Question

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Undefined Behavior and Sequence Points

What is the outcome of the following code:

#include <stdio.h>
int main() {
  int a = 3;
  a= (a = 2) + (a = 3);
  printf("%d", a);
}

Why do I get 6 as the output on gcc? Why not 5?

Answer 1

You’re both writing and reading variable a between sequence points, so the result is formally undefined behavior.

Looking at the assembly code generated by your particular compiler will make it clear why you get a particular result, but the standard makes no guarantees at all.