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IEEE(3)		       FreeBSD Library Functions Manual		       IEEE(3)

NAME
     ieee -- IEEE standard 754 for floating-point arithmetic

DESCRIPTION
     The IEEE Standard 754 for Binary Floating-Point Arithmetic	defines	repre-
     sentations	of floating-point numbers and abstract properties of arith-
     metic operations relating to precision, rounding, and exceptional cases,
     as	described below.

   IEEE	STANDARD 754 Floating-Point Arithmetic
     Radix: Binary.

     Overflow and underflow:
	   Overflow goes by default to a signed	infinity.  Underflow is
	   gradual.

     Zero is represented ambiguously as	+0 or -0.
	   Its sign transforms correctly through multiplication	or division,
	   and is preserved by addition	of zeros with like signs; but x-x
	   yields +0 for every finite x.  The only operations that reveal
	   zero's sign are division by zero and	copysign(x, +-0).  In particu-
	   lar,	comparison (x >	y, x >=	y, etc.) cannot	be affected by the
	   sign	of zero; but if	finite x = y then infinity = 1/(x-y) !=
	   -1/(y-x) = -infinity.

     Infinity is signed.
	   It persists when added to itself or to any finite number.  Its sign
	   transforms correctly	through	multiplication and division, and
	   (finite)/+-infinity = +-0 (nonzero)/0 = +-infinity.	But infin-
	   ity-infinity, infinity*0 and	infinity/infinity are, like 0/0	and
	   sqrt(-3), invalid operations	that produce NaN. ...

     Reserved operands (NaNs):
	   An NaN is (Not a Number).  Some NaNs, called	Signaling NaNs,	trap
	   any floating-point operation	performed upon them; they are used to
	   mark	missing	or uninitialized values, or nonexistent	elements of
	   arrays.  The	rest are Quiet NaNs; they are the default results of
	   Invalid Operations, and propagate through subsequent	arithmetic
	   operations.	If x !=	x then x is NaN; every other predicate (x > y,
	   x = y, x < y, ...) is FALSE if NaN is involved.

     Rounding:
	   Every algebraic operation (+, -, *, /, \/) is rounded by default to
	   within half an ulp, and when	the rounding error is exactly half an
	   ulp then the	rounded	value's	least significant bit is zero.	(An
	   ulp is one Unit in the Last Place.)	This kind of rounding is usu-
	   ally	the best kind, sometimes provably so; for instance, for	every
	   x = 1.0, 2.0, 3.0, 4.0, ...,	2.0**52, we find (x/3.0)*3.0 ==	x and
	   (x/10.0)*10.0 == x and ...  despite that both the quotients and the
	   products have been rounded.	Only rounding like IEEE	754 can	do
	   that.  But no single	kind of	rounding can be	proved best for	every
	   circumstance, so IEEE 754 provides rounding towards zero or towards
	   +infinity or	towards	-infinity at the programmer's option.

     Exceptions:
	   IEEE	754 recognizes five kinds of floating-point exceptions,	listed
	   below in declining order of probable	importance.

		 Exception	      Default Result
		 Invalid Operation    NaN, or FALSE
		 Overflow	      +-infinity
		 Divide	by Zero	      +-infinity
		 Underflow	      Gradual Underflow
		 Inexact	      Rounded value

	   NOTE: An Exception is not an	Error unless handled badly.  What
	   makes a class of exceptions exceptional is that no single default
	   response can	be satisfactory	in every instance.  On the other hand,
	   if a	default	response will serve most instances satisfactorily, the
	   unsatisfactory instances cannot justify aborting computation	every
	   time	the exception occurs.

   Data	Formats
     Single-precision:
	   Type	name: float

	   Wordsize: 32	bits.

	   Precision: 24 significant bits, roughly like	7 significant deci-
	   mals.
		 If x and x' are consecutive positive single-precision numbers
		 (they differ by 1 ulp), then
		 5.9e-08 < 0.5**24 < (x'-x)/x <= 0.5**23 < 1.2e-07.

	   Range: Overflow threshold  =	2.0**128 = 3.4e38
		  Underflow threshold =	0.5**126 = 1.2e-38
		 Underflowed results round to the nearest integer multiple of
		 0.5**149 = 1.4e-45.

     Double-precision:
	   Type	name: double
		 On some architectures,	long double is the same	as double.

	   Wordsize: 64	bits.

	   Precision: 53 significant bits, roughly like	16 significant deci-
	   mals.
		 If x and x' are consecutive positive double-precision numbers
		 (they differ by 1 ulp), then
		 1.1e-16 < 0.5**53 < (x'-x)/x <= 0.5**52 < 2.3e-16.

	   Range: Overflow threshold  =	2.0**1024 = 1.8e308
		  Underflow threshold =	0.5**1022 = 2.2e-308
		 Underflowed results round to the nearest integer multiple of
		 0.5**1074 = 4.9e-324.

     Extended-precision:
	   Type	name: long double (when	supported by the hardware)

	   Wordsize: 96	bits.

	   Precision: 64 significant bits, roughly like	19 significant deci-
	   mals.
		 If x and x' are consecutive positive extended-precision num-
		 bers (they differ by 1	ulp), then
		 1.0e-19 < 0.5**63 < (x'-x)/x <= 0.5**62 < 2.2e-19.

	   Range: Overflow threshold  =	2.0**16384 = 1.2e4932
		  Underflow threshold =	0.5**16382 = 3.4e-4932
		 Underflowed results round to the nearest integer multiple of
		 0.5**16445 = 5.7e-4953.

     Quad-extended-precision:
	   Type	name: long double (when	supported by the hardware)

	   Wordsize: 128 bits.

	   Precision: 113 significant bits, roughly like 34 significant	deci-
	   mals.
		 If x and x' are consecutive positive quad-extended-precision
		 numbers (they differ by 1 ulp), then
		 9.6e-35 < 0.5**113 < (x'-x)/x <= 0.5**112 < 2.0e-34.

	   Range: Overflow threshold  =	2.0**16384 = 1.2e4932
		  Underflow threshold =	0.5**16382 = 3.4e-4932
		 Underflowed results round to the nearest integer multiple of
		 0.5**16494 = 6.5e-4966.

   Additional Information Regarding Exceptions
     For each kind of floating-point exception,	IEEE 754 provides a Flag that
     is	raised each time its exception is signaled, and	stays raised until the
     program resets it.	 Programs may also test, save and restore a flag.
     Thus, IEEE	754 provides three ways	by which programs may cope with	excep-
     tions for which the default result	might be unsatisfactory:

     1.	  Test for a condition that might cause	an exception later, and	branch
	  to avoid the exception.

     2.	  Test a flag to see whether an	exception has occurred since the pro-
	  gram last reset its flag.

     3.	  Test a result	to see whether it is a value that only an exception
	  could	have produced.

	  CAUTION: The only reliable ways to discover whether Underflow	has
	  occurred are to test whether products	or quotients lie closer	to
	  zero than the	underflow threshold, or	to test	the Underflow flag.
	  (Sums	and differences	cannot underflow in IEEE 754; if x != y	then
	  x-y is correct to full precision and certainly nonzero regardless of
	  how tiny it may be.)	Products and quotients that underflow gradu-
	  ally can lose	accuracy gradually without vanishing, so comparing
	  them with zero (as one might on a VAX) will not reveal the loss.
	  Fortunately, if a gradually underflowed value	is destined to be
	  added	to something bigger than the underflow threshold, as is	almost
	  always the case, digits lost to gradual underflow will not be	missed
	  because they would have been rounded off anyway.  So gradual under-
	  flows	are usually provably ignorable.	 The same cannot be said of
	  underflows flushed to	0.

     At	the option of an implementor conforming	to IEEE	754, other ways	to
     cope with exceptions may be provided:

     1.	  ABORT.  This mechanism classifies an exception in advance as an
	  incident to be handled by means traditionally	associated with	error-
	  handling statements like "ON ERROR GO	TO ...".  Different languages
	  offer	different forms	of this	statement, but most share the follow-
	  ing characteristics:

	  -   No means is provided to substitute a value for the offending
	      operation's result and resume computation	from what may be the
	      middle of	an expression.	An exceptional result is abandoned.

	  -   In a subprogram that lacks an error-handling statement, an
	      exception	causes the subprogram to abort within whatever program
	      called it, and so	on back	up the chain of	calling	subprograms
	      until an error-handling statement	is encountered or the whole
	      task is aborted and memory is dumped.

     2.	  STOP.	 This mechanism, requiring an interactive debugging environ-
	  ment,	is more	for the	programmer than	the program.  It classifies an
	  exception in advance as a symptom of a programmer's error; the
	  exception suspends execution as near as it can to the	offending
	  operation so that the	programmer can look around to see how it hap-
	  pened.  Quite	often the first	several	exceptions turn	out to be
	  quite	unexceptionable, so the	programmer ought ideally to be able to
	  resume execution after each one as if	execution had not been
	  stopped.

     3.	  ... Other ways lie beyond the	scope of this document.

     Ideally, each elementary function should act as if	it were	indivisible,
     or	atomic,	in the sense that ...

     1.	  No exception should be signaled that is not deserved by the data
	  supplied to that function.

     2.	  Any exception	signaled should	be identified with that	function
	  rather than with one of its subroutines.

     3.	  The internal behavior	of an atomic function should not be disrupted
	  when a calling program changes from one to another of	the five or so
	  ways of handling exceptions listed above, although the definition of
	  the function may be correlated intentionally with exception han-
	  dling.

     The functions in libm are only approximately atomic.  They	signal no
     inappropriate exception except possibly ...
	   Over/Underflow
		   when	a result, if properly computed,	might have lain	barely
		   within range, and
	   Inexact in cabs(), cbrt(), hypot(), log10() and pow()
		   when	it happens to be exact,	thanks to fortuitous cancella-
		   tion	of errors.
     Otherwise,	...
	   Invalid Operation is	signaled only when
		   any result but NaN would probably be	misleading.
	   Overflow is signaled	only when
		   the exact result would be finite but	beyond the overflow
		   threshold.
	   Divide-by-Zero is signaled only when
		   a function takes exactly infinite values at finite oper-
		   ands.
	   Underflow is	signaled only when
		   the exact result would be nonzero but tinier	than the
		   underflow threshold.
	   Inexact is signaled only when
		   greater range or precision would be needed to represent the
		   exact result.

SEE ALSO
     fenv(3), ieee_test(3), math(3)

     An	explanation of IEEE 754	and its	proposed extension p854	was published
     in	the IEEE magazine MICRO	in August 1984 under the title "A Proposed
     Radix- and	Word-length-independent	Standard for Floating-point Arith-
     metic" by W. J. Cody et al.  The manuals for Pascal, C and	BASIC on the
     Apple Macintosh document the features of IEEE 754 pretty well.  Articles
     in	the IEEE magazine COMPUTER vol.	14 no. 3 (Mar. 1981), and in the ACM
     SIGNUM Newsletter Special Issue of	Oct. 1979, may be helpful although
     they pertain to superseded	drafts of the standard.

STANDARDS
     IEEE Std 754-1985

FreeBSD	10.1		       January 26, 2005			  FreeBSD 10.1

NAME | DESCRIPTION | SEE ALSO | STANDARDS

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