[cfp-interest 3399] nexttoward

[Paul Zimmermann]

       Hi,

I have a question about nexttoward(). The Linux man page says:

       double nexttoward(double x, long double y);

...

       If x is not equal to y, and the correct function result would be subnor‐
       mal, zero, or underflow, a range error occurs, and  either  the  correct
       value (if it can be represented), or 0.0, is returned.

For example nexttoward (0.0, 1.0) will set errno = ERANGE.

I wonder why this is so, since for mathematical functions, underflow is not
raised (and thus errno is not set to ERANGE) when the result is exact.
And nexttoward always delivers an exact result.

Paul

PS: moreover the GNU libc does not seem to follow the man page, it seems
errno is not set, but underflow is raised:

$ cat e.c
#include <stdio.h>
#include <errno.h>
#include <math.h>
#include <fenv.h>

int
main()
{
  double x = 0.0, y = 1.0;
  printf ("errno=%d\n", errno);
  printf ("underflow: %d\n", fetestexcept (FE_UNDERFLOW));
  double z = nexttoward (x, y);
  printf ("errno=%d\n", errno);
  printf ("underflow: %d\n", fetestexcept (FE_UNDERFLOW));
  printf ("z=%la\n", z);
}

$ gcc -fno-builtin e.c -lm && ./a.out
errno=0
underflow: 0
errno=0
underflow: 16
z=0x0.0000000000001p-1022