Red Hat Bugzilla – Bug 241309
huge empty file created after an ungetwc() followed by fwide()
Last modified: 2016-11-24 10:20:59 EST
We're seeing the following unusual behavior of the program below on Red Hat
Advanced Server 4, Update 4:
$ cat t.c && gcc t.c && rm -f foo.out && ./a.out > foo.out && ls -l foo.out &&
du -k foo.out
int main ()
ungetwc ('A', stdout);
fwide (stdout, 0);
-rw-r--r-- 1 sebor devel 182888783729 May 24 16:44 foo.out
The output of strace on the program is in the attached file.
Here's info on libc:
$ ldd a.out && /lib64/tls/libc.so.6
libc.so.6 => /lib64/tls/libc.so.6 (0x000000381b500000)
GNU C Library stable release version 2.3.4, by Roland McGrath et al.
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
Compiled by GNU CC version 3.4.6 20060404 (Red Hat 3.4.6-2).
Compiled on a Linux 2.4.20 system on 2006-07-04.
GNU libio by Per Bothner
crypt add-on version 2.1 by Michael Glad and others
Native POSIX Threads Library by Ulrich Drepper et al
RT using linux kernel aio
The C stubs add-on version 2.1.2.
GNU Libidn by Simon Josefsson
NIS(YP)/NIS+ NSS modules 0.19 by Thorsten Kukuk
Thread-local storage support included.
For bug reporting instructions, please see:
Created attachment 155405 [details]
Output of strace ./a.out >/dev/null 2>/dev/pts/1
This testcase violates ISO C99, 7.19.2(5):
"wide character input/output functions shall not be applied to a byte-oriented
puts is a byte output function, therefore the stream after puts is byte-oriented.
ungetwc is wide input/output function and thus must not be used until you
reorient the stream.
I suspected the program was invalid but didn't look carefully enough for
where the standard says so. Thanks for taking the time to provide the
Even though the program is not valid I thought you might like to know
about this type of behavior and consider making a change. I note that
Enterprise Linux 5 creates a file of 0 size so it looks like the change
may have already been implemented.
Why should we slow all correctly built programs to accomodate an invalid program?