This is Info file libc.info, produced by Makeinfo version 1.68 from the
input file libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.08 DRAFT, last updated 11 Jan 1999, of `The GNU C
Library Reference Manual', for Version 2.1 Beta.

   Copyright (C) 1993, '94, '95, '96, '97, '98, '99 Free Software
Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the section entitled "GNU Library General Public License" is
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the text of the translation of the section
entitled "GNU Library General Public License" must be approved for
accuracy by the Foundation.


File: libc.info,  Node: Floating Point Classes,  Next: Floating Point Errors,  Prev: Floating Point Numbers,  Up: Arithmetic

Floating-Point Number Classification Functions
==============================================

   ISO C 9x defines macros that let you determine what sort of
floating-point number a variable holds.

 - Macro: int fpclassify (*float-type* X)
     This is a generic macro which works on all floating-point types and
     which returns a value of type `int'.  The possible values are:

    `FP_NAN'
          The floating-point number X is "Not a Number" (*note Infinity
          and NaN::.)

    `FP_INFINITE'
          The value of X is either plus or minus infinity (*note
          Infinity and NaN::.)

    `FP_ZERO'
          The value of X is zero.  In floating-point formats like
          IEEE 754, where zero can be signed, this value is also
          returned if X is negative zero.

    `FP_SUBNORMAL'
          Numbers whose absolute value is too small to be represented
          in the normal format are represented in an alternate,
          "denormalized" format (*note Floating Point Concepts::.).
          This format is less precise but can represent values closer
          to zero.  `fpclassify' returns this value for values of X in
          this alternate format.

    `FP_NORMAL'
          This value is returned for all other values of X.  It
          indicates that there is nothing special about the number.


   `fpclassify' is most useful if more than one property of a number
must be tested.  There are more specific macros which only test one
property at a time.  Generally these macros execute faster than
`fpclassify', since there is special hardware support for them.  You
should therefore use the specific macros whenever possible.

 - Macro: int isfinite (*float-type* X)
     This macro returns a nonzero value if X is finite: not plus or
     minus infinity, and not NaN.  It is equivalent to

          (fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)

     `isfinite' is implemented as a macro which accepts any
     floating-point type.

 - Macro: int isnormal (*float-type* X)
     This macro returns a nonzero value if X is finite and normalized.
     It is equivalent to

          (fpclassify (x) == FP_NORMAL)

 - Macro: int isnan (*float-type* X)
     This macro returns a nonzero value if X is NaN.  It is equivalent
     to

          (fpclassify (x) == FP_NAN)

   Another set of floating-point classification functions was provided
by BSD.  The GNU C library also supports these functions; however, we
recommend that you use the C9x macros in new code.  Those are standard
and will be available more widely.  Also, since they are macros, you do
not have to worry about the type of their argument.

 - Function: int isinf (double X)
 - Function: int isinff (float X)
 - Function: int isinfl (long double X)
     This function returns `-1' if X represents negative infinity, `1'
     if X represents positive infinity, and `0' otherwise.

 - Function: int isnan (double X)
 - Function: int isnanf (float X)
 - Function: int isnanl (long double X)
     This function returns a nonzero value if X is a "not a number"
     value, and zero otherwise.

     *Note:* The `isnan' macro defined by ISO C 9x overrides the BSD
     function.  This is normally not a problem, because the two
     routines behave identically.  However, if you really need to get
     the BSD function for some reason, you can write

          (isnan) (x)

 - Function: int finite (double X)
 - Function: int finitef (float X)
 - Function: int finitel (long double X)
     This function returns a nonzero value if X is finite or a "not a
     number" value, and zero otherwise.

 - Function: double infnan (int ERROR)
     This function is provided for compatibility with BSD.  Its
     argument is an error code, `EDOM' or `ERANGE'; `infnan' returns the
     value that a math function would return if it set `errno' to that
     value.  *Note Math Error Reporting::.  `-ERANGE' is also acceptable
     as an argument, and corresponds to `-HUGE_VAL' as a value.

     In the BSD library, on certain machines, `infnan' raises a fatal
     signal in all cases.  The GNU library does not do likewise,
     because that does not fit the ISO C specification.

   *Portability Note:* The functions listed in this section are BSD
extensions.


File: libc.info,  Node: Floating Point Errors,  Next: Rounding,  Prev: Floating Point Classes,  Up: Arithmetic

Errors in Floating-Point Calculations
=====================================

* Menu:

* FP Exceptions::               IEEE 754 math exceptions and how to detect them.
* Infinity and NaN::            Special values returned by calculations.
* Status bit operations::       Checking for exceptions after the fact.
* Math Error Reporting::        How the math functions report errors.


File: libc.info,  Node: FP Exceptions,  Next: Infinity and NaN,  Up: Floating Point Errors

FP Exceptions
-------------

   The IEEE 754 standard defines five "exceptions" that can occur
during a calculation.  Each corresponds to a particular sort of error,
such as overflow.

   When exceptions occur (when exceptions are "raised", in the language
of the standard), one of two things can happen.  By default the
exception is simply noted in the floating-point "status word", and the
program continues as if nothing had happened.  The operation produces a
default value, which depends on the exception (see the table below).
Your program can check the status word to find out which exceptions
happened.

   Alternatively, you can enable "traps" for exceptions.  In that case,
when an exception is raised, your program will receive the `SIGFPE'
signal.  The default action for this signal is to terminate the
program.  *Note Signal Handling::, for how you can change the effect of
the signal.

   In the System V math library, the user-defined function `matherr' is
called when certain exceptions occur inside math library functions.
However, the Unix98 standard deprecates this interface.  We support it
for historical compatibility, but recommend that you do not use it in
new programs.

The exceptions defined in IEEE 754 are:

`Invalid Operation'
     This exception is raised if the given operands are invalid for the
     operation to be performed.  Examples are (see IEEE 754, section 7):
       1. Addition or subtraction: oo - oo.  (But oo + oo = oo).

       2. Multiplication: 0 * oo.

       3. Division: 0/0 or oo/oo.

       4. Remainder: x REM y, where y is zero or x is infinite.

       5. Square root if the operand is less then zero.  More
          generally, any mathematical function evaluated outside its
          domain produces this exception.

       6. Conversion of a floating-point number to an integer or decimal
          string, when the number cannot be represented in the target
          format (due to overflow, infinity, or NaN).

       7. Conversion of an unrecognizable input string.

       8. Comparison via predicates involving < or >, when one or other
          of the operands is NaN.  You can prevent this exception by
          using the unordered comparison functions instead; see *Note
          FP Comparison Functions::.

     If the exception does not trap, the result of the operation is NaN.

`Division by Zero'
     This exception is raised when a finite nonzero number is divided
     by zero.  If no trap occurs the result is either +oo or -oo,
     depending on the signs of the operands.

`Overflow'
     This exception is raised whenever the result cannot be represented
     as a finite value in the precision format of the destination.  If
     no trap occurs the result depends on the sign of the intermediate
     result and the current rounding mode (IEEE 754, section 7.3):
       1. Round to nearest carries all overflows to oo with the sign of
          the intermediate result.

       2. Round toward 0 carries all overflows to the largest
          representable finite number with the sign of the intermediate
          result.

       3. Round toward -oo carries positive overflows to the largest
          representable finite number and negative overflows to -oo.

       4. Round toward oo carries negative overflows to the most
          negative representable finite number and positive overflows
          to oo.

     Whenever the overflow exception is raised, the inexact exception
     is also raised.

`Underflow'
     The underflow exception is raised when an intermediate result is
     too small to be calculated accurately, or if the operation's
     result rounded to the destination precision is too small to be
     normalized.

     When no trap is installed for the underflow exception, underflow is
     signaled (via the underflow flag) only when both tininess and loss
     of accuracy have been detected.  If no trap handler is installed
     the operation continues with an imprecise small value, or zero if
     the destination precision cannot hold the small exact result.

`Inexact'
     This exception is signalled if a rounded result is not exact (such
     as when calculating the square root of two) or a result overflows
     without an overflow trap.


File: libc.info,  Node: Infinity and NaN,  Next: Status bit operations,  Prev: FP Exceptions,  Up: Floating Point Errors

Infinity and NaN
----------------

   IEEE 754 floating point numbers can represent positive or negative
infinity, and "NaN" (not a number).  These three values arise from
calculations whose result is undefined or cannot be represented
accurately.  You can also deliberately set a floating-point variable to
any of them, which is sometimes useful.  Some examples of calculations
that produce infinity or NaN:

     1/0 = oo
     log (0) = -oo
     sqrt (-1) = NaN

   When a calculation produces any of these values, an exception also
occurs; see *Note FP Exceptions::.

   The basic operations and math functions all accept infinity and NaN
and produce sensible output.  Infinities propagate through calculations
as one would expect: for example, 2 + oo = oo, 4/oo = 0, atan (oo) =
pi/2.  NaN, on the other hand, infects any calculation that involves
it.  Unless the calculation would produce the same result no matter
what real value replaced NaN, the result is NaN.

   In comparison operations, positive infinity is larger than all values
except itself and NaN, and negative infinity is smaller than all values
except itself and NaN.  NaN is "unordered": it is not equal to, greater
than, or less than anything, *including itself*. `x == x' is false if
the value of `x' is NaN.  You can use this to test whether a value is
NaN or not, but the recommended way to test for NaN is with the `isnan'
function (*note Floating Point Classes::.).  In addition, `<', `>',
`<=', and `>=' will raise an exception when applied to NaNs.

   `math.h' defines macros that allow you to explicitly set a variable
to infinity or NaN.

 - Macro: float INFINITY
     An expression representing positive infinity.  It is equal to the
     value produced  by mathematical operations like `1.0 / 0.0'.
     `-INFINITY' represents negative infinity.

     You can test whether a floating-point value is infinite by
     comparing it to this macro.  However, this is not recommended; you
     should use the `isfinite' macro instead.  *Note Floating Point
     Classes::.

     This macro was introduced in the ISO C 9X standard.

 - Macro: float NAN
     An expression representing a value which is "not a number".  This
     macro is a GNU extension, available only on machines that support
     the "not a number" value--that is to say, on all machines that
     support IEEE floating point.

     You can use `#ifdef NAN' to test whether the machine supports NaN.
     (Of course, you must arrange for GNU extensions to be visible,
     such as by defining `_GNU_SOURCE', and then you must include
     `math.h'.)

   IEEE 754 also allows for another unusual value: negative zero.  This
value is produced when you divide a positive number by negative
infinity, or when a negative result is smaller than the limits of
representation.  Negative zero behaves identically to zero in all
calculations, unless you explicitly test the sign bit with `signbit' or
`copysign'.


File: libc.info,  Node: Status bit operations,  Next: Math Error Reporting,  Prev: Infinity and NaN,  Up: Floating Point Errors

Examining the FPU status word
-----------------------------

   ISO C 9x defines functions to query and manipulate the
floating-point status word.  You can use these functions to check for
untrapped exceptions when it's convenient, rather than worrying about
them in the middle of a calculation.

   These constants represent the various IEEE 754 exceptions.  Not all
FPUs report all the different exceptions.  Each constant is defined if
and only if the FPU you are compiling for supports that exception, so
you can test for FPU support with `#ifdef'.  They are defined in
`fenv.h'.

`FE_INEXACT'
     The inexact exception.

`FE_DIVBYZERO'
     The divide by zero exception.

`FE_UNDERFLOW'
     The underflow exception.

`FE_OVERFLOW'
     The overflow exception.

`FE_INVALID'
     The invalid exception.

   The macro `FE_ALL_EXCEPT' is the bitwise OR of all exception macros
which are supported by the FP implementation.

   These functions allow you to clear exception flags, test for
exceptions, and save and restore the set of exceptions flagged.

 - Function: void feclearexcept (int EXCEPTS)
     This function clears all of the supported exception flags
     indicated by EXCEPTS.

 - Function: int fetestexcept (int EXCEPTS)
     Test whether the exception flags indicated by the parameter EXCEPT
     are currently set.  If any of them are, a nonzero value is returned
     which specifies which exceptions are set.  Otherwise the result is
     zero.

   To understand these functions, imagine that the status word is an
integer variable named STATUS.  `feclearexcept' is then equivalent to
`status &= ~excepts' and `fetestexcept' is equivalent to `(status &
excepts)'.  The actual implementation may be very different, of course.

   Exception flags are only cleared when the program explicitly
requests it, by calling `feclearexcept'.  If you want to check for
exceptions from a set of calculations, you should clear all the flags
first.  Here is a simple example of the way to use `fetestexcept':

     {
       double f;
       int raised;
       feclearexcept (FE_ALL_EXCEPT);
       f = compute ();
       raised = fetestexcept (FE_OVERFLOW | FE_INVALID);
       if (raised & FE_OVERFLOW) { /* ... */ }
       if (raised & FE_INVALID) { /* ... */ }
       /* ... */
     }

   You cannot explicitly set bits in the status word.  You can, however,
save the entire status word and restore it later.  This is done with the
following functions:

 - Function: void fegetexceptflag (fexcept_t *FLAGP, int EXCEPTS)
     This function stores in the variable pointed to by FLAGP an
     implementation-defined value representing the current setting of
     the exception flags indicated by EXCEPTS.

 - Function: void fesetexceptflag (const fexcept_t *FLAGP, int
     EXCEPTS) This function restores the flags for the exceptions
     indicated by EXCEPTS to the values stored in the variable pointed
     to by FLAGP.

   Note that the value stored in `fexcept_t' bears no resemblance to
the bit mask returned by `fetestexcept'.  The type may not even be an
integer.  Do not attempt to modify an `fexcept_t' variable.


File: libc.info,  Node: Math Error Reporting,  Prev: Status bit operations,  Up: Floating Point Errors

Error Reporting by Mathematical Functions
-----------------------------------------

   Many of the math functions are defined only over a subset of the
real or complex numbers.  Even if they are mathematically defined,
their result may be larger or smaller than the range representable by
their return type.  These are known as "domain errors", "overflows", and
"underflows", respectively.  Math functions do several things when one
of these errors occurs.  In this manual we will refer to the complete
response as "signalling" a domain error, overflow, or underflow.

   When a math function suffers a domain error, it raises the invalid
exception and returns NaN.  It also sets ERRNO to `EDOM'; this is for
compatibility with old systems that do not support IEEE 754 exception
handling.  Likewise, when overflow occurs, math functions raise the
overflow exception and return oo or -oo as appropriate.  They also set
ERRNO to `ERANGE'.  When underflow occurs, the underflow exception is
raised, and zero (appropriately signed) is returned.  ERRNO may be set
to `ERANGE', but this is not guaranteed.

   Some of the math functions are defined mathematically to result in a
complex value over parts of their domains.  The most familiar example of
this is taking the square root of a negative number.  The complex math
functions, such as `csqrt', will return the appropriate complex value
in this case.  The real-valued functions, such as `sqrt', will signal a
domain error.

   Some older hardware does not support infinities.  On that hardware,
overflows instead return a particular very large number (usually the
largest representable number).  `math.h' defines macros you can use to
test for overflow on both old and new hardware.

 - Macro: double HUGE_VAL
 - Macro: float HUGE_VALF
 - Macro: long double HUGE_VALL
     An expression representing a particular very large number.  On
     machines that use IEEE 754 floating point format, `HUGE_VAL' is
     infinity.  On other machines, it's typically the largest positive
     number that can be represented.

     Mathematical functions return the appropriately typed version of
     `HUGE_VAL' or `-HUGE_VAL' when the result is too large to be
     represented.


File: libc.info,  Node: Rounding,  Next: Control Functions,  Prev: Floating Point Errors,  Up: Arithmetic

Rounding Modes
==============

   Floating-point calculations are carried out internally with extra
precision, and then rounded to fit into the destination type.  This
ensures that results are as precise as the input data.  IEEE 754
defines four possible rounding modes:

Round to nearest.
     This is the default mode.  It should be used unless there is a
     specific need for one of the others.  In this mode results are
     rounded to the nearest representable value.  If the result is
     midway between two representable values, the even representable is
     chosen. "Even" here means the lowest-order bit is zero.  This
     rounding mode prevents statistical bias and guarantees numeric
     stability: round-off errors in a lengthy calculation will remain
     smaller than half of `FLT_EPSILON'.

Round toward plus Infinity.
     All results are rounded to the smallest representable value which
     is greater than the result.

Round toward minus Infinity.
     All results are rounded to the largest representable value which
     is less than the result.

Round toward zero.
     All results are rounded to the largest representable value whose
     magnitude is less than that of the result.  In other words, if the
     result is negative it is rounded up; if it is positive, it is
     rounded down.

`fenv.h' defines constants which you can use to refer to the various
rounding modes.  Each one will be defined if and only if the FPU
supports the corresponding rounding mode.

`FE_TONEAREST'
     Round to nearest.

`FE_UPWARD'
     Round toward +oo.

`FE_DOWNWARD'
     Round toward -oo.

`FE_TOWARDZERO'
     Round toward zero.

   Underflow is an unusual case.  Normally, IEEE 754 floating point
numbers are always normalized (*note Floating Point Concepts::.).
Numbers smaller than 2^r (where r is the minimum exponent,
`FLT_MIN_RADIX-1' for FLOAT) cannot be represented as normalized
numbers.  Rounding all such numbers to zero or 2^r would cause some
algorithms to fail at 0.  Therefore, they are left in denormalized
form.  That produces loss of precision, since some bits of the mantissa
are stolen to indicate the decimal point.

   If a result is too small to be represented as a denormalized number,
it is rounded to zero.  However, the sign of the result is preserved; if
the calculation was negative, the result is "negative zero".  Negative
zero can also result from some operations on infinity, such as 4/-oo.
Negative zero behaves identically to zero except when the `copysign' or
`signbit' functions are used to check the sign bit directly.

   At any time one of the above four rounding modes is selected.  You
can find out which one with this function:

 - Function: int fegetround (void)
     Returns the currently selected rounding mode, represented by one
     of the values of the defined rounding mode macros.

To change the rounding mode, use this function:

 - Function: int fesetround (int ROUND)
     Changes the currently selected rounding mode to ROUND.  If ROUND
     does not correspond to one of the supported rounding modes nothing
     is changed.  `fesetround' returns a nonzero value if it changed
     the rounding mode, zero if the mode is not supported.

   You should avoid changing the rounding mode if possible.  It can be
an expensive operation; also, some hardware requires you to compile your
program differently for it to work.  The resulting code may run slower.
See your compiler documentation for details.


File: libc.info,  Node: Control Functions,  Next: Arithmetic Functions,  Prev: Rounding,  Up: Arithmetic

Floating-Point Control Functions
================================

   IEEE 754 floating-point implementations allow the programmer to
decide whether traps will occur for each of the exceptions, by setting
bits in the "control word".  In C, traps result in the program
receiving the `SIGFPE' signal; see *Note Signal Handling::.

   *Note:* IEEE 754 says that trap handlers are given details of the
exceptional situation, and can set the result value.  C signals do not
provide any mechanism to pass this information back and forth.
Trapping exceptions in C is therefore not very useful.

   It is sometimes necessary to save the state of the floating-point
unit while you perform some calculation.  The library provides functions
which save and restore the exception flags, the set of exceptions that
generate traps, and the rounding mode.  This information is known as the
"floating-point environment".

   The functions to save and restore the floating-point environment all
use a variable of type `fenv_t' to store information.  This type is
defined in `fenv.h'.  Its size and contents are implementation-defined.
You should not attempt to manipulate a variable of this type directly.

   To save the state of the FPU, use one of these functions:

 - Function: void fegetenv (fenv_t *ENVP)
     Store the floating-point environment in the variable pointed to by
     ENVP.

 - Function: int feholdexcept (fenv_t *ENVP)
     Store the current floating-point environment in the object pointed
     to by ENVP.  Then clear all exception flags, and set the FPU to
     trap no exceptions.  Not all FPUs support trapping no exceptions;
     if `feholdexcept' cannot set this mode, it returns zero.  If it
     succeeds, it returns a nonzero value.

   The functions which restore the floating-point environment can take
two kinds of arguments:

   * Pointers to `fenv_t' objects, which were initialized previously by
     a call to `fegetenv' or `feholdexcept'.

   * The special macro `FE_DFL_ENV' which represents the floating-point
     environment as it was available at program start.

   * Implementation defined macros with names starting with `FE_'.

     If possible, the GNU C Library defines a macro `FE_NOMASK_ENV'
     which represents an environment where every exception raised
     causes a trap to occur.  You can test for this macro using
     `#ifdef'.  It is only defined if `_GNU_SOURCE' is defined.

     Some platforms might define other predefined environments.

To set the floating-point environment, you can use either of these
functions:

 - Function: void fesetenv (const fenv_t *ENVP)
     Set the floating-point environment to that described by ENVP.

 - Function: void feupdateenv (const fenv_t *ENVP)
     Like `fesetenv', this function sets the floating-point environment
     to that described by ENVP.  However, if any exceptions were
     flagged in the status word before `feupdateenv' was called, they
     remain flagged after the call.  In other words, after `feupdateenv'
     is called, the status word is the bitwise OR of the previous
     status word and the one saved in ENVP.


File: libc.info,  Node: Arithmetic Functions,  Next: Complex Numbers,  Prev: Control Functions,  Up: Arithmetic

Arithmetic Functions
====================

   The C library provides functions to do basic operations on
floating-point numbers.  These include absolute value, maximum and
minimum, normalization, bit twiddling, rounding, and a few others.

* Menu:

* Absolute Value::              Absolute values of integers and floats.
* Normalization Functions::     Extracting exponents and putting them back.
* Rounding Functions::          Rounding floats to integers.
* Remainder Functions::         Remainders on division, precisely defined.
* FP Bit Twiddling::            Sign bit adjustment.  Adding epsilon.
* FP Comparison Functions::     Comparisons without risk of exceptions.
* Misc FP Arithmetic::          Max, min, positive difference, multiply-add.


File: libc.info,  Node: Absolute Value,  Next: Normalization Functions,  Up: Arithmetic Functions

Absolute Value
--------------

   These functions are provided for obtaining the "absolute value" (or
"magnitude") of a number.  The absolute value of a real number X is X
if X is positive, -X if X is negative.  For a complex number Z, whose
real part is X and whose imaginary part is Y, the absolute value is
`sqrt (X*X + Y*Y)'.

   Prototypes for `abs', `labs' and `llabs' are in `stdlib.h';
`imaxabs' is declared in `inttypes.h'; `fabs', `fabsf' and `fabsl' are
declared in `math.h'.  `cabs', `cabsf' and `cabsl' are declared in
`complex.h'.

 - Function: int abs (int NUMBER)
 - Function: long int labs (long int NUMBER)
 - Function: long long int llabs (long long int NUMBER)
 - Function: intmax_t imaxabs (intmax_t NUMBER)
     These functions return the absolute value of NUMBER.

     Most computers use a two's complement integer representation, in
     which the absolute value of `INT_MIN' (the smallest possible `int')
     cannot be represented; thus, `abs (INT_MIN)' is not defined.

     `llabs' and `imaxdiv' are new to ISO C 9x.

 - Function: double fabs (double NUMBER)
 - Function: float fabsf (float NUMBER)
 - Function: long double fabsl (long double NUMBER)
     This function returns the absolute value of the floating-point
     number NUMBER.

 - Function: double cabs (complex double Z)
 - Function: float cabsf (complex float Z)
 - Function: long double cabsl (complex long double Z)
     These functions return the absolute  value of the complex number Z
     (*note Complex Numbers::.).  The absolute value of a complex
     number is:

          sqrt (creal (Z) * creal (Z) + cimag (Z) * cimag (Z))

     This function should always be used instead of the direct formula
     because it takes special care to avoid losing precision.  It may
     also take advantage of hardware support for this operation. See
     `hypot' in *Note Exponents and Logarithms::.


File: libc.info,  Node: Normalization Functions,  Next: Rounding Functions,  Prev: Absolute Value,  Up: Arithmetic Functions

Normalization Functions
-----------------------

   The functions described in this section are primarily provided as a
way to efficiently perform certain low-level manipulations on floating
point numbers that are represented internally using a binary radix; see
*Note Floating Point Concepts::.  These functions are required to have
equivalent behavior even if the representation does not use a radix of
2, but of course they are unlikely to be particularly efficient in
those cases.

   All these functions are declared in `math.h'.

 - Function: double frexp (double VALUE, int *EXPONENT)
 - Function: float frexpf (float VALUE, int *EXPONENT)
 - Function: long double frexpl (long double VALUE, int *EXPONENT)
     These functions are used to split the number VALUE into a
     normalized fraction and an exponent.

     If the argument VALUE is not zero, the return value is VALUE times
     a power of two, and is always in the range 1/2 (inclusive) to 1
     (exclusive).  The corresponding exponent is stored in `*EXPONENT';
     the return value multiplied by 2 raised to this exponent equals
     the original number VALUE.

     For example, `frexp (12.8, &exponent)' returns `0.8' and stores
     `4' in `exponent'.

     If VALUE is zero, then the return value is zero and zero is stored
     in `*EXPONENT'.

 - Function: double ldexp (double VALUE, int EXPONENT)
 - Function: float ldexpf (float VALUE, int EXPONENT)
 - Function: long double ldexpl (long double VALUE, int EXPONENT)
     These functions return the result of multiplying the floating-point
     number VALUE by 2 raised to the power EXPONENT.  (It can be used
     to reassemble floating-point numbers that were taken apart by
     `frexp'.)

     For example, `ldexp (0.8, 4)' returns `12.8'.

   The following functions, which come from BSD, provide facilities
equivalent to those of `ldexp' and `frexp'.

 - Function: double logb (double X)
 - Function: float logbf (float X)
 - Function: long double logbl (long double X)
     These functions return the integer part of the base-2 logarithm of
     X, an integer value represented in type `double'.  This is the
     highest integer power of `2' contained in X.  The sign of X is
     ignored.  For example, `logb (3.5)' is `1.0' and `logb (4.0)' is
     `2.0'.

     When `2' raised to this power is divided into X, it gives a
     quotient between `1' (inclusive) and `2' (exclusive).

     If X is zero, the return value is minus infinity if the machine
     supports infinities, and a very small number if it does not.  If X
     is infinity, the return value is infinity.

     For finite X, the value returned by `logb' is one less than the
     value that `frexp' would store into `*EXPONENT'.

 - Function: double scalb (double VALUE, int EXPONENT)
 - Function: float scalbf (float VALUE, int EXPONENT)
 - Function: long double scalbl (long double VALUE, int EXPONENT)
     The `scalb' function is the BSD name for `ldexp'.

 - Function: long long int scalbn (double X, int n)
 - Function: long long int scalbnf (float X, int n)
 - Function: long long int scalbnl (long double X, int n)
     `scalbn' is identical to `scalb', except that the exponent N is an
     `int' instead of a floating-point number.

 - Function: long long int scalbln (double X, long int n)
 - Function: long long int scalblnf (float X, long int n)
 - Function: long long int scalblnl (long double X, long int n)
     `scalbln' is identical to `scalb', except that the exponent N is a
     `long int' instead of a floating-point number.

 - Function: long long int significand (double X)
 - Function: long long int significandf (float X)
 - Function: long long int significandl (long double X)
     `significand' returns the mantissa of X scaled to the range [1, 2).
     It is equivalent to `scalb (X, (double) -ilogb (X))'.

     This function exists mainly for use in certain standardized tests
     of IEEE 754 conformance.


File: libc.info,  Node: Rounding Functions,  Next: Remainder Functions,  Prev: Normalization Functions,  Up: Arithmetic Functions

Rounding Functions
------------------

   The functions listed here perform operations such as rounding and
truncation of floating-point values. Some of these functions convert
floating point numbers to integer values.  They are all declared in
`math.h'.

   You can also convert floating-point numbers to integers simply by
casting them to `int'.  This discards the fractional part, effectively
rounding towards zero.  However, this only works if the result can
actually be represented as an `int'--for very large numbers, this is
impossible.  The functions listed here return the result as a `double'
instead to get around this problem.

 - Function: double ceil (double X)
 - Function: float ceilf (float X)
 - Function: long double ceill (long double X)
     These functions round X upwards to the nearest integer, returning
     that value as a `double'.  Thus, `ceil (1.5)' is `2.0'.

 - Function: double floor (double X)
 - Function: float floorf (float X)
 - Function: long double floorl (long double X)
     These functions round X downwards to the nearest integer,
     returning that value as a `double'.  Thus, `floor (1.5)' is `1.0'
     and `floor (-1.5)' is `-2.0'.

 - Function: double trunc (double X)
 - Function: float truncf (float X)
 - Function: long double truncl (long double X)
     `trunc' is another name for `floor'

 - Function: double rint (double X)
 - Function: float rintf (float X)
 - Function: long double rintl (long double X)
     These functions round X to an integer value according to the
     current rounding mode.  *Note Floating Point Parameters::, for
     information about the various rounding modes.  The default
     rounding mode is to round to the nearest integer; some machines
     support other modes, but round-to-nearest is always used unless
     you explicitly select another.

     If X was not initially an integer, these functions raise the
     inexact exception.

 - Function: double nearbyint (double X)
 - Function: float nearbyintf (float X)
 - Function: long double nearbyintl (long double X)
     These functions return the same value as the `rint' functions, but
     do not raise the inexact exception if X is not an integer.

 - Function: double round (double X)
 - Function: float roundf (float X)
 - Function: long double roundl (long double X)
     These functions are similar to `rint', but they round halfway
     cases away from zero instead of to the nearest even integer.

 - Function: long int lrint (double X)
 - Function: long int lrintf (float X)
 - Function: long int lrintl (long double X)
     These functions are just like `rint', but they return a `long int'
     instead of a floating-point number.

 - Function: long long int llrint (double X)
 - Function: long long int llrintf (float X)
 - Function: long long int llrintl (long double X)
     These functions are just like `rint', but they return a `long long
     int' instead of a floating-point number.

 - Function: long int lround (double X)
 - Function: long int lroundf (float X)
 - Function: long int lroundl (long double X)
     These functions are just like `round', but they return a `long
     int' instead of a floating-point number.

 - Function: long long int llround (double X)
 - Function: long long int llroundf (float X)
 - Function: long long int llroundl (long double X)
     These functions are just like `round', but they return a `long
     long int' instead of a floating-point number.

 - Function: double modf (double VALUE, double *INTEGER-PART)
 - Function: float modff (float VALUE, float *INTEGER-PART)
 - Function: long double modfl (long double VALUE, long double
          *INTEGER-PART)
     These functions break the argument VALUE into an integer part and a
     fractional part (between `-1' and `1', exclusive).  Their sum
     equals VALUE.  Each of the parts has the same sign as VALUE, and
     the integer part is always rounded toward zero.

     `modf' stores the integer part in `*INTEGER-PART', and returns the
     fractional part.  For example, `modf (2.5, &intpart)' returns
     `0.5' and stores `2.0' into `intpart'.


File: libc.info,  Node: Remainder Functions,  Next: FP Bit Twiddling,  Prev: Rounding Functions,  Up: Arithmetic Functions

Remainder Functions
-------------------

   The functions in this section compute the remainder on division of
two floating-point numbers.  Each is a little different; pick the one
that suits your problem.

 - Function: double fmod (double NUMERATOR, double DENOMINATOR)
 - Function: float fmodf (float NUMERATOR, float DENOMINATOR)
 - Function: long double fmodl (long double NUMERATOR, long double
          DENOMINATOR)
     These functions compute the remainder from the division of
     NUMERATOR by DENOMINATOR.  Specifically, the return value is
     `NUMERATOR - N * DENOMINATOR', where N is the quotient of
     NUMERATOR divided by DENOMINATOR, rounded towards zero to an
     integer.  Thus, `fmod (6.5, 2.3)' returns `1.9', which is `6.5'
     minus `4.6'.

     The result has the same sign as the NUMERATOR and has magnitude
     less than the magnitude of the DENOMINATOR.

     If DENOMINATOR is zero, `fmod' signals a domain error.

 - Function: double drem (double NUMERATOR, double DENOMINATOR)
 - Function: float dremf (float NUMERATOR, float DENOMINATOR)
 - Function: long double dreml (long double NUMERATOR, long double
          DENOMINATOR)
     These functions are like `fmod' except that they rounds the
     internal quotient N to the nearest integer instead of towards zero
     to an integer.  For example, `drem (6.5, 2.3)' returns `-0.4',
     which is `6.5' minus `6.9'.

     The absolute value of the result is less than or equal to half the
     absolute value of the DENOMINATOR.  The difference between `fmod
     (NUMERATOR, DENOMINATOR)' and `drem (NUMERATOR, DENOMINATOR)' is
     always either DENOMINATOR, minus DENOMINATOR, or zero.

     If DENOMINATOR is zero, `drem' signals a domain error.

 - Function: double remainder (double NUMERATOR, double DENOMINATOR)
 - Function: float remainderf (float NUMERATOR, float DENOMINATOR)
 - Function: long double remainderl (long double NUMERATOR, long double
          DENOMINATOR)
     This function is another name for `drem'.


File: libc.info,  Node: FP Bit Twiddling,  Next: FP Comparison Functions,  Prev: Remainder Functions,  Up: Arithmetic Functions

Setting and modifying single bits of FP values
----------------------------------------------

   There are some operations that are too complicated or expensive to
perform by hand on floating-point numbers.  ISO C 9x defines functions
to do these operations, which mostly involve changing single bits.

 - Function: double copysign (double X, double Y)
 - Function: float copysignf (float X, float Y)
 - Function: long double copysignl (long double X, long double Y)
     These functions return X but with the sign of Y.  They work even
     if X or Y are NaN or zero.  Both of these can carry a sign
     (although not all implementations support it) and this is one of
     the few operations that can tell the difference.

     `copysign' never raises an exception.

     This function is defined in IEC 559 (and the appendix with
     recommended functions in IEEE 754/IEEE 854).

 - Function: int signbit (*float-type* X)
     `signbit' is a generic macro which can work on all floating-point
     types.  It returns a nonzero value if the value of X has its sign
     bit set.

     This is not the same as `x < 0.0', because IEEE 754 floating point
     allows zero to be signed.  The comparison `-0.0 < 0.0' is false,
     but `signbit (-0.0)' will return a nonzero value.

 - Function: double nextafter (double X, double Y)
 - Function: float nextafterf (float X, float Y)
 - Function: long double nextafterl (long double X, long double Y)
     The `nextafter' function returns the next representable neighbor of
     X in the direction towards Y.  The size of the step between X and
     the result depends on the type of the result.  If X = Y the
     function simply returns X.  If either value is `NaN', `NaN' is
     returned.  Otherwise a value corresponding to the value of the
     least significant bit in the mantissa is added or subtracted,
     depending on the direction.  `nextafter' will signal overflow or
     underflow if the result goes outside of the range of normalized
     numbers.

     This function is defined in IEC 559 (and the appendix with
     recommended functions in IEEE 754/IEEE 854).

 - Function: double nexttoward (double X, long double Y)
 - Function: float nexttowardf (float X, long double Y)
 - Function: long double nexttowardl (long double X, long double Y)
     These functions are identical to the corresponding versions of
     `nextafter' except that their second argument is a `long double'.

 - Function: double nan (const char *TAGP)
 - Function: float nanf (const char *TAGP)
 - Function: long double nanl (const char *TAGP)
     The `nan' function returns a representation of NaN, provided that
     NaN is supported by the target platform.  `nan
     ("N-CHAR-SEQUENCE")' is equivalent to `strtod
     ("NAN(N-CHAR-SEQUENCE)")'.

     The argument TAGP is used in an unspecified manner.  On IEEE 754
     systems, there are many representations of NaN, and TAGP selects
     one.  On other systems it may do nothing.


File: libc.info,  Node: FP Comparison Functions,  Next: Misc FP Arithmetic,  Prev: FP Bit Twiddling,  Up: Arithmetic Functions

Floating-Point Comparison Functions
-----------------------------------

   The standard C comparison operators provoke exceptions when one or
other of the operands is NaN.  For example,

     int v = a < 1.0;

will raise an exception if A is NaN.  (This does *not* happen with `=='
and `!='; those merely return false and true, respectively, when NaN is
examined.)  Frequently this exception is undesirable.  ISO C 9x
therefore defines comparison functions that do not raise exceptions
when NaN is examined.  All of the functions are implemented as macros
which allow their arguments to be of any floating-point type.  The
macros are guaranteed to evaluate their arguments only once.

 - Macro: int isgreater (*real-floating* X, *real-floating* Y)
     This macro determines whether the argument X is greater than Y.
     It is equivalent to `(X) > (Y)', but no exception is raised if X
     or Y are NaN.

 - Macro: int isgreaterequal (*real-floating* X, *real-floating* Y)
     This macro determines whether the argument X is greater than or
     equal to Y.  It is equivalent to `(X) >= (Y)', but no exception is
     raised if X or Y are NaN.

 - Macro: int isless (*real-floating* X, *real-floating* Y)
     This macro determines whether the argument X is less than Y.  It
     is equivalent to `(X) < (Y)', but no exception is raised if X or Y
     are NaN.

 - Macro: int islessequal (*real-floating* X, *real-floating* Y)
     This macro determines whether the argument X is less than or equal
     to Y.  It is equivalent to `(X) <= (Y)', but no exception is
     raised if X or Y are NaN.

 - Macro: int islessgreater (*real-floating* X, *real-floating* Y)
     This macro determines whether the argument X is less or greater
     than Y.  It is equivalent to `(X) < (Y) || (X) > (Y)' (although it
     only evaluates X and Y once), but no exception is raised if X or Y
     are NaN.

     This macro is not equivalent to `X != Y', because that expression
     is true if X or Y are NaN.

 - Macro: int isunordered (*real-floating* X, *real-floating* Y)
     This macro determines whether its arguments are unordered.  In
     other words, it is true if X or Y are NaN, and false otherwise.

   Not all machines provide hardware support for these operations.  On
machines that don't, the macros can be very slow.  Therefore, you should
not use these functions when NaN is not a concern.

   *Note:* There are no macros `isequal' or `isunequal'.  They are
unnecessary, because the `==' and `!=' operators do *not* throw an
exception if one or both of the operands are NaN.


File: libc.info,  Node: Misc FP Arithmetic,  Prev: FP Comparison Functions,  Up: Arithmetic Functions

Miscellaneous FP arithmetic functions
-------------------------------------

   The functions in this section perform miscellaneous but common
operations that are awkward to express with C operators.  On some
processors these functions can use special machine instructions to
perform these operations faster than the equivalent C code.

 - Function: double fmin (double X, double Y)
 - Function: float fminf (float X, float Y)
 - Function: long double fminl (long double X, long double Y)
     The `fmin' function returns the lesser of the two values X and Y.
     It is similar to the expression
          ((x) < (y) ? (x) : (y))
     except that X and Y are only evaluated once.

     If an argument is NaN, the other argument is returned.  If both
     arguments are NaN, NaN is returned.

 - Function: double fmax (double X, double Y)
 - Function: float fmaxf (float X, float Y)
 - Function: long double fmaxl (long double X, long double Y)
     The `fmax' function returns the greater of the two values X and Y.

     If an argument is NaN, the other argument is returned.  If both
     arguments are NaN, NaN is returned.

 - Function: double fdim (double X, double Y)
 - Function: float fdimf (float X, float Y)
 - Function: long double fdiml (long double X, long double Y)
     The `fdim' function returns the positive difference between X and
     Y.  The positive difference is X - Y if X is greater than Y, and 0
     otherwise.

     If X, Y, or both are NaN, NaN is returned.

 - Function: double fma (double X, double Y, double Z)
 - Function: float fmaf (float X, float Y, float Z)
 - Function: long double fmal (long double X, long double Y, long
          double Z)
     The `fma' function performs floating-point multiply-add.  This is
     the operation (X * Y) + Z, but the intermediate result is not
     rounded to the destination type.  This can sometimes improve the
     precision of a calculation.

     This function was introduced because some processors have a special
     instruction to perform multiply-add.  The C compiler cannot use it
     directly, because the expression `x*y + z' is defined to round the
     intermediate result.  `fma' lets you choose when you want to round
     only once.

     On processors which do not implement multiply-add in hardware,
     `fma' can be very slow since it must avoid intermediate rounding.
     `math.h' defines the symbols `FP_FAST_FMA', `FP_FAST_FMAF', and
     `FP_FAST_FMAL' when the corresponding version of `fma' is no
     slower than the expression `x*y + z'.  In the GNU C library, this
     always means the operation is implemented in hardware.


File: libc.info,  Node: Complex Numbers,  Next: Operations on Complex,  Prev: Arithmetic Functions,  Up: Arithmetic

Complex Numbers
===============

   ISO C 9x introduces support for complex numbers in C.  This is done
with a new type qualifier, `complex'.  It is a keyword if and only if
`complex.h' has been included.  There are three complex types,
corresponding to the three real types:  `float complex', `double
complex', and `long double complex'.

   To construct complex numbers you need a way to indicate the imaginary
part of a number.  There is no standard notation for an imaginary
floating point constant.  Instead, `complex.h' defines two macros that
can be used to create complex numbers.

 - Macro: const float complex _Complex_I
     This macro is a representation of the complex number "0+1i".
     Multiplying a real floating-point value by `_Complex_I' gives a
     complex number whose value is purely imaginary.  You can use this
     to construct complex constants:

          3.0 + 4.0i = `3.0 + 4.0 * _Complex_I'

     Note that `_Complex_I * _Complex_I' has the value `-1', but the
     type of that value is `complex'.

`_Complex_I' is a bit of a mouthful.  `complex.h' also defines a
shorter name for the same constant.

 - Macro: const float complex I
     This macro has exactly the same value as `_Complex_I'.  Most of the
     time it is preferable.  However, it causes problems if you want to
     use the identifier `I' for something else.  You can safely write

          #include <complex.h>
          #undef I

     if you need `I' for your own purposes.  (In that case we recommend
     you also define some other short name for `_Complex_I', such as
     `J'.)


