On: The Little C Function From Hell

[DanielBMarkham]

I confess to just scanning the code and being cold on C right now, but isn't ++x a post increment? That is, it occurs after the rest of the expression has been evaluated? (as opposed to x++, which is a pre-increment) Just guessing that the perhaps the issue is when the actual overflow is occurring.

[acqq → DanielBMarkham]

Mnemonic: read as when you would read the text, left to right:

   ++x increment than use the value x. 
   x++ use the value x then increment.

[vhf → DanielBMarkham]

It is the opposite, like in many other languages. e.g.

  int i = 0;
  printf("%i %i", i++, ++i); // prints "0 2"

Same goes in C, C++, Java, PHP, ...

[EDIT] Turns out this is a bad example, as "the order in which function arguments are evaluated is undefined" (cf. below) Correct is :

  int i = 0;
  printf("%i", i++); // prints 0
  printf("%i", ++i); // prints 2

[decklin → vhf]

Actually, the order in which function arguments are evaluated is undefined. http://c-faq.com/expr/comma.html

You can use a comma in other expressions to introduce a sequence point: http://c-faq.com/~scs/cgi-bin/faqcat.cgi?sec=expr#seqpoints

[vhf → decklin]

Thanks for your clever comment and for those emitting similar concerns.

You surely are right. I wanted to give a quick example, turns out it was a bad one. Next time I'll write :

  int i = 0;
  printf("%i", i++); // prints 0
  printf("%i", ++i); // prints 2

[hythloday → vhf]

turns out it was a bad one

Given that you were making a point on an article about the complexity of C, I'd say it was an unintentionally excellent example.

[sirclueless → hythloday]

If you really want to make your brain hurt, there was an article on HN a while ago about the following statement:

    i = i++;

Evaluating that expression takes a real dive into the guts of the C spec.

[simias → vhf]

I believe you trigger an undefined behaviour there. You modify i twice in the same statement.

[jim_kaiser → vhf]

The standard says, evaluation order of function arguments is undefined.. but in general, printf evaluates right to left. This was a common puzzle as I remember.

[excuse-me → vhf]

Is that defined behaviour?

Is there any rule saying that args to a function have to be evaluated in a particular order - ie is ',' a function point?

[CodeMage → DanielBMarkham]

It's precisely the other way around: ++x is a pre-increment and x++ is a post-increment.

[dag11 → DanielBMarkham]

++x evaluates to x's new (incremented) value.

x++ evaluates to x's old (pre-incremented) value.

[loeg → DanielBMarkham]

Nope, you've got the two backwards.