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#### 20.8.3 Rounding Functions

The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating point numbers to integer values. They are all declared in math.h.

You can also convert floating-point numbers to integers simply by casting them to `int`. This discards the fractional part, effectively rounding towards zero. However, this only works if the result can actually be represented as an `int`—for very large numbers, this is impossible. The functions listed here return the result as a `double` instead to get around this problem.

— Function: double ceil (double x)
— Function: float ceilf (float x)
— Function: long double ceill (long double x)

These functions round x upwards to the nearest integer, returning that value as a `double`. Thus, `ceil (1.5)` is `2.0`.

— Function: double floor (double x)
— Function: float floorf (float x)
— Function: long double floorl (long double x)

These functions round x downwards to the nearest integer, returning that value as a `double`. Thus, ```floor (1.5)``` is `1.0` and `floor (-1.5)` is `-2.0`.

— Function: double trunc (double x)
— Function: float truncf (float x)
— Function: long double truncl (long double x)

The `trunc` functions round x towards zero to the nearest integer (returned in floating-point format). Thus, `trunc (1.5)` is `1.0` and `trunc (-1.5)` is `-1.0`.

— Function: double rint (double x)
— Function: float rintf (float x)
— Function: long double rintl (long double x)

These functions round x to an integer value according to the current rounding mode. See Floating Point Parameters, for information about the various rounding modes. The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicitly select another.

If x was not initially an integer, these functions raise the inexact exception.

— Function: double nearbyint (double x)
— Function: float nearbyintf (float x)
— Function: long double nearbyintl (long double x)

These functions return the same value as the `rint` functions, but do not raise the inexact exception if x is not an integer.

— Function: double round (double x)
— Function: float roundf (float x)
— Function: long double roundl (long double x)

These functions are similar to `rint`, but they round halfway cases away from zero instead of to the nearest even integer.

— Function: long int lrint (double x)
— Function: long int lrintf (float x)
— Function: long int lrintl (long double x)

These functions are just like `rint`, but they return a `long int` instead of a floating-point number.

— Function: long long int llrint (double x)
— Function: long long int llrintf (float x)
— Function: long long int llrintl (long double x)

These functions are just like `rint`, but they return a `long long int` instead of a floating-point number.

— Function: long int lround (double x)
— Function: long int lroundf (float x)
— Function: long int lroundl (long double x)

These functions are just like `round`, but they return a `long int` instead of a floating-point number.

— Function: long long int llround (double x)
— Function: long long int llroundf (float x)
— Function: long long int llroundl (long double x)

These functions are just like `round`, but they return a `long long int` instead of a floating-point number.

— Function: double modf (double value, double *integer-part)
— Function: float modff (float value, float *integer-part)
— Function: long double modfl (long double value, long double *integer-part)

These functions break the argument value into an integer part and a fractional part (between `-1` and `1`, exclusive). Their sum equals value. Each of the parts has the same sign as value, and the integer part is always rounded toward zero.

`modf` stores the integer part in `*`integer-part, and returns the fractional part. For example, `modf (2.5, &intpart)` returns `0.5` and stores `2.0` into `intpart`.