1/* fflush.c -- allow flushing input streams
2   Copyright (C) 2007 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#include "fpurge.h"
29
30#undef fflush
31
32/* Flush all pending data on STREAM according to POSIX rules.  Both
33   output and seekable input streams are supported.  */
34int
35rpl_fflush (FILE *stream)
36{
37  int result;
38  off_t pos;
39
40  /* When stream is NULL, POSIX and C99 only require flushing of "output
41     streams and update streams in which the most recent operation was not
42     input", and all implementations do this.
43
44     When stream is "an output stream or an update stream in which the most
45     recent operation was not input", POSIX and C99 requires that fflush
46     writes out any buffered data, and all implementations do this.
47
48     When stream is, however, an input stream or an update stream in
49     which the most recent operation was input, C99 specifies nothing,
50     and POSIX only specifies behavior if the stream is seekable.
51     mingw, in particular, drops the input buffer, leaving the file
52     descriptor positioned at the end of the input buffer. I.e. ftell
53     (stream) is lost.  We don't want to call the implementation's
54     fflush in this case.
55
56     We test ! freading (stream) here, rather than fwriting (stream), because
57     what we need to know is whether the stream holds a "read buffer", and on
58     mingw this is indicated by _IOREAD, regardless of _IOWRT.  */
59  if (stream == NULL || ! freading (stream))
60    return fflush (stream);
61
62  /* POSIX does not specify fflush behavior for non-seekable input
63     streams.  Some implementations purge unread data, some return
64     EBADF, some do nothing.  */
65  pos = ftello (stream);
66  if (pos == -1)
67    {
68      errno = EBADF;
69      return EOF;
70    }
71
72  /* To get here, we must be flushing a seekable input stream, so the
73     semantics of fpurge are now appropriate to clear the buffer.  To
74     avoid losing data, the lseek is also necessary.  */
75  result = fpurge (stream);
76  if (result != 0)
77    return result;
78
79#if defined __sferror && defined __SNPT /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
80
81  {
82    /* Disable seek optimization for the next fseeko call.  This tells the
83       following fseeko call to seek to the desired position directly, rather
84       than to seek to a block-aligned boundary.  */
85    int saved_flags = stream->_flags & (__SOPT | __SNPT);
86    stream->_flags = (stream->_flags & ~__SOPT) | __SNPT;
87
88    result = fseeko (stream, pos, SEEK_SET);
89
90    stream->_flags = (stream->_flags & ~(__SOPT | __SNPT)) | saved_flags;
91  }
92  return result;
93
94#else
95
96  pos = lseek (fileno (stream), pos, SEEK_SET);
97  if (pos == -1)
98    return EOF;
99  /* After a successful lseek, update the file descriptor's position cache
100     in the stream.  */
101# if defined __sferror           /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
102  stream->_offset = pos;
103  stream->_flags |= __SOFF;
104# endif
105
106  return 0;
107
108#endif
109}
110