On 01/03/2010 04:54 AM, Andris Kalnozols wrote: > Since multiple assignments are legal and evaluated from right to left, > one could expect the following to work: > > pcptr->code = pcptr = nop; I wouldn't. :-) > It does *not* work, however, using the gcc 3.3 or 4.3 compilers. > To summarize: > > pcptr = pcptr->code = nop; /* crashes with no compiler warning */ > pcptr->code = pcptr = nop; /* crashes with the warning: > * operation on `pcptr' may be undefined > */ > pcptr->code = (pcptr = nop); /* same as above */ > pcptr = nop; pcptr->code = nop; /* this works */ > > FWIW, the HP-UX 11.31 compiler warns/not warns in the same > way but the compiled program is "luckier" at run time. > > Perhaps the compiler should regard the above bootstrapping-style > multiple assignments as ambiguous and issue the warning regardless > of the order in which the targets appear. Well, gcc doesn't necessarily know. We do a fair bit of analysis in an attempt to help the programmer, and we warn when we reasonably can, but gcc isn't omniscient. In this case, though, you were burnt by a pointlessly obscure coding style, combined with a misunderstanding of the language. If you write the above code as pcptr = nop; pcptr->code = nop; it'll always work, and its semantics are obvious. Andrew.
