lib/fflush.c

   1 /* fflush.c -- allow flushing input streams
   2    Copyright (C) 2007-2020 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, 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 }