Thread: help required

Please help me in understanding this code snippet-

Code:

main()
{
int a=2,*f1,*f2;
f1=f2=&a;
*f2+=*f2+=a+=2.5;
printf("\n%d %d %d",a,*f1,*f2);
}

the output is 16 16 16 but how is it working???????

The code exhibits undefined behavior, which means that anything can happen; practically speaking for this code it means there is no correct output; anything the compiler generates is acceptable because you broke the rules.

Briefly, you're not allowed to modify something multiple times in between sequence points. There are lots of things that count as sequence points, but for this snippet you can think of a sequence point as happening wherever there is a semicolon.

So you have:

Code:

*f2 += *f2 += a += 2.5;

*f2 and a are the same object, and you're modifying it three times here without any sequence points, thus the behavior is undefined.

A number of my compilers generate code that outputs "16 16 16", but I also have one that does "8 8 8" and another that does "12 12 12". These are all "correct" in the sense that anything is correct once you've done something undefined.

actually the Q has been asked in written test. and there was no option like compiler dependent. Anyway thanks for the reply...

actually the Q has been asked in written test. and there was no option like compiler dependent.

If this is the case, be warned that some of what you're learning in this course is incorrect. Your instructor likely does not know C especially well and is teaching you a variant of C that he learned, which is tied to a particular compiler.

I can't be certain about this, of course, but it's nowhere near as uncommon as one might hope. Be prepared to relearn some aspects of the language after you leave this class.