int fesetround (int ROUND) returns 0 if ok, 1 if not ok

[Conrado Badenas]

Dear glibc maintainer,

I sent this same bug report to the maintainer of the Debian packages
libc6 and glibc-doc, but I have not obtained an answer in six weeks.
Today I have decided to download the file glibc-2.1.3.tar.gz (Feb 25
2000 8803k) from a GNU mirror site. I have examined the source files,
and now I know what happens:

$ info libc Rounding
[...]
 - Function: int fesetround (int ROUND)
     Changes the currently selected rounding mode to ROUND.  If ROUND
     does not correspond to one of the supported rounding modes nothing
     is changed.  `fesetround' returns a nonzero value if it changed
     the rounding mode, zero if the mode is not supported.
[...]

but...

$ cat nothing.c
#include <stdio.h>
#include <fenv.h>
void setandprintround(int i) {
        printf ("%.8x (%d) ==> ",i,fesetround(i));
        i=fegetround();
        if (i==FE_TONEAREST)       printf("To nearest\n");
        else if (i==FE_UPWARD)     printf("Upward\n");
        else if (i==FE_DOWNWARD)   printf("Downward\n");
        else if (i==FE_TOWARDZERO) printf("Toward 0\n");
        else                       printf("UNKNOWN!!!\n");
}
main() {
        int i;
        setandprintround(rand());
        setandprintround(FE_TOWARDZERO);
        setandprintround(FE_DOWNWARD);
        setandprintround(FE_UPWARD);
        setandprintround(FE_TONEAREST);
}
$ gcc nothing.c -o nothing -lm
$ nothing
6b8b4567 (1) ==> To nearest
00000c00 (0) ==> Toward 0
00000400 (0) ==> Downward
00000800 (0) ==> Upward
00000000 (0) ==> To nearest

fesetround() returns 0 if it changes the rounding mode, and 1 (not zero)
if it can't change the rounding mode.

The file glibc-2.1.3/sysdeps/i386/fpu/fesetround.c contains:
fesetround (int round)
{
  unsigned short int cw;

  if ((round & ~0xc00) != 0)
    /* ROUND is no valid rounding mode.  */
    return 1;

  __asm__ ("fnstcw %0" : "=m" (*&cw));
  cw &= ~0xc00;
  cw |= round;
  __asm__ ("fldcw %0" : : "m" (*&cw));

  return 0;
}

which means that fesetround returns 0 if ok, and 1 if not ok. The files
fesetround.c for other architectures do the same (0 if ok, 1 of not ok).

Thus, the glibc documentation should be changed to contain
 - Function: int fesetround (int ROUND)
     Changes the currently selected rounding mode to ROUND.  If ROUND
     does not correspond to one of the supported rounding modes nothing
     is changed.  `fesetround' returns zero if it changed the rounding
     mode, a nonzero value if the mode is not supported.

Thank you very much for your patiente.

-- 
Conrado Badenas <address@hidden>
PhD student                  | Assistant Lecturer
Department of Thermodynamics | Department of Exp. Sciences
Faculty of Physics           | Univ. School of Tech. and Exp. Sciences
University of Valencia       | University Jaume I
c/. Dr. Moliner, 50          | Campus del Riu Sec
46100 Burjassot (Valencia)   | 12071 Castellón
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