I have seen some very weird for loops when reading other people's code. I have been trying to search for a full syntax explanation for the for loop in C but it is very hard because the word "for" appears in unrelated sentences making the search almost impossible to Google effectively.

This question came to my mind after reading this thread which made me curious again.

The for here:

for(p=0;p+=(a&1)*b,a!=1;a>>=1,b<<=1);

In the middle condition there is a comma separating the two pieces of code, what does this comma do? The comma on the right side I understand as it makes both a>>=1 and b<<=1.

But within a loop exit condition, what happens? Does it exit when p==0, when a==1 or when both happen?

It would be great if anyone could help me understand this and maybe point me in the direction of a full for loop syntax description.