lib/fflush.c

   1 /* fflush.c -- allow flushing input streams
   2    Copyright (C) 2007-2017 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 #include "unused-parameter.h"
  32
  33 #undef fflush
  34
  35
  36 #if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
  37
  38 /* Clear the stream's ungetc buffer, preserving the value of ftello (fp).  */
  39 static void
  40 clear_ungetc_buffer_preserving_position (FILE *fp)
  41 {
  42   if (fp->_flags & _IO_IN_BACKUP)
  43     /* _IO_free_backup_area is a bit complicated.  Simply call fseek.  */
  44     fseeko (fp, 0, SEEK_CUR);
  45 }
  46
  47 #else
  48
  49 /* Clear the stream's ungetc buffer.  May modify the value of ftello (fp).  */
  50 static void
  51 clear_ungetc_buffer (FILE *fp)
  52 {
  53 # if defined __sferror || defined __DragonFly__ || defined __ANDROID__
  54   /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
  55   if (HASUB (fp))
  56     {
  57       fp_->_p += fp_->_r;
  58       fp_->_r = 0;
  59     }
  60 # elif defined __EMX__              /* emx+gcc */
  61   if (fp->_ungetc_count > 0)
  62     {
  63       fp->_ungetc_count = 0;
  64       fp->_rcount = - fp->_rcount;
  65     }
  66 # elif defined _IOERR               /* Minix, AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw, MSVC, NonStop Kernel */
  67   /* Nothing to do.  */
  68 # else                              /* other implementations */
  69   fseeko (fp, 0, SEEK_CUR);
  70 # endif
  71 }
  72
  73 #endif
  74
  75 #if ! (defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */)
  76
  77 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
  78 /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
  79
  80 static int
  81 disable_seek_optimization (FILE *fp)
  82 {
  83   int saved_flags = fp_->_flags & (__SOPT | __SNPT);
  84   fp_->_flags = (fp_->_flags & ~__SOPT) | __SNPT;
  85   return saved_flags;
  86 }
  87
  88 static void
  89 restore_seek_optimization (FILE *fp, int saved_flags)
  90 {
  91   fp_->_flags = (fp_->_flags & ~(__SOPT | __SNPT)) | saved_flags;
  92 }
  93
  94 # else
  95
  96 static void
  97 update_fpos_cache (FILE *fp _GL_UNUSED_PARAMETER,
  98                    off_t pos _GL_UNUSED_PARAMETER)
  99 {
 100 #  if defined __sferror || defined __DragonFly__ || defined __ANDROID__
 101   /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
 102 #   if defined __CYGWIN__
 103   /* fp_->_offset is typed as an integer.  */
 104   fp_->_offset = pos;
 105 #   else
 106   /* fp_->_offset is an fpos_t.  */
 107   /* Use a union, since on NetBSD, the compilation flags determine
 108      whether fpos_t is typedef'd to off_t or a struct containing a
 109      single off_t member.  */
 110   union
 111     {
 112       fpos_t f;
 113       off_t o;
 114     } u;
 115   u.o = pos;
 116   fp_->_offset = u.f;
 117 #   endif
 118   fp_->_flags |= __SOFF;
 119 #  endif
 120 }
 121 # endif
 122 #endif
 123
 124 /* Flush all pending data on STREAM according to POSIX rules.  Both
 125    output and seekable input streams are supported.  */
 126 int
 127 rpl_fflush (FILE *stream)
 128 {
 129   /* When stream is NULL, POSIX and C99 only require flushing of "output
 130      streams and update streams in which the most recent operation was not
 131      input", and all implementations do this.
 132
 133      When stream is "an output stream or an update stream in which the most
 134      recent operation was not input", POSIX and C99 requires that fflush
 135      writes out any buffered data, and all implementations do this.
 136
 137      When stream is, however, an input stream or an update stream in
 138      which the most recent operation was input, C99 specifies nothing,
 139      and POSIX only specifies behavior if the stream is seekable.
 140      mingw, in particular, drops the input buffer, leaving the file
 141      descriptor positioned at the end of the input buffer. I.e. ftell
 142      (stream) is lost.  We don't want to call the implementation's
 143      fflush in this case.
 144
 145      We test ! freading (stream) here, rather than fwriting (stream), because
 146      what we need to know is whether the stream holds a "read buffer", and on
 147      mingw this is indicated by _IOREAD, regardless of _IOWRT.  */
 148   if (stream == NULL || ! freading (stream))
 149     return fflush (stream);
 150
 151 #if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
 152
 153   clear_ungetc_buffer_preserving_position (stream);
 154
 155   return fflush (stream);
 156
 157 #else
 158   {
 159     /* Notes about the file-position indicator:
 160        1) The file position indicator is incremented by fgetc() and decremented
 161           by ungetc():
 162           <http://www.opengroup.org/susv3/functions/fgetc.html>
 163             "... the fgetc() function shall ... advance the associated file
 164              position indicator for the stream ..."
 165           <http://www.opengroup.org/susv3/functions/ungetc.html>
 166             "The file-position indicator is decremented by each successful
 167              call to ungetc()..."
 168        2) <http://www.opengroup.org/susv3/functions/ungetc.html> says:
 169             "The value of the file-position indicator for the stream after
 170              reading or discarding all pushed-back bytes shall be the same
 171              as it was before the bytes were pushed back."
 172           Here we are discarding all pushed-back bytes.  But more specifically,
 173        3) <http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt> says:
 174             "[After fflush(),] the file offset of the underlying open file
 175              description shall be set to the file position of the stream, and
 176              any characters pushed back onto the stream by ungetc() ... shall
 177              be discarded."  */
 178
 179     /* POSIX does not specify fflush behavior for non-seekable input
 180        streams.  Some implementations purge unread data, some return
 181        EBADF, some do nothing.  */
 182     off_t pos = ftello (stream);
 183     if (pos == -1)
 184       {
 185         errno = EBADF;
 186         return EOF;
 187       }
 188
 189     /* Clear the ungetc buffer.  */
 190     clear_ungetc_buffer (stream);
 191
 192     /* To get here, we must be flushing a seekable input stream, so the
 193        semantics of fpurge are now appropriate to clear the buffer.  To
 194        avoid losing data, the lseek is also necessary.  */
 195     {
 196       int result = fpurge (stream);
 197       if (result != 0)
 198         return result;
 199     }
 200
 201 # if (defined __sferror || defined __DragonFly__ || defined __ANDROID__) && defined __SNPT
 202     /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
 203
 204     {
 205       /* Disable seek optimization for the next fseeko call.  This tells the
 206          following fseeko call to seek to the desired position directly, rather
 207          than to seek to a block-aligned boundary.  */
 208       int saved_flags = disable_seek_optimization (stream);
 209       int result = fseeko (stream, pos, SEEK_SET);
 210
 211       restore_seek_optimization (stream, saved_flags);
 212       return result;
 213     }
 214
 215 # else
 216
 217     pos = lseek (fileno (stream), pos, SEEK_SET);
 218     if (pos == -1)
 219       return EOF;
 220     /* After a successful lseek, update the file descriptor's position cache
 221        in the stream.  */
 222     update_fpos_cache (stream, pos);
 223
 224     return 0;
 225
 226 # endif
 227   }
 228 #endif
 229 }