X-Git-Url: http://erislabs.net/gitweb/?a=blobdiff_plain;f=lib%2Ffflush.c;h=459a7107072d457f01b2e8a0feaff15f78e2d564;hb=4228bc8a5ec1846f6a12c81a46e3dd864e027196;hp=9361c09f3fa15d5855aa9caf366168e14a53679c;hpb=9b87488be4a30247c1eabd71408e1e953b797c22;p=gnulib.git

diff --git a/lib/fflush.c b/lib/fflush.c
index 9361c09f3..459a71070 100755
--- a/lib/fflush.c
+++ b/lib/fflush.c
@@ -38,15 +38,31 @@ rpl_fflush (FILE *stream)
   int result;
   off_t pos;
 
-  /* When stream is NULL, POSIX only requires flushing of output
-     streams.  C89 guarantees behavior of output streams, and fflush
-     should be safe on read-write streams that are not currently
-     reading.  */
-  if (! stream || ! freading (stream))
+  /* When stream is NULL, POSIX and C99 only require flushing of "output
+     streams and update streams in which the most recent operation was not
+     input", and all implementations do this.
+
+     When stream is "an output stream or an update stream in which the most
+     recent operation was not input", POSIX and C99 requires that fflush
+     writes out any buffered data, and all implementations do this.
+
+     When stream is, however, an input stream or an update stream in
+     which the most recent operation was input, C99 specifies nothing,
+     and POSIX only specifies behavior if the stream is seekable.
+     mingw, in particular, drops the input buffer, leaving the file
+     descriptor positioned at the end of the input buffer. I.e. ftell
+     (stream) is lost.  We don't want to call the implementation's
+     fflush in this case.
+
+     We test ! freading (stream) here, rather than fwriting (stream), because
+     what we need to know is whether the stream holds a "read buffer", and on
+     mingw this is indicated by _IOREAD, regardless of _IOWRT.  */
+  if (stream == NULL || ! freading (stream))
     return fflush (stream);
 
   /* POSIX does not specify fflush behavior for non-seekable input
-     streams.  */
+     streams.  Some implementations purge unread data, some return
+     EBADF, some do nothing.  */
   pos = ftello (stream);
   if (pos == -1)
     {
@@ -60,12 +76,16 @@ rpl_fflush (FILE *stream)
   result = fpurge (stream);
   if (result != 0)
     return result;
+
   pos = lseek (fileno (stream), pos, SEEK_SET);
   if (pos == -1)
     return EOF;
-#if defined __sferror               /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
+  /* After a successful lseek, update the file descriptor's position cache
+     in the stream.  */
+#if defined __sferror           /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
   stream->_offset = pos;
   stream->_flags |= __SOFF;
 #endif
+
   return 0;
 }