nexttoward(), nexttowardf(), nexttowardl()

Compute the next representable floating-point number

Synopsis:

#include <math.h>

double nexttoward ( double x,
                    long double y);

float nexttowardf ( float x,
                    long double y );

long double nexttowardl ( 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 nexttoward(), nexttowardf(), and nexttowardl() 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():

Returns:

The next machine floating-point number of x in the direction towards y, converted to the type that the function returns.

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 = nexttoward(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 = nexttoward(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:

ANSI, POSIX 1003.1

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