NAME

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

SYNOPSIS

#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);


DESCRIPTION

^[CX] 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.1-2024 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.

^[MX] Even though underflow or overflow can occur, the returned value shall be independent of the current rounding direction mode.

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.

^[MX] If x or y is NaN, a NaN shall be returned.

^[MX] If x!=y and the correct function value is subnormal, zero, or underflows, a range error shall occur, and
^[MXX] the correct function value (if representable) or
^[MX] 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

^[MX] 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.

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

XBD 4.23 Treatment of Error Conditions for Mathematical Functions, <math.h>, <tgmath.h>

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 6

The nextafter() function is no longer marked as an extension.

The nextafterf(), nextafterl(), nexttoward(), nexttowardf(), and nexttowardl() functions are added for alignment with the ISO/IEC 9899:1999 standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0407 [68] and XSH/TC1-2008/0408 [357] are applied.

Issue 8

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.