Is the second int in a multiple declaration always set to 1?

Question

In this Code Golf post, there is the claim that “the second variable in a definition is always set to 1“, making this a well-formed line:

int i=-1,c,o,w,b,e=b=w=o=c;

And supposedly everything except i is set to 1 because c is automatically 1.

I thought I knew some C, and thought this was illegal (being UB and resulting if anything in random stack contents).

Does C really set c to 1?

Answer 1

I'm the OP from CodeGolf. It seems I simply had a typo, I meant to say int i=-1,c,o,w,b,e=b=w=o=c=1; That way the second defined int is always set to 1 and the others can be set to it. The confusion is that I originally had the variable that comes next (L=3) as just l (undefined) and I was setting all of the other variables to e=b=w=o=c=(L=3); which in my mind was going to set L equal to 3, return true for that (1), then set the rest to 1.

A few tests later I realized this was just setting them all to 3 and only worked with the specific string I was using to test my code. So I deleted them and changed it to just be L=3 hard coded and the others to be e=b=w=o=c=1;L=3. At some point I must have pressed cmd+z one too many times and removed the "=" and the "1" so I was just left with e=b=w=o=c;. Due to the consistent undefined nature of this (at least on my IDE) it was always defining them as 0 and therefor the bug went un-noticed.

Now that I've corrected it back, thanks to this post, the byte lengths are the same and there was no need for any of this tricky e=b=w=o=c=1 code anyways, I only thought the byte length was different because when I copy pasted my function into a byte counter it showed it was 2 bytes smaller (I didn't know I just had a typo and was missing 2 bytes).

My IDE is always defining those variables as 0. My code is designed to work with all of the variables being defined as 1, the fact that it works w/ 0 is coincidence. Also just because it happens on my IDE doesn't mean it will on others, though I have tested it on a few IDEs now online and run many loops and it does seem to always return 0. In any event, I've still updated my original code to set them to 1 as it should be (adding 2 bytes to my program).

Thanks for everyone's input

Answer 2

This code exhibits undefined behavior.

The variables c, o, b, and w are uninitialized. That means their contents are indeterminate.

From section 6.7.9 of the C standard:

10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

The indeterminate value of c is then assigned to several other variables. By reading the value of an uninitialized variable, the code invokes undefined behavior.

The initial value of c could be 1, but if so it's not a predictable value.

Also note that the above statement contains both initialization (for i and e) and assignment (for c, o, b, and w), so this statement won't compile at file scope.

I attempted to run the function in the linked post and it didn't pass the first test input. Undefined behavior.