1 /* fflush.c -- allow flushing input streams
2 Copyright (C) 2007-2022 Free Software Foundation, Inc.
3
4 This file is free software: you can redistribute it and/or modify
5 it under the terms of the GNU Lesser General Public License as
6 published by the Free Software Foundation; either version 2.1 of the
7 License, or (at your option) any later version.
8
9 This file 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 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser 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 #undef fflush
32
33
34 #if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
35 /* GNU libc, BeOS, Haiku, Linux libc5 */
36
37 /* Clear the stream's ungetc buffer, preserving the value of ftello (fp). */
38 static void
39 clear_ungetc_buffer_preserving_position (FILE *fp)
40 {
41 if (fp->_flags & _IO_IN_BACKUP)
42 /* _IO_free_backup_area is a bit complicated. Simply call fseek. */
43 fseeko (fp, 0, SEEK_CUR);
44 }
45
46 #else
47
48 /* Clear the stream's ungetc buffer. May modify the value of ftello (fp). */
49 static void
50 clear_ungetc_buffer (FILE *fp)
51 {
52 # if defined __sferror || defined __DragonFly__ || defined __ANDROID__
53 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
54 if (HASUB (fp))
55 {
56 fp_->_p += fp_->_r;
57 fp_->_r = 0;
58 }
59 # elif defined __EMX__ /* emx+gcc */
60 if (fp->_ungetc_count > 0)
61 {
62 fp->_ungetc_count = 0;
63 fp->_rcount = - fp->_rcount;
64 }
65 # elif defined _IOERR /* Minix, AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
66 /* Nothing to do. */
67 # else /* other implementations */
68 fseeko (fp, 0, SEEK_CUR);
69 # endif
70 }
71
72 #endif
73
74 #if ! (defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1)
75 /* GNU libc, BeOS, Haiku, Linux libc5 */
76
77 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
78 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
79
80 static int
81 disable_seek_optimization (FILE *fp)
82 {
83 int saved_flags = fp_->_flags & (__SOPT | __SNPT);
84 fp_->_flags = (fp_->_flags & ~__SOPT) | __SNPT;
85 return saved_flags;
86 }
87
88 static void
89 restore_seek_optimization (FILE *fp, int saved_flags)
90 {
91 fp_->_flags = (fp_->_flags & ~(__SOPT | __SNPT)) | saved_flags;
92 }
93
94 # else
95
96 static void
97 update_fpos_cache (_GL_ATTRIBUTE_MAYBE_UNUSED FILE *fp,
98 _GL_ATTRIBUTE_MAYBE_UNUSED off_t pos)
99 {
100 # if defined __sferror || defined __DragonFly__ || defined __ANDROID__
101 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
102 # if defined __CYGWIN__
103 /* fp_->_offset is typed as an integer. */
104 fp_->_offset = pos;
105 # else
106 /* fp_->_offset is an fpos_t. */
107 /* Use a union, since on NetBSD, the compilation flags determine
108 whether fpos_t is typedef'd to off_t or a struct containing a
109 single off_t member. */
110 union
111 {
112 fpos_t f;
113 off_t o;
114 } u;
115 u.o = pos;
116 fp_->_offset = u.f;
117 # endif
118 fp_->_flags |= __SOFF;
119 # endif
120 }
121 # endif
122 #endif
123
124 /* Flush all pending data on STREAM according to POSIX rules. Both
125 output and seekable input streams are supported. */
126 int
127 rpl_fflush (FILE *stream)
128 {
129 /* When stream is NULL, POSIX and C99 only require flushing of "output
130 streams and update streams in which the most recent operation was not
131 input", and all implementations do this.
132
133 When stream is "an output stream or an update stream in which the most
134 recent operation was not input", POSIX and C99 requires that fflush
135 writes out any buffered data, and all implementations do this.
136
137 When stream is, however, an input stream or an update stream in
138 which the most recent operation was input, C99 specifies nothing,
139 and POSIX only specifies behavior if the stream is seekable.
140 mingw, in particular, drops the input buffer, leaving the file
141 descriptor positioned at the end of the input buffer. I.e. ftell
142 (stream) is lost. We don't want to call the implementation's
143 fflush in this case.
144
145 We test ! freading (stream) here, rather than fwriting (stream), because
146 what we need to know is whether the stream holds a "read buffer", and on
147 mingw this is indicated by _IOREAD, regardless of _IOWRT. */
148 if (stream == NULL || ! freading (stream))
149 return fflush (stream);
150
151 #if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
152 /* GNU libc, BeOS, Haiku, Linux libc5 */
153
154 clear_ungetc_buffer_preserving_position (stream);
155
156 return fflush (stream);
157
158 #else
159 {
160 /* What POSIX says:
161 1) About the file-position indicator (-> fseeko, ftello):
162 The file position indicator is incremented by fgetc() and decremented
163 by ungetc():
164 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fgetc.html>
165 "... the fgetc() function shall ... advance the associated file
166 position indicator for the stream ..."
167 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/ungetc.html>
168 "The file-position indicator is decremented by each successful
169 call to ungetc()..."
170 2) fflush discards bytes pushed back by ungetc:
171 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html>
172 "...any characters pushed back onto the stream by ungetc()
173 or ungetwc() that have not subsequently been read from the
174 stream shall be discarded..."
175 This implies implicitly: fflush does not change the file position
176 indicator.
177 3) Effects on the file descriptor, if the file descriptor is capable of
178 seeking:
179 <https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html>
180 "...the file offset of the underlying open file description shall
181 be set to the file position of the stream..." */
182
183 /* POSIX does not specify fflush behavior for non-seekable input
184 streams. Some implementations purge unread data, some return
185 EBADF, some do nothing. */
186 off_t pos = ftello (stream);
187 if (pos == -1)
188 {
189 errno = EBADF;
190 return EOF;
191 }
192
193 /* Clear the ungetc buffer. */
194 clear_ungetc_buffer (stream);
195
196 /* To get here, we must be flushing a seekable input stream, so the
197 semantics of fpurge are now appropriate to clear the buffer. To
198 avoid losing data, the lseek is also necessary. */
199 {
200 int result = fpurge (stream);
201 if (result != 0)
202 return result;
203 }
204
205 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
206 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
207
208 {
209 /* Disable seek optimization for the next fseeko call. This tells the
210 following fseeko call to seek to the desired position directly, rather
211 than to seek to a block-aligned boundary. */
212 int saved_flags = disable_seek_optimization (stream);
213 int result = fseeko (stream, pos, SEEK_SET);
214
215 restore_seek_optimization (stream, saved_flags);
216 return result;
217 }
218
219 # else
220
221 pos = lseek (fileno (stream), pos, SEEK_SET);
222 if (pos == -1)
223 return EOF;
224 /* After a successful lseek, update the file descriptor's position cache
225 in the stream. */
226 update_fpos_cache (stream, pos);
227
228 return 0;
229
230 # endif
231 }
232 #endif
233 }