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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'.
20.1 Integers Basic integer types and concepts 20.2 Integer Division Integer division with guaranteed rounding. 20.3 Floating Point Numbers Basic concepts. IEEE 754. 20.4 Floating-Point Number Classification Functions The five kinds of floating-point number. 20.5 Errors in Floating-Point Calculations When something goes wrong in a calculation. 20.6 Rounding Modes Controlling how results are rounded. 20.7 Floating-Point Control Functions Saving and restoring the FPU's state. 20.8 Arithmetic Functions Fundamental operations provided by the library. 20.9 Complex Numbers The types. Writing complex constants. 20.10 Projections, Conjugates, and Decomposing of Complex Numbers Projection, conjugation, decomposition. 20.11 Parsing of Numbers Converting strings to numbers. 20.12 Old-fashioned System V number-to-string functions An archaic way to convert numbers to strings.
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The C language defines several integer data types: integer, short integer, long integer, and character, all in both signed and unsigned varieties. The GNU C compiler extends the language to contain long long integers as well.
The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines. The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on.
To address this problem, the GNU C library contains C type definitions you can use to declare integers that meet your exact needs. Because the GNU C library header files are customized to a specific machine, your program source code doesn't have to be.
These typedefs are in `stdint.h'.
If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness:
If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist.
If you don't need a specific storage size, but want the smallest data structure with at least N bits, use one of these:
If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these:
If you want an integer with the widest range possible on the platform on which it is being used, use one of the following. If you use these, you should write code that takes into account the variable size and range of the integer.
The GNU C library also provides macros that tell you the maximum and
minimum possible values for each integer data type. The macro names
follow these examples: INT32_MAX, UINT8_MAX,
INT_FAST32_MIN, INT_LEAST64_MIN, UINTMAX_MAX,
INTMAX_MAX, INTMAX_MIN. Note that there are no macros for
unsigned integer minima. These are always zero.
There are similar macros for use with C's built in integer types which should come with your C compiler. These are described in A.5 Data Type Measurements.
Don't forget you can use the C sizeof function with any of these
data types to get the number of bytes of storage each uses.
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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.
div
function. It has the following members:
int quot
int rem
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.
ldiv
function. It has the following members:
long int quot
long int rem
(This is identical to div_t except that the components are of
type long int rather than int.)
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.
lldiv
function. It has the following members:
long long int quot
long long int rem
(This is identical to div_t except that the components are of
type long long int rather than int.)
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 C99.
imaxdiv
function. It has the following members:
intmax_t quot
intmax_t rem
(This is identical to div_t except that the components are of
type intmax_t rather than int.)
See 20.1 Integers for a description of the intmax_t type.
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.
See 20.1 Integers for a description of the intmax_t type.
The imaxdiv function was added in ISO C99.
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Most computer hardware has support for two different kinds of numbers:
integers (
) 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
where 
is the sign bit, 
the exponent, and 
the mantissa. See section A.5.3.1 Floating Point Representation Concepts, for details. (It is
possible to have a different base for the exponent, but all modern
hardware uses 
.)
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
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).
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ISO C99 defines macros that let you determine what sort of floating-point number a variable holds.
int. The possible values are:
FP_NAN
FP_INFINITE
FP_ZERO
FP_SUBNORMAL
fpclassify returns this value
for values of x in this alternate format.
FP_NORMAL
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.
(fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE) |
isfinite is implemented as a macro which accepts any
floating-point type.
(fpclassify (x) == FP_NORMAL) |
(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 ISO C99 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.
-1 if x represents negative infinity,
1 if x represents positive infinity, and 0 otherwise.
Note: The isnan macro defined by ISO C99 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) |
EDOM or ERANGE; infnan returns the
value that a math function would return if it set errno to that
value. See section 20.5.4 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.
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20.5.1 FP Exceptions IEEE 754 math exceptions and how to detect them. 20.5.2 Infinity and NaN Special values returned by calculations. 20.5.3 Examining the FPU status word Checking for exceptions after the fact. 20.5.4 Error Reporting by Mathematical Functions How the math functions report errors.
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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 24. 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:
. (But
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 20.4 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.
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 20.4 Floating-Point Number Classification Functions.
This macro was introduced in the ISO C99 standard.
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.
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ISO C99 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
FE_DIVBYZERO
FE_UNDERFLOW
FE_OVERFLOW
FE_INVALID
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.
The function returns zero in case the operation was successful, a non-zero value otherwise.
FE_OVERFLOW) or underflow (FE_UNDERFLOW) are
raised before inexact (FE_INEXACT). Whether for overflow or
underflow the inexact exception is also raised is also implementation
dependent.
The function returns zero in case the operation was successful, a non-zero value otherwise.
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:
The function returns zero in case the operation was successful, a non-zero value otherwise.
The function returns zero in case the operation was successful, a non-zero value otherwise.
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.
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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;
754} exception handling. Likewise, when overflow occurs, math
(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'.
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 C99.
See 20.1 Integers for a description of the intmax_t type.
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 19.4 Exponentiation and Logarithms.
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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 A.5.3.1 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'.
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.
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.
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.
scalb function is the BSD name for ldexp.
scalbn is identical to scalb, except that the exponent
n is an int instead of a floating-point number.
scalbln is identical to scalb, except that the exponent
n is a long int instead of a floating-point number.
significand returns the mantissa of x scaled to the range
.
It is equivalent to scalb (x, (double) -ilogb (x)).
This function exists mainly for use in certain standardized tests of IEEE 754 conformance.
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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.
double. Thus, ceil (1.5)
is 2.0.
1.0 and floor (-1.5) is -2.0.
trunc functions round x towards zero to the nearest
integer (returned in floating-point format). Thus, trunc (1.5)
is 1.0 and trunc (-1.5) is -1.0.
If x was not initially an integer, these functions raise the inexact exception.
rint functions, but
do not raise the inexact exception if x is not an integer.
rint, but they round halfway
cases away from zero instead of to the nearest even integer.
rint, but they return a
long int instead of a floating-point number.
rint, but they return a
long long int instead of a floating-point number.
round, but they return a
long int instead of a floating-point number.
round, but they return a
long long int instead of a floating-point number.
-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.
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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.
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.
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 (numerator, denominator)} is always either denominator, minus denominator, or zero.
If denominator is zero, drem signals a domain error.
drem.
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There are some operations that are too complicated or expensive to perform by hand on floating-point numbers. ISO C99 defines functions to do these operations, which mostly involve changing single bits.
copysign never raises an exception.
This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).
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.
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
the function simply returns y. 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).
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)").
754} systems, there are many representations of NaN, and tagp selects one. On other systems it may do nothing.
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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 C99 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.
(x) > (y), but no
exception is raised if x or y are NaN.
(x) >= (y), but no
exception is raised if x or y are NaN.
(x) < (y), but no exception is
raised if x or y are NaN.
(x) <= (y), 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.
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.
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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.
fmin function returns the lesser of the two values x
and y. It is similar to the expression
((x) < (y) ? (x) : (y)) |
If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.
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.
fdim function returns the positive difference between
y} if x is greater than y, and 
otherwise.
If x, y, or both are NaN, NaN is returned.
fma function performs floating-point multiply-add. This is
the operation
, 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.
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ISO C99 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.
".
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:
|
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.
= _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.)
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ISO C99 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.
long int}.
The atoll function was introduced in ISO C99. 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 7. 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 12.12.4 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;
}
|
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These functions are declared in `stdlib.h'.
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:
isspace function
(see section 4.1 Classification of Characters). These are discarded.
The hexadecimal format is as follows:
*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 20.5.4 Error Reporting by Mathematical Functions.
strtod recognizes four special input strings. The strings
double} is a separate type).
These functions have been GNU extensions and are new to ISO C99.
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 these functions, which take an additional argument, the locale to use in conversion. See section 20.11.1 Parsing of Integers.
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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'.
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.
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
'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.
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.
ecvt except that it takes a
long double for the first parameter and that ndigit is
restricted by the precision of a long double.
fcvt except that it
takes a long double for the first parameter and that ndigit is
restricted by the precision of a long double.
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.
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.
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.
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.
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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