path: root/manual/=limits.texinfo

@node Representation Limits, System Configuration Limits, System Information, Top
@chapter Representation Limits

This chapter contains information about constants and parameters that
characterize the representation of the various integer and
floating-point types supported by the GNU C library.

@menu
* Integer Representation Limits::       Determining maximum and minimum
                                         representation values of
                                         various integer subtypes.
* Floating-Point Limits ::              Parameters which characterize
                                         supported floating-point
                                         representations on a particular
                                         system. 
@end menu

@node Integer Representation Limits, Floating-Point Limits ,  , Representation Limits
@section Integer Representation Limits
@cindex integer representation limits
@cindex representation limits, integer
@cindex limits, integer representation

Sometimes it is necessary for programs to know about the internal
representation of various integer subtypes.  For example, if you want
your program to be careful not to overflow an @code{int} counter
variable, you need to know what the largest representable value that
fits in an @code{int} is.  These kinds of parameters can vary from
compiler to compiler and machine to machine.  Another typical use of
this kind of parameter is in conditionalizing data structure definitions
with @samp{#ifdef} to select the most appropriate integer subtype that
can represent the required range of values.

Macros representing the minimum and maximum limits of the integer types
are defined in the header file @file{limits.h}.  The values of these
macros are all integer constant expressions.
@pindex limits.h

@comment limits.h
@comment ANSI
@deftypevr Macro int CHAR_BIT
This is the number of bits in a @code{char}, usually eight.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int SCHAR_MIN
This is the minimum value that can be represented by a @code{signed char}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int SCHAR_MAX
This is the maximum value that can be represented by a @code{signed char}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int UCHAR_MAX
This is the maximum value that can be represented by a @code{unsigned char}.
(The minimum value of an @code{unsigned char} is zero.)
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int CHAR_MIN
This is the minimum value that can be represented by a @code{char}.
It's equal to @code{SCHAR_MIN} if @code{char} is signed, or zero
otherwise.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int CHAR_MAX
This is the maximum value that can be represented by a @code{char}.
It's equal to @code{SCHAR_MAX} if @code{char} is signed, or
@code{UCHAR_MAX} otherwise.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int SHRT_MIN
This is the minimum value that can be represented by a @code{signed
short int}.  On most machines that the GNU C library runs on,
@code{short} integers are 16-bit quantities.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int SHRT_MAX
This is the maximum value that can be represented by a @code{signed
short int}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int USHRT_MAX
This is the maximum value that can be represented by an @code{unsigned
short int}.  (The minimum value of an @code{unsigned short int} is zero.)
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int INT_MIN
This is the minimum value that can be represented by a @code{signed
int}.  On most machines that the GNU C system runs on, an @code{int} is
a 32-bit quantity.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro int INT_MAX
This is the maximum value that can be represented by a @code{signed
int}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro {unsigned int} UINT_MAX
This is the maximum value that can be represented by an @code{unsigned
int}.  (The minimum value of an @code{unsigned int} is zero.)
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro {long int} LONG_MIN
This is the minimum value that can be represented by a @code{signed long
int}.  On most machines that the GNU C system runs on, @code{long}
integers are 32-bit quantities, the same size as @code{int}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro {long int} LONG_MAX
This is the maximum value that can be represented by a @code{signed long
int}.
@end deftypevr

@comment limits.h
@comment ANSI
@deftypevr Macro {unsigned long int} ULONG_MAX
This is the maximum value that can be represented by an @code{unsigned
long int}.  (The minimum value of an @code{unsigned long int} is zero.)
@end deftypevr

@strong{Incomplete:}  There should be corresponding limits for the GNU
C Compiler's @code{long long} type, too.  (But they are not now present
in the header file.)

The header file @file{limits.h} also defines some additional constants
that parameterize various operating system and file system limits.  These
constants are described in @ref{System Parameters} and @ref{File System
Parameters}.
@pindex limits.h


@node Floating-Point Limits ,  , Integer Representation Limits, Representation Limits
@section Floating-Point Limits
@cindex floating-point number representation
@cindex representation, floating-point number
@cindex limits, floating-point representation

Because floating-point numbers are represented internally as approximate
quantities, algorithms for manipulating floating-point data often need
to be parameterized in terms of the accuracy of the representation.
Some of the functions in the C library itself need this information; for
example, the algorithms for printing and reading floating-point numbers
(@pxref{I/O on Streams}) and for calculating trigonometric and
irrational functions (@pxref{Mathematics}) use information about the
underlying floating-point representation to avoid round-off error and
loss of accuracy.  User programs that implement numerical analysis
techniques also often need to be parameterized in this way in order to
minimize or compute error bounds.

The specific representation of floating-point numbers varies from
machine to machine.  The GNU C library defines a set of parameters which
characterize each of the supported floating-point representations on a
particular system.

@menu
* Floating-Point Representation::       Definitions of terminology.
* Floating-Point Parameters::           Descriptions of the library
                                         facilities. 
* IEEE Floating Point::                 An example of a common
                                         representation. 
@end menu

@node Floating-Point Representation, Floating-Point Parameters,  , Floating-Point Limits
@subsection Floating-Point Representation

This section introduces the terminology used to characterize the
representation of floating-point numbers.

You are probably already familiar with most of these concepts in terms
of scientific or exponential notation for floating-point numbers.  For
example, the number @code{123456.0} could be expressed in exponential
notation as @code{1.23456e+05}, a shorthand notation indicating that the
mantissa @code{1.23456} is multiplied by the base @code{10} raised to
power @code{5}.

More formally, the internal representation of a floating-point number
can be characterized in terms of the following parameters:

@itemize @bullet
@item
The @dfn{sign} is either @code{-1} or @code{1}.
@cindex sign (of floating-point number)

@item
The @dfn{base} or @dfn{radix} for exponentiation; an integer greater
than @code{1}.  This is a constant for the particular representation.
@cindex base (of floating-point number)
@cindex radix (of floating-point number)

@item
The @dfn{exponent} to which the base is raised.  The upper and lower
bounds of the exponent value are constants for the particular
representation.
@cindex exponent (of floating-point number)

Sometimes, in the actual bits representing the floating-point number,
the exponent is @dfn{biased} by adding a constant to it, to make it
always be represented as an unsigned quantity.  This is only important
if you have some reason to pick apart the bit fields making up the
floating-point number by hand, which is something for which the GNU
library provides no support.  So this is ignored in the discussion that
follows.
@cindex bias (of floating-point number exponent)

@item
The value of the @dfn{mantissa} or @dfn{significand}, which is an
unsigned integer.
@cindex mantissa (of floating-point number)
@cindex significand (of floating-point number)

@item 
The @dfn{precision} of the mantissa.  If the base of the representation
is @var{b}, then the precision is the number of base-@var{b} digits in
the mantissa.  This is a constant for the particular representation.

Many floating-point representations have an implicit @dfn{hidden bit} in
the mantissa.  Any such hidden bits are counted in the precision.
Again, the GNU library provides no facilities for dealing with such low-level
aspects of the representation.
@cindex precision (of floating-point number)
@cindex hidden bit (of floating-point number mantissa)
@end itemize

The mantissa of a floating-point number actually represents an implicit
fraction whose denominator is the base raised to the power of the
precision.  Since the largest representable mantissa is one less than
this denominator, the value of the fraction is always strictly less than
@code{1}.  The mathematical value of a floating-point number is then the
product of this fraction; the sign; and the base raised to the exponent.

If the floating-point number is @dfn{normalized}, the mantissa is also
greater than or equal to the base raised to the power of one less
than the precision (unless the number represents a floating-point zero,
in which case the mantissa is zero).  The fractional quantity is
therefore greater than or equal to @code{1/@var{b}}, where @var{b} is
the base.
@cindex normalized floating-point number

@node Floating-Point Parameters, IEEE Floating Point, Floating-Point Representation, Floating-Point Limits
@subsection Floating-Point Parameters

@strong{Incomplete:}  This section needs some more concrete examples
of what these parameters mean and how to use them in a program.

These macro definitions can be accessed by including the header file
@file{float.h} in your program.
@pindex float.h

Macro names starting with @samp{FLT_} refer to the @code{float} type,
while names beginning with @samp{DBL_} refer to the @code{double} type
and names beginning with @samp{LDBL_} refer to the @code{long double}
type.  (In implementations that do not support @code{long double} as
a distinct data type, the values for those constants are the same
as the corresponding constants for the @code{double} type.)@refill
@cindex @code{float} representation limits
@cindex @code{double} representation limits
@cindex @code{long double} representation limits

Of these macros, only @code{FLT_RADIX} is guaranteed to be a constant
expression.  The other macros listed here cannot be reliably used in
places that require constant expressions, such as @samp{#if}
preprocessing directives or array size specifications.

Although the ANSI C standard specifies minimum and maximum values for
most of these parameters, the GNU C implementation uses whatever
floating-point representations are supported by the underlying hardware.
So whether GNU C actually satisfies the ANSI C requirements depends on
what machine it is running on.

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_ROUNDS
This value characterizes the rounding mode for floating-point addition.
The following values indicate standard rounding modes:

@table @code
@item -1
The mode is indeterminable.
@item 0
Rounding is towards zero.
@item 1
Rounding is to the nearest number.
@item 2
Rounding is towards positive infinity.
@item 3
Rounding is towards negative infinity.
@end table

@noindent
Any other value represents a machine-dependent nonstandard rounding
mode.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_RADIX
This is the value of the base, or radix, of exponent representation.
This is guaranteed to be a constant expression, unlike the other macros
described in this section.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_MANT_DIG
This is the number of base-@code{FLT_RADIX} digits in the floating-point
mantissa for the @code{float} data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_MANT_DIG
This is the number of base-@code{FLT_RADIX} digits in the floating-point
mantissa for the @code{double} data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_MANT_DIG
This is the number of base-@code{FLT_RADIX} digits in the floating-point
mantissa for the @code{long double} data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_DIG
This is the number of decimal digits of precision for the @code{float}
data type.  Technically, if @var{p} and @var{b} are the precision and
base (respectively) for the representation, then the decimal precision
@var{q} is the maximum number of decimal digits such that any floating
point number with @var{q} base 10 digits can be rounded to a floating
point number with @var{p} base @var{b} digits and back again, without
change to the @var{q} decimal digits.

The value of this macro is guaranteed to be at least @code{6}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_DIG
This is similar to @code{FLT_DIG}, but is for the @code{double} data
type.  The value of this macro is guaranteed to be at least @code{10}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_DIG
This is similar to @code{FLT_DIG}, but is for the @code{long double}
data type.  The value of this macro is guaranteed to be at least
@code{10}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_MIN_EXP
This is the minimum negative integer such that the mathematical value
@code{FLT_RADIX} raised to this power minus 1 can be represented as a
normalized floating-point number of type @code{float}.  In terms of the
actual implementation, this is just the smallest value that can be
represented in the exponent field of the number.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_MIN_EXP
This is similar to @code{FLT_MIN_EXP}, but is for the @code{double} data
type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_MIN_EXP
This is similar to @code{FLT_MIN_EXP}, but is for the @code{long double}
data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_MIN_10_EXP
This is the minimum negative integer such that the mathematical value
@code{10} raised to this power minus 1 can be represented as a
normalized floating-point number of type @code{float}.  This is
guaranteed to be no greater than @code{-37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_MIN_10_EXP
This is similar to @code{FLT_MIN_10_EXP}, but is for the @code{double}
data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_MIN_10_EXP
This is similar to @code{FLT_MIN_10_EXP}, but is for the @code{long
double} data type.
@end deftypevr



@comment float.h
@comment ANSI
@deftypevr Macro int FLT_MAX_EXP
This is the maximum negative integer such that the mathematical value
@code{FLT_RADIX} raised to this power minus 1 can be represented as a
floating-point number of type @code{float}.  In terms of the actual
implementation, this is just the largest value that can be represented
in the exponent field of the number.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_MAX_EXP
This is similar to @code{FLT_MAX_EXP}, but is for the @code{double} data
type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_MAX_EXP
This is similar to @code{FLT_MAX_EXP}, but is for the @code{long double}
data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int FLT_MAX_10_EXP
This is the maximum negative integer such that the mathematical value
@code{10} raised to this power minus 1 can be represented as a
normalized floating-point number of type @code{float}.  This is
guaranteed to be at least @code{37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int DBL_MAX_10_EXP
This is similar to @code{FLT_MAX_10_EXP}, but is for the @code{double}
data type.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro int LDBL_MAX_10_EXP
This is similar to @code{FLT_MAX_10_EXP}, but is for the @code{long
double} data type.
@end deftypevr


@comment float.h
@comment ANSI
@deftypevr Macro double FLT_MAX
The value of this macro is the maximum representable floating-point
number of type @code{float}, and is guaranteed to be at least
@code{1E+37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro double DBL_MAX
The value of this macro is the maximum representable floating-point
number of type @code{double}, and is guaranteed to be at least
@code{1E+37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro {long double} LDBL_MAX
The value of this macro is the maximum representable floating-point
number of type @code{long double}, and is guaranteed to be at least
@code{1E+37}.
@end deftypevr


@comment float.h
@comment ANSI
@deftypevr Macro double FLT_MIN
The value of this macro is the minimum normalized positive
floating-point number that is representable by type @code{float}, and is
guaranteed to be no more than @code{1E-37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro double DBL_MIN
The value of this macro is the minimum normalized positive
floating-point number that is representable by type @code{double}, and
is guaranteed to be no more than @code{1E-37}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro {long double} LDBL_MIN
The value of this macro is the minimum normalized positive
floating-point number that is representable by type @code{long double},
and is guaranteed to be no more than @code{1E-37}.
@end deftypevr


@comment float.h
@comment ANSI
@deftypevr Macro double FLT_EPSILON
This is the minimum positive floating-point number of type @code{float}
such that @code{1.0 + FLT_EPSILON != 1.0} is true.  It's guaranteed to
be no greater than @code{1E-5}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro double DBL_EPSILON
This is similar to @code{FLT_EPSILON}, but is for the @code{double}
type.  The maximum value is @code{1E-9}.
@end deftypevr

@comment float.h
@comment ANSI
@deftypevr Macro {long double} LDBL_EPSILON
This is similar to @code{FLT_EPSILON}, but is for the @code{long double}
type.  The maximum value is @code{1E-9}.
@end deftypevr


@node IEEE Floating Point,  , Floating-Point Parameters, Floating-Point Limits
@subsection IEEE Floating Point
@cindex IEEE floating-point representation 
@cindex floating-point, IEEE
@cindex IEEE Std 754


Here is an example showing how these parameters work for a common
floating point representation, specified by the @cite{IEEE Standard for
Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985)}.  Nearly
all computers today use this format.

The IEEE single-precision float representation uses a base of 2.  There
is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total
precision is 24 base-2 digits), and an 8-bit exponent that can represent
values in the range -125 to 128, inclusive.

So, for an implementation that uses this representation for the
@code{float} data type, appropriate values for the corresponding
parameters are:

@example
FLT_RADIX                             2
FLT_MANT_DIG                         24
FLT_DIG                               6
FLT_MIN_EXP                        -125
FLT_MIN_10_EXP                      -37
FLT_MAX_EXP                         128
FLT_MAX_10_EXP                      +38
FLT_MIN                 1.17549435E-38F
FLT_MAX                 3.40282347E+38F
FLT_EPSILON             1.19209290E-07F
@end example

Here are the values for the @code{double} data type:

@example
DBL_MANT_DIG                         53
DBL_DIG                              15
DBL_MIN_EXP                       -1021
DBL_MIN_10_EXP                     -307
DBL_MAX_EXP                        1024
DBL_MAX_10_EXP                      308
DBL_MAX         1.7976931348623157E+308
DBL_MIN         2.2250738585072014E-308
DBL_EPSILON     2.2204460492503131E-016
@end example