[vorbis-dev] Libvorbis suggestion & floating point exception

Jan Lellmann Jan.Lellmann at web.de
Wed Sep 25 16:30:41 PDT 2002

Since nobody likes to reply, I'll do it myself :)

The floating point exception is a "divide by zero", which happens quite
often (and in several places) in libvorbis, the most prominent place being
fit_line called with one or zero points to fit.

I figured this does not affect some (most) people because of the floating
point exception handling turned off by their compilers/on their systems, so
I tried to just disable it and it actually seems to work.

So for anyone having the same problem, I inserted these two functions:


unsigned short FPU_Disable_Exceptions()
  unsigned short cw, cwnew;
  asm { FSTCW cw };
  cwnew = cw | 0x0005; // Disable divide by zero/invalid operation
  asm { FLDCW cwnew };
  return cw;

void FPU_Restore_Exceptions(unsigned short cw)
  asm { FCLEX; FLDCW cw };

--- snip ---

Then just put

  unsigned short fpucw = FPU_Disable_Exceptions();

at the beginning and


at the end of the function calling vorbis_analysis(). This will disable
exception handling and just return NaN when dividing by zero. The encoded
streams seem to be correct and flawless, though.

Note this is just a workaround and in no way portable code, I tested it only
under W2K, with Borland C++ Builder 5.


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