SVR2 (at least) printf bug

[Curtis Jackson]

This bug is known to the Unix Hotline and the appropriate PIC (People
In Charge), but wasn't known to me so I thought I'd share it.

If you open a file for simple update (mode "r+" on fopen), then read
some stuff in the file, then try to write on the file, the writes
will appear to work perfectly (printf returns good bytecounts and
everything), but no writes will take place in the real world.

This happens because the code to check for _IORW (read/write permissions)
is wrong in printf.c, fprintf.c, vprintf.c, and vfprintf.c.  This
causes the write permissions (_IOWRT) turnon code to be skipped and
_doprnt() returns a negative number; so the _cnt field in the FILE
descriptor decrements rather than increments, and the fflush() therefore
has no effect -- thus silently no writes.

The errant lines are:

	if (!(stdout->_flag | _IOWRT)) {
		/* if no write flag */
		if (stdout->_flag | _IORW) {

since |'ing with a constant is not a very good idea, change them to &'s:

	if (!(stdout->_flag & _IOWRT)) {
		/* if no write flag */
		if (stdout->_flag & _IORW) {

and everything works fine.

I found this bug very interesting to track down since (apparently) the
initial modes on the file are both _IOWRT and _IORW.  This means that
if you just write a file fopened with "r+" everything works, but the
first read turns off _IOWRT and any subsequent writes silently fail.

Don't forget to remake the library!!
-- 

The MAD Programmer -- 919-228-3313 (Cornet 291)
alias: Curtis Jackson	...![ ihnp4 ulysses cbosgd mgnetp ]!burl!rcj
			...![ ihnp4 cbosgd akgua masscomp ]!clyde!rcj