.
This chapter contains information about functions for doing basic
arithmetic operations, such as splitting a float into its integer and
fractional parts or retrieving the imaginary part of a complex value.
These functions are declared in the header files `math.h' and
`complex.h'.
Most computer hardware has support for two different kinds of numbers:
integers (...-3, -2, -1, 0, 1, 2, 3...) and
floating-point numbers. Floating-point numbers have three parts: the
mantissa, the exponent, and the sign bit. The real
number represented by a floating-point value is given by
@ifnottex
(s ? -1 : 1) @mul{ 2^e @mul{} M}
where s is the sign bit, e the exponent, and M
the mantissa. See section Floating Point Representation Concepts, for details. (It is
possible to have a different base for the exponent, but all modern
hardware uses 2.)
Floating-point numbers can represent a finite subset of the real
numbers. While this subset is large enough for most purposes, it is
important to remember that the only reals that can be represented
exactly are rational numbers that have a terminating binary expansion
shorter than the width of the mantissa. Even simple fractions such as
1/5 can only be approximated by floating point.
Mathematical operations and functions frequently need to produce values
that are not representable. Often these values can be approximated
closely enough for practical purposes, but sometimes they can't.
Historically there was no way to tell when the results of a calculation
were inaccurate. Modern computers implement the IEEE 754 standard
for numerical computations, which defines a framework for indicating to
the program when the results of calculation are not trustworthy. This
framework consists of a set of exceptions that indicate why a
result could not be represented, and the special values infinity
and not a number (NaN).
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" (see section Infinity and NaN)
FP_INFINITE
-
The value of x is either plus or minus infinity (see section 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
(see section Floating Point Representation 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. See section Error Reporting by Mathematical Functions. -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.
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. See section 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):
-
Addition or subtraction: @infinity{ - @infinity{}}. (But
@infinity{ + @infinity{} = @infinity{}}).
-
Multiplication: 0 @mul{ @infinity{}}.
-
Division: 0/0 or @infinity{/@infinity{}}.
-
Remainder: x REM y, where y is zero or x is
infinite.
-
Square root if the operand is less then zero. More generally, any
mathematical function evaluated outside its domain produces this
exception.
-
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).
-
Conversion of an unrecognizable input string.
-
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 section Floating-Point 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 +@infinity{} or
-@infinity{}, 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):
-
Round to nearest carries all overflows to @infinity{}
with the sign of the intermediate result.
-
Round toward 0 carries all overflows to the largest representable
finite number with the sign of the intermediate result.
-
Round toward -@infinity{} carries positive overflows to the
largest representable finite number and negative overflows to
-@infinity{}.
-
Round toward @infinity{} carries negative overflows to the
most negative representable finite number and positive overflows
to @infinity{}.
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.
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:
@ifnottex
1/0 = @infinity{}
log (0) = -@infinity{}
sqrt (-1) = NaN
When a calculation produces any of these values, an exception also
occurs; see section 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 + @infinity{ = @infinity{}},
4/@infinity{ = 0}, atan @math{(@infinity{}) = @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 (see section Floating-Point Number Classification Functions). 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. See section Floating-Point Number Classification Functions.
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.
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.
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 @infinity{} or
-@infinity{} 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.
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 +@infinity{}.
FE_DOWNWARD
-
Round toward -@infinity{}.
FE_TOWARDZERO
-
Round toward zero.
Underflow is an unusual case. Normally, IEEE 754 floating point
numbers are always normalized (see section Floating Point Representation 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/-@infinity{}. 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.
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 section 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.
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.
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
(see section 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 section Exponentiation and Logarithms.
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 section Floating Point Representation 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.
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. See section 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.
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.
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.
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.
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 @mul{ 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.
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.)
ISO C 9x also defines functions that perform basic operations on
complex numbers, such as decomposition and conjugation. The prototypes
for all these functions are in `complex.h'. All functions are
available in three variants, one for each of the three complex types.
- Function: double creal (complex double z)
-
- Function: float crealf (complex float z)
-
- Function: long double creall (complex long double z)
-
These functions return the real part of the complex number z.
- Function: double cimag (complex double z)
-
- Function: float cimagf (complex float z)
-
- Function: long double cimagl (complex long double z)
-
These functions return the imaginary part of the complex number z.
- Function: complex double conj (complex double z)
-
- Function: complex float conjf (complex float z)
-
- Function: complex long double conjl (complex long double z)
-
These functions return the conjugate value of the complex number
z. The conjugate of a complex number has the same real part and a
negated imaginary part. In other words, `conj(a + bi) = a + -bi'.
- Function: double carg (complex double z)
-
- Function: float cargf (complex float z)
-
- Function: long double cargl (complex long double z)
-
These functions return the argument of the complex number z.
The argument of a complex number is the angle in the complex plane
between the positive real axis and a line passing through zero and the
number. This angle is measured in the usual fashion and ranges from 0
to 2@pi{}.
carg has a branch cut along the positive real axis.
- Function: complex double cproj (complex double z)
-
- Function: complex float cprojf (complex float z)
-
- Function: complex long double cprojl (complex long double z)
-
These functions return the projection of the complex value z onto
the Riemann sphere. Values with a infinite imaginary part are projected
to positive infinity on the real axis, even if the real part is NaN. If
the real part is infinite, the result is equivalent to
INFINITY + I * copysign (0.0, cimag (z))
This section describes functions for performing integer division. These
functions are redundant when GNU CC is used, because in GNU C the
`/' operator always rounds towards zero. But in other C
implementations, `/' may round differently with negative arguments.
div and ldiv are useful because they specify how to round
the quotient: towards zero. The remainder has the same sign as the
numerator.
These functions are specified to return a result r such that the value
r.quot*denominator + r.rem equals
numerator.
To use these facilities, you should include the header file
`stdlib.h' in your program.
- Data Type: div_t
-
This is a structure type used to hold the result returned by the
div
function. It has the following members:
int quot
-
The quotient from the division.
int rem
-
The remainder from the division.
- Function: div_t div (int numerator, int denominator)
-
This function
div computes the quotient and remainder from
the division of numerator by denominator, returning the
result in a structure of type div_t.
If the result cannot be represented (as in a division by zero), the
behavior is undefined.
Here is an example, albeit not a very useful one.
div_t result;
result = div (20, -6);
Now result.quot is -3 and result.rem is 2.
- Data Type: ldiv_t
-
This is a structure type used to hold the result returned by the
ldiv
function. It has the following members:
long int quot
-
The quotient from the division.
long int rem
-
The remainder from the division.
(This is identical to div_t except that the components are of
type long int rather than int.)
- Function: ldiv_t ldiv (long int numerator, long int denominator)
-
The
ldiv function is similar to div, except that the
arguments are of type long int and the result is returned as a
structure of type ldiv_t.
- Data Type: lldiv_t
-
This is a structure type used to hold the result returned by the
lldiv
function. It has the following members:
long long int quot
-
The quotient from the division.
long long int rem
-
The remainder from the division.
(This is identical to div_t except that the components are of
type long long int rather than int.)
- Function: lldiv_t lldiv (long long int numerator, long long int denominator)
-
The
lldiv function is like the div function, but the
arguments are of type long long int and the result is returned as
a structure of type lldiv_t.
The lldiv function was added in ISO C 9x.
- Data Type: imaxdiv_t
-
This is a structure type used to hold the result returned by the
imaxdiv
function. It has the following members:
intmax_t quot
-
The quotient from the division.
intmax_t rem
-
The remainder from the division.
(This is identical to div_t except that the components are of
type intmax_t rather than int.)
- Function: imaxdiv_t imaxdiv (intmax_t numerator, intmax_t denominator)
-
The
imaxdiv function is like the div function, but the
arguments are of type intmax_t and the result is returned as
a structure of type imaxdiv_t.
The imaxdiv function was added in ISO C 9x.
This section describes functions for "reading" integer and
floating-point numbers from a string. It may be more convenient in some
cases to use sscanf or one of the related functions; see
section Formatted Input. But often you can make a program more robust by
finding the tokens in the string by hand, then converting the numbers
one by one.
These functions are declared in `stdlib.h'.
- Function: long int strtol (const char *string, char **tailptr, int base)
-
The
strtol ("string-to-long") function converts the initial
part of string to a signed integer, which is returned as a value
of type long int.
This function attempts to decompose string as follows:
-
A (possibly empty) sequence of whitespace characters. Which characters
are whitespace is determined by the
isspace function
(see section Classification of Characters). These are discarded.
-
An optional plus or minus sign (`+' or `-').
-
A nonempty sequence of digits in the radix specified by base.
If base is zero, decimal radix is assumed unless the series of
digits begins with `0' (specifying octal radix), or `0x' or
`0X' (specifying hexadecimal radix); in other words, the same
syntax used for integer constants in C.
Otherwise base must have a value between
2 and 35.
If base is 16, the digits may optionally be preceded by
`0x' or `0X'. If base has no legal value the value returned
is 0l and the global variable errno is set to EINVAL.
-
Any remaining characters in the string. If tailptr is not a null
pointer,
strtol stores a pointer to this tail in
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for an integer in the
specified base, no conversion is performed. In this case,
strtol returns a value of zero and the value stored in
*tailptr is the value of string.
In a locale other than the standard "C" locale, this function
may recognize additional implementation-dependent syntax.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtol returns either
LONG_MAX or LONG_MIN (see section Range of an Integer Type), as
appropriate for the sign of the value. It also sets errno
to ERANGE to indicate there was overflow.
You should not check for errors by examining the return value of
strtol, because the string might be a valid representation of
0l, LONG_MAX, or LONG_MIN. Instead, check whether
tailptr points to what you expect after the number
(e.g. '\0' if the string should end after the number). You also
need to clear errno before the call and check it afterward, in
case there was overflow.
There is an example at the end of this section.
- Function: unsigned long int strtoul (const char *string, char **tailptr, int base)
-
The
strtoul ("string-to-unsigned-long") function is like
strtol except it returns an unsigned long int value. If
the number has a leading `-' sign, the return value is negated.
The syntax is the same as described above for strtol. The value
returned on overflow is ULONG_MAX (see section Range of an Integer Type).
strtoul sets errno to EINVAL if base is out of
range, or ERANGE on overflow.
- Function: long long int strtoll (const char *string, char **tailptr, int base)
-
The
strtoll function is like strtol except that it returns
a long long int value, and accepts numbers with a correspondingly
larger range.
If the string has valid syntax for an integer but the value is not
representable because of overflow, strtoll returns either
LONG_LONG_MAX or LONG_LONG_MIN (see section Range of an Integer Type), as
appropriate for the sign of the value. It also sets errno to
ERANGE to indicate there was overflow.
The strtoll function was introduced in ISO C 9x.
- Function: long long int strtoq (const char *string, char **tailptr, int base)
-
strtoq ("string-to-quad-word") is the BSD name for strtoll.
- Function: unsigned long long int strtoull (const char *string, char **tailptr, int base)
-
The
strtoull function is like strtoul except that it
returns an unsigned long long int. The value returned on overflow
is ULONG_LONG_MAX (see section Range of an Integer Type).
The strtoull function was introduced in ISO C 9x.
- Function: unsigned long long int strtouq (const char *string, char **tailptr, int base)
-
strtouq is the BSD name for strtoull.
- Function: long int atol (const char *string)
-
This function is similar to the
strtol function with a base
argument of 10, except that it need not detect overflow errors.
The atol function is provided mostly for compatibility with
existing code; using strtol is more robust.
- Function: int atoi (const char *string)
-
This function is like
atol, except that it returns an int.
The atoi function is also considered obsolete; use strtol
instead.
- Function: long long int atoll (const char *string)
-
This function is similar to
atol, except it returns a long
long int.
The atoll function was introduced in ISO C 9x. It too is
obsolete (despite having just been added); use strtoll instead.
Some locales specify a printed syntax for numbers other than the one
that these functions understand. If you need to read numbers formatted
in some other locale, you can use the strtoX_l functions. Each
of the strtoX functions has a counterpart with `_l' added to
its name. The `_l' counterparts take an additional argument: a
pointer to an locale_t structure, which describes how the numbers
to be read are formatted. See section Locales and Internationalization.
Portability Note: These functions are all GNU extensions. You
can also use scanf or its relatives, which have the `'' flag
for parsing numeric input according to the current locale
(see section Numeric Input Conversions). This feature is standard.
Here is a function which parses a string as a sequence of integers and
returns the sum of them:
int
sum_ints_from_string (char *string)
{
int sum = 0;
while (1) {
char *tail;
int next;
/* Skip whitespace by hand, to detect the end. */
while (isspace (*string)) string++;
if (*string == 0)
break;
/* There is more nonwhitespace, */
/* so it ought to be another number. */
errno = 0;
/* Parse it. */
next = strtol (string, &tail, 0);
/* Add it in, if not overflow. */
if (errno)
printf ("Overflow\n");
else
sum += next;
/* Advance past it. */
string = tail;
}
return sum;
}
These functions are declared in `stdlib.h'.
- Function: double strtod (const char *string, char **tailptr)
-
The
strtod ("string-to-double") function converts the initial
part of string to a floating-point number, which is returned as a
value of type double.
This function attempts to decompose string as follows:
-
A (possibly empty) sequence of whitespace characters. Which characters
are whitespace is determined by the
isspace function
(see section Classification of Characters). These are discarded.
-
An optional plus or minus sign (`+' or `-').
-
A nonempty sequence of digits optionally containing a decimal-point
character--normally `.', but it depends on the locale
(see section Generic Numeric Formatting Parameters).
-
An optional exponent part, consisting of a character `e' or
`E', an optional sign, and a sequence of digits.
-
Any remaining characters in the string. If tailptr is not a null
pointer, a pointer to this tail of the string is stored in
*tailptr.
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for a floating-point
number, no conversion is performed. In this case, strtod returns
a value of zero and the value returned in *tailptr is the
value of string.
In a locale other than the standard "C" or "POSIX" locales,
this function may recognize additional locale-dependent syntax.
If the string has valid syntax for a floating-point number but the value
is outside the range of a double, strtod will signal
overflow or underflow as described in section Error Reporting by Mathematical Functions.
strtod recognizes four special input strings. The strings
"inf" and "infinity" are converted to @infinity{},
or to the largest representable value if the floating-point format
doesn't support infinities. You can prepend a "+" or "-"
to specify the sign. Case is ignored when scanning these strings.
The strings "nan" and "nan(chars...)" are converted
to NaN. Again, case is ignored. If chars... are provided, they
are used in some unspecified fashion to select a particular
representation of NaN (there can be several).
Since zero is a valid result as well as the value returned on error, you
should check for errors in the same way as for strtol, by
examining errno and tailptr.
- Function: float strtof (const char *string, char **tailptr)
-
- Function: long double strtold (const char *string, char **tailptr)
-
These functions are analogous to
strtod, but return float
and long double values respectively. They report errors in the
same way as strtod. strtof can be substantially faster
than strtod, but has less precision; conversely, strtold
can be much slower but has more precision (on systems where long
double is a separate type).
These functions are GNU extensions.
- Function: double atof (const char *string)
-
This function is similar to the
strtod function, except that it
need not detect overflow and underflow errors. The atof function
is provided mostly for compatibility with existing code; using
strtod is more robust.
The GNU C library also provides `_l' versions of thse functions,
which take an additional argument, the locale to use in conversion.
See section Parsing of Integers.
The old System V C library provided three functions to convert
numbers to strings, with unusual and hard-to-use semantics. The GNU C
library also provides these functions and some natural extensions.
These functions are only available in glibc and on systems descended
from AT&T Unix. Therefore, unless these functions do precisely what you
need, it is better to use sprintf, which is standard.
All these functions are defined in `stdlib.h'.
- Function: char * ecvt (double value, int ndigit, int *decpt, int *neg)
-
The function
ecvt converts the floating-point number value
to a string with at most ndigit decimal digits. The
returned string contains no decimal point or sign. The first digit of
the string is non-zero (unless value is actually zero) and the
last digit is rounded to nearest. *decpt is set to the
index in the string of the first digit after the decimal point.
*neg is set to a nonzero value if value is negative,
zero otherwise.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call
to ecvt.
If value is zero, it is implementation defined whether
*decpt is 0 or 1.
For example: ecvt (12.3, 5, &d, &n) returns "12300"
and sets d to 2 and n to 0.
- Function: char * fcvt (double value, int ndigit, int *decpt, int *neg)
-
The function
fcvt is like ecvt, but ndigit specifies
the number of digits after the decimal point. If ndigit is less
than zero, value is rounded to the ndigit+1'th place to the
left of the decimal point. For example, if ndigit is -1,
value will be rounded to the nearest 10. If ndigit is
negative and larger than the number of digits to the left of the decimal
point in value, value will be rounded to one significant digit.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
The returned string is statically allocated and overwritten by each call
to fcvt.
- Function: char * gcvt (double value, int ndigit, char *buf)
-
gcvt is functionally equivalent to `sprintf(buf, "%*g",
ndigit, value'. It is provided only for compatibility's sake. It
returns buf.
If ndigit decimal digits would exceed the precision of a
double it is reduced to a system-specific value.
As extensions, the GNU C library provides versions of these three
functions that take long double arguments.
- Function: char * qecvt (long double value, int ndigit, int *decpt, int *neg)
-
This function is equivalent to
ecvt except that it takes a
long double for the first parameter and that ndigit is
restricted by the precision of a long double.
- Function: char * qfcvt (long double value, int ndigit, int *decpt, int *neg)
-
This function is equivalent to
fcvt except that it
takes a long double for the first parameter and that ndigit is
restricted by the precision of a long double.
- Function: char * qgcvt (long double value, int ndigit, char *buf)
-
This function is equivalent to
gcvt except that it takes a
long double for the first parameter and that ndigit is
restricted by the precision of a long double.
The ecvt and fcvt functions, and their long double
equivalents, all return a string located in a static buffer which is
overwritten by the next call to the function. The GNU C library
provides another set of extended functions which write the converted
string into a user-supplied buffer. These have the conventional
_r suffix.
gcvt_r is not necessary, because gcvt already uses a
user-supplied buffer.
- Function: char * ecvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
-
The
ecvt_r function is the same as ecvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
- Function: char * fcvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
-
The
fcvt_r function is the same as fcvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
- Function: char * qecvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
-
The
qecvt_r function is the same as qecvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
- Function: char * qfcvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
-
The
qfcvt_r function is the same as qfcvt, except
that it places its result into the user-specified buffer pointed to by
buf, with length len.
This function is a GNU extension.
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