Updated: 2003

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**#include <math.h>
**

**
double nextafter(double** *x***, double** *y***);
float nextafterf(float**

long double nextafterl(long double

double nexttoward(double

float nexttowardf(float

long double nexttowardl(long double

The *nextafter*(), *nextafterf*(), and *nextafterl*()
functions shall compute the next representable
floating-point value following *x* in the direction of *y*.
Thus, if *y* is less than *x*, *nextafter*()
shall return the largest representable floating-point number less
than *x*. The *nextafter*(), *nextafterf*(), and
*nextafterl*() functions shall return *y* if *x* equals
*y*.

The *nexttoward*(), *nexttowardf*(), and *nexttowardl*()
functions shall be equivalent to the corresponding
*nextafter*() functions, except that the second parameter shall
have type **long double** and the functions shall return
*y* converted to the type of the function if *x* equals *y*.

An application wishing to check for error situations should set *errno*
to zero and call
*feclearexcept*(FE_ALL_EXCEPT) before calling these functions.
On return, if *errno* is non-zero or
*fetestexcept*(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is non-zero, an error has occurred.

Upon successful completion, these functions shall return the next
representable floating-point value following *x* in the
direction of *y*.

If *x*== *y*, *y* (of the type *x*) shall be returned.

If *x* is finite and the correct function value would overflow,
a range error shall occur and ±HUGE_VAL,
±HUGE_VALF, and ±HUGE_VALL (with the same sign as *x*) shall
be returned as appropriate for the return type of
the function.

If
*x* or *y* is NaN, a NaN shall be returned.

If *x*!= *y* and the correct function value is subnormal,
zero, or underflows, a range error shall occur, and either
the correct function value (if representable) or 0.0 shall be returned.

These functions shall fail if:

- Range Error
- The correct value overflows.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then *errno* shall be set to [ERANGE]. If the
integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the overflow floating-point exception shall be
raised.

- Range Error
- The correct value is subnormal or underflows.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then *errno* shall be set to [ERANGE]. If the
integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the underflow floating-point exception shall be
raised.

*The following sections are informative.*

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.

*feclearexcept*(), *fetestexcept*(), the Base
Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment
of
Error Conditions for Mathematical Functions, *<math.h>*