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.\" from: @(#)floor.3 6.5 (Berkeley) 4/19/91
.\" $Id: float.3,v 1.3 2004/12/02 18:29:12 scp Exp $
.\"
.Dd March 20, 2007
.Dt FLOAT 3
.ds up \fIulp\fR
.ds nn \fINaN\fR
.Os
.Sh NAME
.Nm float
.Nd description of floating-point types available on OS X
.Sh DESCRIPTION
This page describes the available C floating-point types. For a list of math library functions
that operate on these types, see the page on the math library, "man math".
.Sh TERMINOLOGY
Floating point numbers are represented in three parts: a \fBsign\fR, a \fBmantissa\fR (or \fBsignificand\fR),
and an \fBexponent\fR. Given such a representation with sign
.Fa s ,
mantissa
.Fa m ,
and exponent
.Fa e ,
the corresponding numerical value is
.Fa s*m*2**e .
.Pp
Floating-point types differ in the number of bits of accuracy in the mantissa (called the \fBprecision\fR),
and set of available exponents (the \fBexponent range\fR).
.Pp
Floating-point numbers with the maximum available exponent are reserved operands, denoting an \fBinfinity\fR if the
significand is precisely zero, and a Not-a-Number, or \fBNaN\fR, otherwise.
.Pp
Floating-point numbers with the minimum available exponent are either \fBzero\fR if the significand is precisely zero,
and \fBdenormal\fR otherwise. Note that zero is signed: +0 and -0 are distinct floating point numbers.
.Pp
Floating-point numbers with exponents other than the maximum and minimum available are called \fBnormal\fR numbers.
.Sh PROPERTIES OF IEEE-754 FLOATING-POINT
Basic arithmetic operations in IEEE-754 floating-point are \fBcorrectly rounded\fR: this means that the result delivered
is the same as the result that would be achieved by computing the exact real-number operation on the operands,
then rounding the real-number result to a floating-point value.
.Pp
\fBOverflow\fR occurs when the value of the exact result is too large in magnitude to be represented in the
floating-point type in which the computation is being performed; doing so would require an exponent outside of the
exponent range of the type. By default, computations that result in overflow return a signed infinity.
.Pp
\fBUnderflow\fR occurs when the value of the exact result is too small in magnitude to be represented as a normal
number in the floating-point type in which the computation is being performed. By default, underflow is gradual,
and produces a denormal number or a zero.
.Pp
All floating-points number of a given type are integer multiples of the smallest non-zero floating-point number of
that type; however, the converse is not true. This means that, in the default mode, (x-y) = 0 only if x = y.
.Pp
The sign of zero transforms correctly through multiplication and division, and is preserved by addition of
zeros with like signs, but x - x yields +0 for every finite floating-point number x. The only operations that
reveal the sign of a zero are x/(±0) and copysign(x,±0). In particular, comparisons (x > y, x != y, etc) are not
affected by the sign of zero.
.Pp
The sign of infinity transforms correctly through multiplication and division, and infinities are unaffected by addition
or subtraction of any finite floating-point number. But Inf-Inf, Inf*0, and Inf/Inf are, like 0/0 or sqrt(-3), invalid
operations that produce NaN.
.Pp
NaNs are the default results of invalid operations, and they propagate through subsequent arithmetic operations.
If x is a NaN, then x != x is TRUE, and every other comparison predicate (x > y, x = y, x <= y, etc) evaluates to
FALSE, regardless of the value of y. Additionally, predicates that entail an ordered comparison (rather than mere
equality or inequality) signal Invalid Operation when one of the arguments is NaN.
.Pp
IEEE-754 provides five kinds of floating-point \fBexceptions\fR, listed below:
.Pp
.nf
.ta \w'Invalid Operation'u+6n +\w'Gradual Underflow'u+2n
Exception Default Result
.tc \(ru
.tc
Invalid Operation NaN or FALSE
Overflow ±Infinity
Divide by Zero ±Infinity
Underflow Gradual Underflow
Inexact Rounded Value
.ta
.fi
.Pp
NOTE: An exception is not an error unless it is handled incorrectly. What makes a class of exceptions exceptional
is that no single default response can be satisfactory in every instance. On the other hand, because a default
response will serve most instances of the exception satisfactorily, simply aborting the computation cannot be
justified.
.Pp
For each kind of floating-point exception, IEEE-754 provides a flag that is raised each time its exception is
signaled, and remains raised until the program resets it. Programs may test, save, and restore the flags, or a subset
thereof.
.Sh PRECISION AND EXPONENT RANGE OF SPECIFIC FLOATING-POINT TYPES
On both Intel and PPC macs, the type
.Fa float
corresponds to IEEE-754 single precision. A single-precision number is represented in 32 bits, and has a precision
of 24 significant bits, roughly like 7 significant decimal digits. 8 bits are used to encode the exponent, which gives
an exponent range from -126 to 127, inclusive.
.Pp
The header defines several useful constants for the float type:
.br
.Fa FLT_MANT_DIG
- The number of binary digits in the significand of a float.
.br
.Fa FLT_MIN_EXP
- One more than the smallest exponent available in the float type.
.br
.Fa FLT_MAX_EXP
- One more than the largest exponent available in the float type.
.br
.Fa FLT_DIG
- the precision in decimal digits of a float. A decimal value with this many digits, stored as a float, always
yields the same value up to this many digits when converted back to decimal notation.
.br
.Fa FLT_MIN_10_EXP
- the smallest n such that 10**n is a non-zero normal number as a float.
.br
.Fa FLT_MAX_10_EXP
- the largest n such that 10**n is finite as a float.
.br
.Fa FLT_MIN
- the smallest positive normal float.
.br
.Fa FLT_MAX
- the largest finite float.
.br
.Fa FLT_EPSILON
- the difference between 1.0 and the smallest float bigger than 1.0.
.Pp
On both Intel and PPC macs, the type
.Fa double
corresponds to IEEE-754 double precision. A double-precision number is represented in 64 bits, and has a precision
of 53 significant bits, roughly like 16 significant decimal digits. 11 bits are used to encode the exponent, which gives
an exponent range from -1022 to 1023, inclusive.
.Pp
The header defines several useful constants for the double type:
.br
.Fa DBL_MANT_DIG
- The number of binary digits in the significand of a double.
.br
.Fa DBL_MIN_EXP
- One more than the smallest exponent available in the double type.
.br
.Fa DBL_MAX_EXP
- One more than the exponent available in the double type.
.br
.Fa DBL_DIG
- the precision in decimal digits of a double. A decimal value with this many digits, stored as a double, always
yields the same value up to this many digits when converted back to decimal notation.
.br
.Fa DBL_MIN_10_EXP
- the smallest n such that 10**n is a non-zero normal number as a double.
.br
.Fa DBL_MAX_10_EXP
- the largest n such that 10**n is finite as a double.
.br
.Fa DBL_MIN
- the smallest positive normal double.
.br
.Fa DBL_MAX
- the largest finite double.
.br
.Fa DBL_EPSILON
- the difference between 1.0 and the smallest double bigger than 1.0.
.Pp
On Intel macs, the type
.Fa long double
corresponds to IEEE-754 double extended precision. A double extended number is represented in 80 bits, and has a
precision of 64 significant bits, roughly like 19 significant decimal digits. 15 bits are used to encode the exponent,
which gives an exponent range from -16383 to 16384, inclusive.
.Pp
The header defines several useful constants for the long double type:
.br
.Fa LDBL_MANT_DIG
- The number of binary digits in the significand of a long double.
.br
.Fa LDBL_MIN_EXP
- One more than the smallest exponent available in the long double type.
.br
.Fa LDBL_MAX_EXP
- One more than the exponent available in the long double type.
.br
.Fa LDBL_DIG
- the precision in decimal digits of a long double. A decimal value with this many digits, stored as a long double,
always yields the same value up to this many digits when converted back to decimal notation.
.br
.Fa LDBL_MIN_10_EXP
- the smallest n such that 10**n is a non-zero normal number as a long double.
.br
.Fa LDBL_MAX_10_EXP
- the largest n such that 10**n is finite as a long double.
.br
.Fa LDBL_MIN
- the smallest positive normal long double.
.br
.Fa LDBL_MAX
- the largest finite long double.
.br
.Fa LDBL_EPSILON
- the difference between 1.0 and the smallest long double bigger than 1.0.
.Sh LONG DOUBLE ON POWERPC MACS
On PowerPC macs, by default the type
.Fa long double
is mapped to IEEE-754 double precision, described above. If additional precision is required, a non-IEEE-754 128
bit long double format is also available. To use this format, compile with the
.Fa -mlong-double-128
flag. If you wish to use the functions, you must include the linker flag
.Fa -lmx
as well as the usual
.Fa -lm .
The -mlong-double-128 flag is only valid when the target architecture is ppc or ppc64.
.Pp
Each 128-bit long double is made up of two IEEE doubles (head and tail). The value of the long double is the sum of the
values of the two parts (unless the head double has value -0.0, in which case the value of the long double is -0.0).
The value of the head is required to be the value of the long double rounded to the nearest double. If the head is an
infinity, the value of the long double is the value of the head, and the tail must be ±0.0. The tail of a NaN can be
any double value. There are many 128-bit bit patterns that are not valid as long doubles. These do not represet any
value.
.Pp
The 128-bit long double format provides 106 significant bits, which is roughly like 31 significant decimal digits. It
has the same exponent range as double, from -1022 to 1023, inclusive. The usual constants are provided from ,
as described above.
.Pp
In the 128-bit long double format, addition and subtraction have a relative error bound of one \fBulp\fR, or 2**-106.
Multiplication has a relative error within 2 ulps, and division a relative error within 3 ulps.
.Sh SEE ALSO
.Xr math 3 ,
.Xr complex 3
.Sh STANDARDS
Floating-point arithmetic conforms to the ISO/IEC 9899:1999(E) standard.