# nextafter - Man Page

next representable floating-point number

## Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

## Synopsis

#include <math.h> double nextafter(doublex, doubley); float nextafterf(floatx, floaty); long double nextafterl(long doublex, long doubley); double nexttoward(doublex, long doubley); float nexttowardf(floatx, long doubley); long double nexttowardl(long doublex, long doubley);

## Description

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

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.

## Return Value

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

the correct function value (if representable) or

0.0 shall be returned.

## Errors

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.*

## Examples

None.

## Application Usage

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.

When *<tgmath.h>* is included, note that the return type of *nextafter*() depends on the generic typing deduced from both arguments, while the return type of *nexttoward*() depends only on the generic typing of the first argument.

## Rationale

None.

## Future Directions

None.

## See Also

feclearexcept(), fetestexcept()

The Base Definitions volume of POSIX.1-2017, *Section 4.20*, *Treatment of Error Conditions for Mathematical Functions*, <math.h>, <tgmath.h>

## Copyright

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

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