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

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 that you want the next number after.
y
A number that specifies the direction you want to go; see below.

## Library:

libm

Use the -l m option to qcc to link against this library.

## 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():

• Call feclearexcept(FE_ALL_EXCEPT) before calling nextafter(), nextafterf(), or nextafterl().
• On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, then an error has occurred.

## Returns:

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

• If x equals y, y (of the appropriate type) is returned.
• If x is finite, and the correct function value would overflow, a range error occurs, and the functions return ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL (with the same sign as x), depending on the function.
• If x or y is NaN, NaN is returned.
• If x != y, and the correct function value is subnormal, zero, or underflows, a range error occurs, and the correct function value (if representable) or 0.0 is returned.

## Errors:

FE_OVERFLOW
The result would cause an overflow.
FE_UNDERFLOW
The value of x isn't equal to y, and the correct function value is subnormal, zero, or underflows.

## 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_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW);
if(except_flags) {
/* An error occurred; handle it appropriately. */
}

feclearexcept(FE_ALL_EXCEPT);
c = nextafter(a, 1);
except_flags = fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW);
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
Interrupt handler No
Signal handler No