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Date: Thu, 08 Apr 2004 18:53:59 -0700
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Subject: Access Violations / Corruptions / Refs to temps
From: truth@proposaltech.com (Todd Ruth)

> > I have a feeling that PHP has a memory problem somewhere :/.

I'm not sure if it caused any of the issues either of you spoke of,
but I can report that PHP does have some pointer problems.  I've
been putting off this email until I had written up something nicer,
but it's probably better to just get the word out (assuming this isn't
already known)...

PHP has problems related to using a temporary as the source for a
reference.  An obvious no-no is to write "$x =& NULL;" or
"$x =& @$y[5];".  These are parse errors.  On the other hand, PHP won't
complain about:

function &f() {
   $y = array();
   return @$y[5];
}
$x =& f();

It even acts OK.  The above code could be a mistake or it could be
a misguided attempt at optimization (believing that keeping a ref
to a temporary is better than setting $x equal to the temporary).
Maybe there was a desire to modify $y if applicable, but otherwise
be OK with modifying a temporary.  In any case, if you have a large 
enough system and code like that lying around, there's a good chance 
you'll get crashes.  You may not get a crash but instead see unrelated
data mysteriously changed in functions that get called later.
There are at least 2 ways to adjust the code above to make PHP
happy.  One is to remove the "&" in the assignment.  Another is
to create a "real" temporary variable instead of letting PHP make its
own temporary.  ie

function &f() {
   $y = array();
   $temp = @$y[5];
   return $temp;
}
$x =& f();

I've had crashes go away and mysterious data overwrites go away with
the only change to the system being replacing a piece of code like
the first block of this email with a piece of code like the second
block.  This has happened for just about every kind of "=& temp" you
can think of.  Here are a few examples (in addition to the case above)
of using temporaries as sources for references.  I think we've had a
crash / mysterious data overwrite for just about all of these (if not
all).

function f1() {
   $y = 5;
   return $y;
}
$x =& f1();  // Get rid of the & or add an & to f1

function &f2() {
   return NULL;   // Get rid of & below or use "$r = NULL; return $r;"
}
$x =& f2();

function g() {
   return NULL;
}
function h(&$x) {
   $x = 5;
}
h(g());

That last one could use some combination of the above recommendations
to be fixed.  For the record, I agree that there are few (if any) cases
where the above code is the best code for a design problem, but I also
dislike that these code blocks can lead to crashes / corruptions.

BTW, we have never had a problem with using a temporary reference as
the source for a non-reference copy.  That is to say the following has
never caused us a problem:

function &f() {
   $y = 5;
   return $y;
}
$x = f();

Best of luck,
Todd