lib/fflush.c

   1 /* fflush.c -- allow flushing input streams
   2    Copyright (C) 2007-2011 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 Lesser 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 Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser 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).  */
  37 static inline void
  38 clear_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).  */
  48 static inline void
  49 clear_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, NonStop Kernel */
  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
  74 static inline int
  75 disable_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
  82 static inline void
  83 restore_seek_optimization (FILE *fp, int saved_flags)
  84 {
  85   fp_->_flags = (fp_->_flags & ~(__SOPT | __SNPT)) | saved_flags;
  86 }
  87
  88 #endif
  89
  90 static inline void
  91 update_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.  */
 116 int
 117 rpl_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 }