1/* fflush.c -- allow flushing input streams
2   Copyright (C) 2007-2010 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 <http://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_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
35
36/* Clear the stream's ungetc buffer, preserving the value of ftello (fp).  */
37static inline void
38clear_ungetc_buffer_preserving_position (FILE *fp)
39{
40  if (fp->_flags & _IO_IN_BACKUP)
41    /* _IO_free_backup_area is a bit complicated.  Simply call fseek.  */
42    fseeko (fp, 0, SEEK_CUR);
43}
44
45#else
46
47/* Clear the stream's ungetc buffer.  May modify the value of ftello (fp).  */
48static inline void
49clear_ungetc_buffer (FILE *fp)
50{
51# if defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
52  if (HASUB (fp))
53    {
54      fp_->_p += fp_->_r;
55      fp_->_r = 0;
56    }
57# elif defined __EMX__              /* emx+gcc */
58  if (fp->_ungetc_count > 0)
59    {
60      fp->_ungetc_count = 0;
61      fp->_rcount = - fp->_rcount;
62    }
63# elif defined _IOERR               /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw */
64  /* Nothing to do.  */
65# else                              /* other implementations */
66  fseeko (fp, 0, SEEK_CUR);
67# endif
68}
69
70#endif
71
72#if (defined __sferror || defined __DragonFly__) && defined __SNPT /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
73
74static inline int
75disable_seek_optimization (FILE *fp)
76{
77  int saved_flags = fp_->_flags & (__SOPT | __SNPT);
78  fp_->_flags = (fp_->_flags & ~__SOPT) | __SNPT;
79  return saved_flags;
80}
81
82static inline void
83restore_seek_optimization (FILE *fp, int saved_flags)
84{
85  fp_->_flags = (fp_->_flags & ~(__SOPT | __SNPT)) | saved_flags;
86}
87
88#endif
89
90static inline void
91update_fpos_cache (FILE *fp, off_t pos)
92{
93#if defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
94# if defined __CYGWIN__
95  /* fp_->_offset is typed as an integer.  */
96  fp_->_offset = pos;
97# else
98  /* fp_->_offset is an fpos_t.  */
99  /* Use a union, since on NetBSD, the compilation flags determine
100     whether fpos_t is typedef'd to off_t or a struct containing a
101     single off_t member.  */
102  union
103    {
104      fpos_t f;
105      off_t o;
106    } u;
107  u.o = pos;
108  fp_->_offset = u.f;
109# endif
110  fp_->_flags |= __SOFF;
111#endif
112}
113
114/* Flush all pending data on STREAM according to POSIX rules.  Both
115   output and seekable input streams are supported.  */
116int
117rpl_fflush (FILE *stream)
118{
119  /* When stream is NULL, POSIX and C99 only require flushing of "output
120     streams and update streams in which the most recent operation was not
121     input", and all implementations do this.
122
123     When stream is "an output stream or an update stream in which the most
124     recent operation was not input", POSIX and C99 requires that fflush
125     writes out any buffered data, and all implementations do this.
126
127     When stream is, however, an input stream or an update stream in
128     which the most recent operation was input, C99 specifies nothing,
129     and POSIX only specifies behavior if the stream is seekable.
130     mingw, in particular, drops the input buffer, leaving the file
131     descriptor positioned at the end of the input buffer. I.e. ftell
132     (stream) is lost.  We don't want to call the implementation's
133     fflush in this case.
134
135     We test ! freading (stream) here, rather than fwriting (stream), because
136     what we need to know is whether the stream holds a "read buffer", and on
137     mingw this is indicated by _IOREAD, regardless of _IOWRT.  */
138  if (stream == NULL || ! freading (stream))
139    return fflush (stream);
140
141#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
142
143  clear_ungetc_buffer_preserving_position (stream);
144
145  return fflush (stream);
146
147#else
148  {
149    /* Notes about the file-position indicator:
150       1) The file position indicator is incremented by fgetc() and decremented
151          by ungetc():
152          <http://www.opengroup.org/susv3/functions/fgetc.html>
153            "... the fgetc() function shall ... advance the associated file
154             position indicator for the stream ..."
155          <http://www.opengroup.org/susv3/functions/ungetc.html>
156            "The file-position indicator is decremented by each successful
157             call to ungetc()..."
158       2) <http://www.opengroup.org/susv3/functions/ungetc.html> says:
159            "The value of the file-position indicator for the stream after
160             reading or discarding all pushed-back bytes shall be the same
161             as it was before the bytes were pushed back."
162          Here we are discarding all pushed-back bytes.  But more specifically,
163       3) <http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt> says:
164            "[After fflush(),] the file offset of the underlying open file
165             description shall be set to the file position of the stream, and
166             any characters pushed back onto the stream by ungetc() ... shall
167             be discarded."  */
168
169    /* POSIX does not specify fflush behavior for non-seekable input
170       streams.  Some implementations purge unread data, some return
171       EBADF, some do nothing.  */
172    off_t pos = ftello (stream);
173    if (pos == -1)
174      {
175        errno = EBADF;
176        return EOF;
177      }
178
179    /* Clear the ungetc buffer.  */
180    clear_ungetc_buffer (stream);
181
182    /* To get here, we must be flushing a seekable input stream, so the
183       semantics of fpurge are now appropriate to clear the buffer.  To
184       avoid losing data, the lseek is also necessary.  */
185    {
186      int result = fpurge (stream);
187      if (result != 0)
188        return result;
189    }
190
191# if (defined __sferror || defined __DragonFly__) && defined __SNPT /* FreeBSD, NetBSD, OpenBSD, DragonFly, MacOS X, Cygwin */
192
193    {
194      /* Disable seek optimization for the next fseeko call.  This tells the
195         following fseeko call to seek to the desired position directly, rather
196         than to seek to a block-aligned boundary.  */
197      int saved_flags = disable_seek_optimization (stream);
198      int result = fseeko (stream, pos, SEEK_SET);
199
200      restore_seek_optimization (stream, saved_flags);
201      return result;
202    }
203
204# else
205
206    pos = lseek (fileno (stream), pos, SEEK_SET);
207    if (pos == -1)
208      return EOF;
209    /* After a successful lseek, update the file descriptor's position cache
210       in the stream.  */
211    update_fpos_cache (stream, pos);
212
213    return 0;
214
215# endif
216  }
217#endif
218}
219