nextafter, nextafterf, nextafterl, nexttoward, nexttowardf, nexttowardl — floating-point number manipulation


#include <math.h>
double nextafter( double x,
  double y);
float nextafterf( float x,
  float y);
long double nextafterl( long double x,
  long double y);
double nexttoward( double x,
  long double y);
float nexttowardf( float x,
  long double y);
long double nexttowardl( long double x,
  long double y);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
or cc -std=c99
nextafterf(), nextafterl():
or cc -std=c99
nexttoward(), nexttowardf(), nexttowardl():
_XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99
[Note] Note

Link with −lm.


The nextafter() functions return the next representable floating-point value following x in the direction of y. If y is less than x, these functions will return the largest representable number less than x.

If x equals y, the functions return y.

The nexttoward() functions do the same as the nextafter() functions, except that they have a long double second argument.


On success, these functions return the next representable floating-point value after x in the direction of y.

If x equals y, then y (cast to the same type as x) is returned.

If x or y is a NaN, a NaN is returned.

If x is finite, and the result would overflow, a range error occurs, and the functions return HUGE_VAL, HUGE_VALF, or HUGE_VALL, respectively, with the correct mathematical sign.

If x is not equal to y, and the correct function result would be subnormal, zero, or underflow, a range error occurs, and either the correct value (if it can be represented), or 0.0, is returned.


See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

Range error: result overflow

An overflow floating-point exception (FE_OVERFLOW) is raised.

Range error: result is subnormal or underflows

An underflow floating-point exception (FE_UNDERFLOW) is raised.

These functions do not set errno.


C99, POSIX.1-2001. This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).


In glibc version 2.5 and earlier, these functions do not raise an underflow floating-point (FE_UNDERFLOW) exception when an underflow occurs.




This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright 2002 Walter Harms (
and Copyright 2008, Linux Foundation, written by Michael Kerrisk

Distributed under GPL

Based on glibc infopages