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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 the GNU C Library 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`) Preliminary: | MT-Unsafe race:ecvt | AS-Unsafe | AC-Safe | See POSIX Safety Concepts.

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.`*`

is set to the index in the string of the first digit after the decimal point.`decpt``*`

is set to a nonzero value if`neg``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`*`

is`decpt``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`) Preliminary: | MT-Unsafe race:fcvt | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

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’th place to the left of the decimal point. For example, if`ndigit`+1`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`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

`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`) Preliminary: | MT-Unsafe race:qecvt | AS-Unsafe | AC-Safe | See POSIX Safety Concepts.

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`) Preliminary: | MT-Unsafe race:qfcvt | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

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`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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:
*int***ecvt_r***(double*`value`, int`ndigit`, int *`decpt`, int *`neg`, char *`buf`, size_t`len`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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`. The return value is`-1`

in case of an error and zero otherwise.This function is a GNU extension.

- Function:
*int***fcvt_r***(double*`value`, int`ndigit`, int *`decpt`, int *`neg`, char *`buf`, size_t`len`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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`. The return value is`-1`

in case of an error and zero otherwise.This function is a GNU extension.

- Function:
*int***qecvt_r***(long double*`value`, int`ndigit`, int *`decpt`, int *`neg`, char *`buf`, size_t`len`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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`. The return value is`-1`

in case of an error and zero otherwise.This function is a GNU extension.

- Function:
*int***qfcvt_r***(long double*`value`, int`ndigit`, int *`decpt`, int *`neg`, char *`buf`, size_t`len`) Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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`. The return value is`-1`

in case of an error and zero otherwise.This function is a GNU extension.

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