1 /* fflush.c -- allow flushing input streams
2 Copyright (C) 2007-2019 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
clear_ungetc_buffer_preserving_position(FILE * fp)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
clear_ungetc_buffer(FILE * fp)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, 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
disable_seek_optimization(FILE * fp)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
restore_seek_optimization(FILE * fp,int saved_flags)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
update_fpos_cache(FILE * fp _GL_UNUSED_PARAMETER,off_t pos _GL_UNUSED_PARAMETER)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
rpl_fflush(FILE * stream)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 /* Notes about the file-position indicator:
163 1) The file position indicator is incremented by fgetc() and decremented
164 by ungetc():
165 <http://www.opengroup.org/susv3/functions/fgetc.html>
166 "... the fgetc() function shall ... advance the associated file
167 position indicator for the stream ..."
168 <http://www.opengroup.org/susv3/functions/ungetc.html>
169 "The file-position indicator is decremented by each successful
170 call to ungetc()..."
171 2) <http://www.opengroup.org/susv3/functions/ungetc.html> says:
172 "The value of the file-position indicator for the stream after
173 reading or discarding all pushed-back bytes shall be the same
174 as it was before the bytes were pushed back."
175 Here we are discarding all pushed-back bytes. But more specifically,
176 3) <http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt> says:
177 "[After fflush(),] the file offset of the underlying open file
178 description shall be set to the file position of the stream, and
179 any characters pushed back onto the stream by ungetc() ... shall
180 be discarded." */
181
182 /* POSIX does not specify fflush behavior for non-seekable input
183 streams. Some implementations purge unread data, some return
184 EBADF, some do nothing. */
185 off_t pos = ftello (stream);
186 if (pos == -1)
187 {
188 errno = EBADF;
189 return EOF;
190 }
191
192 /* Clear the ungetc buffer. */
193 clear_ungetc_buffer (stream);
194
195 /* To get here, we must be flushing a seekable input stream, so the
196 semantics of fpurge are now appropriate to clear the buffer. To
197 avoid losing data, the lseek is also necessary. */
198 {
199 int result = fpurge (stream);
200 if (result != 0)
201 return result;
202 }
203
204 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
205 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
206
207 {
208 /* Disable seek optimization for the next fseeko call. This tells the
209 following fseeko call to seek to the desired position directly, rather
210 than to seek to a block-aligned boundary. */
211 int saved_flags = disable_seek_optimization (stream);
212 int result = fseeko (stream, pos, SEEK_SET);
213
214 restore_seek_optimization (stream, saved_flags);
215 return result;
216 }
217
218 # else
219
220 pos = lseek (fileno (stream), pos, SEEK_SET);
221 if (pos == -1)
222 return EOF;
223 /* After a successful lseek, update the file descriptor's position cache
224 in the stream. */
225 update_fpos_cache (stream, pos);
226
227 return 0;
228
229 # endif
230 }
231 #endif
232 }
233