lib/fflush.c

   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
  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     /* 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 }