Obscure C code (was Re: Excessive optimization)

On Thu, Dec 2, 2010 at 11:15 PM, Chuck Guzis <cclist at sydex.com> wrote: There was a good example of that sort of thing and an intentionally obsoleted feature of C interacting badly in a VAX-C tutorial... It was something like... main() { int i; i=-1; printf("i is %d\n", i); } ... it worked *if* you loaded the program with the debugger, otherwise you got a seemingly large and random value for i. The trick is the "=-" was originally (like c. 1972) the syntax for what's now "-=" and that took precedence over the seemingly obvious interpretation of "i = -1" (note the spaces). The reason it worked in the debugger was that as part of the debugging process, the program loader would clear out one page of the stack frame with zeros so you could see autovariables populate the stack area, so the result was a zero in memory "minus equals" one, or negative one (-1). Later versions of VAX-C (and other C compilers) complain about non-initialized autovariables and obsolete language features (like =- and =+), but the compiler at the time did something odd but quasi-legal and did it quietly. -ethan