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### 20.12 Old-fashioned System V number-to-string functions

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)

> 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: int 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. 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)

> 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)

> 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)

> 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.