How is this behaviour of C explained?

Question

so I was playing around with recursion in C and I am not sure why is this happening:
code A

int foo(int x)
{
 if (x==0) return 0;
 else return foo(--x)+x;
}

int main() { printf("%d\n", foo(10));

code B

int foo(int x)
{
 if (x==0) return 0;
 else return foo(x--)+x;
}

int main() { printf("%d\n", foo(10));

Code A prints 45 instead of 55 and I figured that out. It is because the recursive call gets unwounded this way: 9+8+7+...+0 = 45

Code B on the other hand gets stuck and never returns to the prompt!! i have to ctrl+c it. Why does it get stuck? Is it because it never decrements past 10?

Answer 1

That's because in snippet B x gets decremented after the recursive call so the function is always called with foo(10)

EDIT:
As James McNellis pointed out in its comment x gets decremented before calling the function but its original value is used in the call instead of the decremented one.
Basically the expression x-- is evaluated before foo(x--) but its value is actually x, therefore the function is called with x = 10