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)
{
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;
}