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  HYPOT(3) OpenBSD Programmer's Manual HYPOT(3)
  NAME
  hypot, hypotf, cabs, cabsf - euclidean distance and complex absolute val-
  ue functions
  SYNOPSIS
  #include <math.h>
  double
  hypot(double x, double y);
  float
  hypotf(float x, float y);
  double
  cabs(struct complex { double x; double y; } z);
  float
  cabsf(struct complex { double x; double y; } z);
  DESCRIPTION
  The hypot() and cabs() functions computes the sqrt(x*x+y*y) in such a way
  that underflow will not happen, and overflow occurs only if the final re-
  sult deserves it.
  hypot(infinity, v) = hypot(v, infinity) = +infinity for all v, including
  NaN.
  ERROR (due to Roundoff, etc.)
  Below 0.97 ulps. Consequently hypot(5.0, 12.0) = 13.0 exactly; in gener-
  al, hypot and cabs return an integer whenever an integer might be expect-
  ed.
  The same cannot be said for the shorter and faster version of hypot and
  cabs that is provided in the comments in cabs.c; its error can exceed 1.2
  ulps.
  NOTES
  As might be expected, hypot(v, NaN) and hypot(NaN, v) are NaN for all
  finite v; with "reserved operand" in place of "NaN", the same is true on
  a VAX. But programmers on machines other than a VAX (if has no infinity)
  might be surprised at first to discover that hypot(+-infinity, NaN) =
  +infinity. This is intentional; it happens because hypot(infinity, v) =
  +infinity for all v, finite or infinite. Hence hypot(infinity, v) is in-
  dependent of v. Unlike the reserved operand fault on a VAX, the IEEE NaN
  is designed to disappear when it turns out to be irrelevant, as it does
  in hypot(infinity, NaN).
  BUGS
  The cabs() and cabsf() functions use structures that are not defined in
  any header and need to be defined by the user. As such they cannot be
  prototyped properly.
  SEE ALSO
  math(3), sqrt(3)
  HISTORY
  Both a hypot() function and a cabs() function appeared in Version 7 AT&T
  UNIX.
  OpenBSD 2.6 May 6, 1991 1