Comma and assignment operators in C

Question

I have the following:

int a = 1, b = 2, c = 3, d = 4;
a = b, c = d;
printf("%d, %d, %d, %d", a, b, c, d);

The output is:

2, 2, 4, 4

How does the comma operator work with assignment operators? From what I have known it would evaluate and return the second expression if we have,

(exp1, exp2)

So, why would it evaluate a = b ?

Answer 1

The first operand is evaluated and the result discarded. The second operand is then evaluated, and the result returned as the overall result of the expression.

The standard says:

The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

Answer 2

The comma operator is lower precedence than assignment. All expressions in a comma operator are evaluated, but only the last is used as the resulting value. So both assignments are performed. The result of the comma operator in your case would be the result of c = d. This result is not used.