C ternary operator branches evaluation?

Question

I always assumed the ternary operator in C did not evaluate the branch that failed the test. Why does it in this case? a is less than b so only c should be assigned to 1, d should remain 2. Thank your for tips and suggestions. I have compiled with both gcc-9 and clang.

#include <stdio.h>

int main() {
  int a = 42;
  int b = 99;
  int c = 0;
  int d = 2;

  // Both branches are evaluated?
  a < b ? c, c = 1 : d, d = 1;

  printf("c %d, d %d.\n", c, d);
  // Prints c 1, d 1.
}

Answer 1

The comma operator has lower precedence than the conditional operator, so your expression is equivalent to:

(a < b ? c, c = 1 : d), d = 1;