Obscure C code (was Re: Excessive optimization)

Eric Smith eric at brouhaha.com
Thu Dec 2 18:24:43 CST 2010

Fred Cisin wrote:
> Of course your example would work.
> I'm assuming that it was created by mistake, not a deliberate attempt to
> print 14 characters from pseudo-random places in memory.
> When you shoot yourself in the foot, C has no "gun-control".
> What would an excessive optimization compiler change it to?
By definition, if it is "excessive" optimization, it might do anything!

> A[i++] = i++;
> Many people think that they know what that will do.  They may even be
> right for THEIR compiler, but I don't think that even ANSI C is a rigid
> enough specification to pin down the sequence of certain actions.
I can't quote chapter and verse, but I'm fairly sure the standard will 
say that the result of that assignment is undefined.

In pre-standard C, it is also undefined, but for a different reason.

Many programmers would say "I would never write code like that", but 
I've seen statements like that creep into code as a result of 
cut-and-paste or global-search-and-replace operations that weren't 
carefully checked.  It's also quite possible when writing complex 
expressions to come up with things like that which are not necessarily 
obvious to casual inspection.  It is good that many compilers will 
detect these things and generate warnings.  When I have the power to do 
so, I generally treat warnings from the compiler as fatal errors 
requiring a fix, and not acceptable in released code.


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