
54 # if defined __sferror || defined __DragonFly__ || defined __ANDROID__
55 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
56 if (HASUB (fp))
57 {
58 fp_->_p += fp_->_r;
59 fp_->_r = 0;
60 }
61 # elif defined __EMX__ /* emx+gcc */
62 if (fp->_ungetc_count > 0)
63 {
64 fp->_ungetc_count = 0;
65 fp->_rcount = - fp->_rcount;
66 }
67 # elif defined _IOERR /* Minix, AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
68 /* Nothing to do. */
69 # else /* other implementations */
70 fseeko (fp, 0, SEEK_CUR);
71 # endif
72 }
73
74 #endif
75
76 #if ! (defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1)
77 /* GNU libc, BeOS, Haiku, Linux libc5 */
78
79 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
80 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
81
82 static int
83 disable_seek_optimization (FILE *fp)
84 {
85 int saved_flags = fp_->_flags & (__SOPT | __SNPT);
86 fp_->_flags = (fp_->_flags & ~__SOPT) | __SNPT;
87 return saved_flags;
88 }
89
90 static void
91 restore_seek_optimization (FILE *fp, int saved_flags)
92 {
93 fp_->_flags = (fp_->_flags & ~(__SOPT | __SNPT)) | saved_flags;
94 }
95
96 # else
97
98 static void
99 update_fpos_cache (FILE *fp _GL_UNUSED_PARAMETER,
100 off_t pos _GL_UNUSED_PARAMETER)
101 {
102 # if defined __sferror || defined __DragonFly__ || defined __ANDROID__
103 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
104 # if defined __CYGWIN__
105 /* fp_->_offset is typed as an integer. */
106 fp_->_offset = pos;
107 # else
108 /* fp_->_offset is an fpos_t. */
109 /* Use a union, since on NetBSD, the compilation flags determine
110 whether fpos_t is typedef'd to off_t or a struct containing a
111 single off_t member. */
112 union
113 {
114 fpos_t f;
115 off_t o;
116 } u;
117 u.o = pos;
118 fp_->_offset = u.f;
119 # endif
120 fp_->_flags |= __SOFF;
121 # endif
122 }
123 # endif
124 #endif
125
126 /* Flush all pending data on STREAM according to POSIX rules. Both
127 output and seekable input streams are supported. */
128 int
129 rpl_fflush (FILE *stream)
130 {
131 /* When stream is NULL, POSIX and C99 only require flushing of "output
132 streams and update streams in which the most recent operation was not
133 input", and all implementations do this.
134
135 When stream is "an output stream or an update stream in which the most
136 recent operation was not input", POSIX and C99 requires that fflush
137 writes out any buffered data, and all implementations do this.
138
139 When stream is, however, an input stream or an update stream in
140 which the most recent operation was input, C99 specifies nothing,
141 and POSIX only specifies behavior if the stream is seekable.
142 mingw, in particular, drops the input buffer, leaving the file
143 descriptor positioned at the end of the input buffer. I.e. ftell
144 (stream) is lost. We don't want to call the implementation's
145 fflush in this case.
146
147 We test ! freading (stream) here, rather than fwriting (stream), because
148 what we need to know is whether the stream holds a "read buffer", and on
149 mingw this is indicated by _IOREAD, regardless of _IOWRT. */
150 if (stream == NULL || ! freading (stream))
151 return fflush (stream);
152
153 #if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
154 /* GNU libc, BeOS, Haiku, Linux libc5 */
155
156 clear_ungetc_buffer_preserving_position (stream);
157
158 return fflush (stream);
159
160 #else
161 {
162 /* What POSIX says:
163 1) About the file-position indicator (-> fseeko, ftello):
164 The file position indicator is incremented by fgetc() and decremented
165 by ungetc():
166 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fgetc.html>
167 "... the fgetc() function shall ... advance the associated file
168 position indicator for the stream ..."
169 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/ungetc.html>
170 "The file-position indicator is decremented by each successful
171 call to ungetc()..."
172 2) fflush discards bytes pushed back by ungetc:
173 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html>
174 "...any characters pushed back onto the stream by ungetc()
175 or ungetwc() that have not subsequently been read from the
176 stream shall be discarded..."
177 This implies implicitly: fflush does not change the file position
178 indicator.
179 3) Effects on the file descriptor, if the file descriptor is capable of
180 seeking:
181 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html>
182 "...the file offset of the underlying open file description shall
183 be set to the file position of the stream..." */
184
185 /* POSIX does not specify fflush behavior for non-seekable input
186 streams. Some implementations purge unread data, some return
187 EBADF, some do nothing. */
188 off_t pos = ftello (stream);
189 if (pos == -1)
190 {
191 errno = EBADF;
192 return EOF;
193 }
194
195 /* Clear the ungetc buffer. */
196 clear_ungetc_buffer (stream);
197
198 /* To get here, we must be flushing a seekable input stream, so the
199 semantics of fpurge are now appropriate to clear the buffer. To
200 avoid losing data, the lseek is also necessary. */
201 {
202 int result = fpurge (stream);
203 if (result != 0)
204 return result;
205 }
206
207 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
208 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
209
210 {
211 /* Disable seek optimization for the next fseeko call. This tells the
212 following fseeko call to seek to the desired position directly, rather
213 than to seek to a block-aligned boundary. */
214 int saved_flags = disable_seek_optimization (stream);
215 int result = fseeko (stream, pos, SEEK_SET);
216
217 restore_seek_optimization (stream, saved_flags);
218 return result;
219 }
220
221 # else
222
223 pos = lseek (fileno (stream), pos, SEEK_SET);
224 if (pos == -1)
225 return EOF;
226 /* After a successful lseek, update the file descriptor's position cache
227 in the stream. */
228 update_fpos_cache (stream, pos);
229
230 return 0;
231
232 # endif
233 }
234 #endif
235 }