# nextafter(), nextafterf(), nextafterl()

QNX SDP8.0C Library ReferenceAPIDeveloper

*Compute the next representable double-precision 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`);

## Arguments:

`x`- The number for which you want the next number after.
`y`- A number that specifies the direction you want to go; see below.

## Library:

- libm
- The general-purpose math library.
- libm-sve
- A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

- If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
- If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.

Note:

Compile your program with the -fno-builtin option to prevent the compiler from using a
built-in version of the function.

## Description:

The nextafter(), nextafterf(), and nextafterl() functions
compute the next representable double-precision floating-point value
following `x` in the direction of `y`.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

- Call
`feclearexcept(FE_ALL_EXCEPT)`

before calling nextafter(), nextafterf(), or nextafterl(). - On return, if
`fetestexcept(FE_ALL_EXCEPT)`

is nonzero, then an error has occurred.

## Returns:

The next machine floating-point number of `x` in the
direction towards `y`.

If | These functions return: | Errors: |
---|---|---|

x equals y |
y |
— |

x is finite, and the correct value would overflow |
Inf | FE_OVERFLOW |

x or y is NaN |
NaN | — |

x != y, and the correct value is subnormal, zero, or underflows |
The correct value (if representable), or 0.0 | FE_UNDERFLOW |

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

## Examples:

```
#include <stdio.h>
#include <inttypes.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>
void dump_to_hex(double d) {
printf("0x%08x %08x \n",
(uint32_t)(*((uint64_t*)&d) >> 32),
(uint32_t)(*((uint64_t*)&d)));
}
int main(int argc, char** argv)
{
double a, b, c;
int except_flags;
a = 0;
feclearexcept(FE_ALL_EXCEPT);
b = nextafter(a, -1);
except_flags = fetestexcept(FE_ALL_EXCEPT);
if(except_flags) {
/* An error occurred; handle it appropriately. */
}
feclearexcept(FE_ALL_EXCEPT);
c = nextafter(a, 1);
except_flags = fetestexcept(FE_ALL_EXCEPT);
if(except_flags) {
/* An error occurred; handle it appropriately. */
}
printf("Next possible value before %f is %f \n", a, b);
printf("-->"); dump_to_hex(a);
printf("-->"); dump_to_hex(b);
printf("Next possible value after %f is %f \n", a, c);
printf("-->"); dump_to_hex(a);
printf("-->"); dump_to_hex(c);
return EXIT_SUCCESS;
}
```

produces the output:

```
Next possible value before 0.000000 is 0.000000
-->0x00000000 00000000
-->0x80000000 00000001
Next possible value after 0.000000 is 0.000000
-->0x00000000 00000000
```

## Classification:

Safety: | |
---|---|

Cancellation point | No |

Signal handler | Yes |

Thread | Yes |

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