Path: utzoo!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!nosc!ucsd!ucbvax!hoptoad!tim
From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: serious code generation bug in Lightspeed C
Message-ID: <6275@hoptoad.uucp>
Date: 12 Jan 89 15:02:44 GMT
Organization: Eclectic Software, San Francisco
Lines: 29

There is a serious bug in how Lightspeed C handles the assignment
operation.  The lvalue is evaluated before the value is computed.  This
means that assigments to relocatable structure members do not work
correctly.

For example, suppose you have a relocatable data structure and a handle
to that structure in "h".  Further suppose that one member of the
structure referred to by "h" is of type Ptr, called "p".  Then this
straightforward statement gives an incorrect result:

	(*h)->p = NewPtr(size);

The address of "(*h)->p", the lvalue, is computed before the right-hand
side is evaluated.  The address of "(*h)->p" may change as a result of
the NewPtr trap, therefore it may be wrong on return from the trap and
the result of NewPtr may be assigned into invalid memory.

This is pretty amazing, even for a compiler like Think's that
deliberately produces low-budget, non-optimized code.  There is a
work-around, assigning the result of NewPtr into an intermediate
variable and then assigning it to the lvalue, but it is unwieldy, and
it is difficult to spot all places where this compiler bug could bite.
-- 
Tim Maroney, Consultant, Eclectic Software, sun!hoptoad!tim
"Gangsters would kidnap my math teacher, Miss Albertine, and I'd track them
 down and kill them one by one until she was free, and then she'd break off
 her engagement with my sarcastic English teacher, Mr. Richardson, because
 she'd fallen hopelessly in love with her grim-faced and silent
 fourteen-year-old savior." -- Nite Owl, in WATCHMEN by Alan Moore