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 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.

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.

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.

Published under the terms of the GNU General Public License