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 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: (QNX Neutrino 7.1 or later) 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:

C11, POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	Yes
Signal handler	Yes
Thread	Yes