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Discussion on: Is "C Programming language" Still Worth Learning in 2021?

stokesm profile image
Matthew Stokes

Interesting. As far as I could read up because I didn't think it was either; in most cases the compiler will handle it as you expect, but it doesn't have to according to the spec which is why it is undefined?

There is no guarantee in the specification for c that the increment of i will be done when you use it as the third argument to printf(). So you could reasonably get 1, 1?

I may well have misunderstood though!

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pentacular profile image

I think you're imagining that the operations occur in an unspecified order, as would be the case for

foo(a(), b());

There is a sequence point when a call is executed, so a(), and b() occur in some distinct, if unspecified, order.

The program will not have undefined behavior, but may have unspecified behavior (if it depends on the order of those calls), but we can continue to reason about the C Abstract Machine for both cases.

foo(i, i++);

There is no sequence point between i and i++, so they occur at the same time, leading to a violation of the C Abstract Machine, producing undefined behavior.

We cannot reason about the program from this point onward.