Added description of 'x' mode character (exclusive open).

man3/fopen.3

@@ -229,6 +229,23 @@ and
 functions conform to ANSI X3.159-1989 (``ANSI C'').  The
 .BR fdopen ()
 function conforms to IEEE Std1003.1-1988 (``POSIX.1'').
+.SH "GLIBC EXTENSIONS"
+The GNU C libary allows the following extension for the string specified in
+.IR mode :
+.TP
+.B x
+Open the file exclusively
+(like the 
+.B O_EXCL
+flag of
+.BR open (2)).
+If the file already exists,
+.BR fopen ()
+fails, and sets
+.I errno
+to
+.BR EEXIST .
+.\" FIXME: document /,ccs= charset/
 .SH "SEE ALSO"
 .BR open (2),
 .BR fclose (3),