[Insight-developers] nrrd assumptions and signed overflow

[Sean McBride]

Hi all,

Many of the "nrrd" tests are failing on my clang dashboard, and I took a look at why... There is this function:

****** nrrdSanity
**
** makes sure that all the basic assumptions of nrrd hold for
** the architecture/etc which we're currently running on.  
** 
** returns 1 if all is okay, 0 if there is a problem

which does this:

  tmpLLI = NRRD_LLONG_MAX;
  if (tmpLLI != NRRD_LLONG_MAX) {
    return 0;
  }

  tmpLLI += 1; // problem here
  
  if (NRRD_LLONG_MIN != tmpLLI) {
    return 0;
  }
  tmpULLI = NRRD_ULLONG_MAX;

"tmpLLI" is a signed long long.  They are adding 1 to the largest possible signed long long, and signed overflow is undefined in C/C++.  The compiler can do whatever it wants in the case of undefined behaviour, and with the flags I'm using it deliberately generates an illegal instruction.

I stopped investigating here, since from the comments of this method, it seems that nrrd requires/expects that the compiler wraps signed values.

Anyone know this nrrd stuff well?

Cheers,

-- 
____________________________________________________________
Sean McBride, B. Eng                 sean at rogue-research.com
Rogue Research                        www.rogue-research.com 
Mac Software Developer              Montréal, Québec, Canada