See ioctl, if the return value is true, its not an error.

My manpage says:

RETURN VALUE
       Usually, on success zero is returned.  A few ioctl() requests  
+use  the
       return  value as an output parameter and return a non-negative 
+value on
       success.  On error, -1 is returned, and errno is set appropriat
+ely.
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Note the phrase: "Usually, on success, zero is returned."... Some return a value as an output param, but this call isn't one of them. My error check code appears to be valid according to the above, though I noticed an oddity if you print the value returning from ioctl in this code:

 require 'sys/ioctl.ph';
    die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
    open(TTY, "+</dev/tty")                     or die "No tty: $!";
    my $ret=ioctl(TTY, &TIOCGWINSZ, $winsize='');
    printf "ret num=%d, retstring=%s\n",$ret,$ret;

    unless ($ret) {
            die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWIN
+SZ;
    }
    ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
    print "(row,col) = ($row,$col)";
    print "  (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixe
+l;

    print "\n";
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The output is:

ret num=0, retstring=0 but true
(row,col) = (79,80)
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That is bizarre! 0 = true?!

In any event, that's a red herring, as my code works when I use "/dev/tty" as a device instead of STDOUT. Same goes for the unpack format.

I used S!*, which is likely "more correct" than S4 shown above.

The "!" after the "S" says to use the platform's native size for a short instead of fixed a fixed 16-bit size. Since the struct is declared as a short on my platform, I figured it was best to use my platform's native "short" size rather than a fixed 16 bit value -- wouldn't you agree??

The "*" vs. "4", makes no difference in this case as there are only 4 values to receive the output. "*" tells unpack to unpack as many as are in the struct -- so if the struct was 6 shorts long instead of 4, it would return 6 values, the last 2 of which would be thrown away as I only have an array of 4 vars to receive it. It would be like doing:

my @ar=(1,2,3,4,5,6);
my ($x,$y,$z)=@ar;
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$x-$z get values 1 through 3, and values 4,5,6 get ignored.

All of the above covers the stuff that "doesn't make a difference". Then comes the part that doesn't make sense.

As you note, fileno() on STD(IN,OUT,ERR) all return 0,1,2 as expected. Then why can't I use ioctl(STDOUT) with ioctl and have it work?