div() in C: Getting Quotient and Remainder from a Single Division Operation

div takes an integer numerator and denominator and returns a div_t struct containing both the quotient and the remainder. On most hardware, the division instruction produces both results simultaneously. If you write x / y and x % y separately, the compiler might optimize them into one instruction, or it might not. div guarantees one operation. For long and long long, use ldiv and lldiv.

The quotient truncates toward zero, matching the / operator. The remainder takes the sign of the dividend, matching the % operator. The invariant is: numerator = quot * denominator + rem. This holds for all inputs except division by zero, which is undefined behavior. div does not check for zero. Neither does the hardware.

div is a micro-optimization that also improves readability when you need both results. Instead of computing the division twice, you compute it once and destructure the result. Next: modf, which splits a floating-point number into integer and fractional parts.