Compute the next representable double-precision floating-point number
#include <math.h> double nextafter ( double x, double y); float nextafterf ( float x, float y ); long double nextafterl ( long double x, long double y);
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:
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.
#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
Safety: | |
---|---|
Cancellation point | No |
Interrupt handler | No |
Signal handler | No |
Thread | Yes |