C language policy on Pre-increment and Post-increment

Question

Consider the following code-snippet in C:

int v = 10;
int z = v;
v = v++ + ++v;
printf("v = %d\n",v); // gives 23
printf("z = %d\n",z++ + ++z); // gives 22

Why does this happen?

Answer 1

Because of undefined behavior, missing sequence point.