fabs, floor, ceil, rint - absolute value, floor, ceiling, and
round-to-nearest functions

#include <math.h>

double floor(x)
double x;

double ceil(x)
double x;

double fabs(x)
double x;

double rint(x)
double x;

Fabs returns the absolute value |x|.

Floor returns the largest integer no greater than x.

Ceil returns the smallest integer no less than x.

Rint returns the integer (represented as a double precision number)
nearest x in the direction of the prevailing rounding mode.

On a VAX, rint(x) is equivalent to adding half to the magnitude and
then rounding towards zero.

In the default rounding mode, to nearest, on a machine that
conforms to IEEE 754, rint(x) is the integer nearest x with the
additional stipulation that if |rint(x)-x|=1/2 then rint(x) is
even. Other rounding modes can make rint act like floor, or like
ceil, or round towards zero.

Another way to obtain an integer near x is to declare (in C)
double x; int k; k = x;
Most C compilers round x towards 0 to get the integer k, but some
do otherwise. If in doubt, use floor, ceil, or rint first,
whichever you intend. Also note that, if x is larger than k can
accommodate, the value of k and the presence or absence of an
integer overflow are hard to predict.

abs(3), ieee(3), libm(3)