extern void abort (void);

typedef struct foo_t
{ 
  unsigned int blksz;
  unsigned int bf_cnt; 
} foo_t;

#define _RNDUP(x, unit)  ((((x) + (unit) - 1) / (unit)) * (unit))
#define _RNDDOWN(x, unit)  ((x) - ((x)%(unit)))

long long
foo (foo_t *const pxp,  long long offset, unsigned int extent)
{
  long long blkoffset = _RNDDOWN(offset, (long long )pxp->blksz);
  unsigned int diff = (unsigned int)(offset - blkoffset);
  unsigned int blkextent = _RNDUP(diff + extent, pxp->blksz);

  if (pxp->blksz < blkextent)
    return -1LL;

  if (pxp->bf_cnt > pxp->blksz)
    pxp->bf_cnt = pxp->blksz;

  return blkoffset;
}

int
main ()
{
  foo_t x;
  long long xx;

  x.blksz = 8192;
  x.bf_cnt = 0;
  xx = foo (&x, 0, 4096);
  if (xx != 0LL)
    abort ();
  return 0;
}