Question

Reading through the C specs I found this function:

double remquo(double x, double y, int *quo);
float remquof(float x, float y, int *quo);
long double remquol(long double x, long double y,
    int *quo);

The remquo functions compute the same remainder as the remainder functions. In
the object pointed to by quo they store a value whose sign is the sign of x/y and whose
magnitude is congruent modulo 2^n
to the magnitude of the integral quotient of x/y, where
n is an implementation-defined integer greater than or equal to 3.

The remquo functions return x REM y. If y is zero, the value stored in the object
pointed to by quo is unspecified and whether a domain error occurs or the functions
return zero is implementation defined.

I understand what it returns, it returns fmod(x, y), but I don’t understand the whole quo part. Is it semantically equal to this?

*quo = (int) x/y;
*quo %= n; /* n implementation defined */

And my last question, for what could this function be useful?

Answer 1

EDIT: As Jeffrey Scofield said in his answer, the returned quotient really isn’t x/y, but the low 3 bits (plus sign) of the quotient.

It’s equivalent to (up to type differences):

quo = x/y;
rem = x%y;

Where rem is the return value, and quo is returned as an output parameter.
Its advantage over the above syntax is that it does just one division operation.