fcvt(3) -- Linux man page
NAMEecvt, fcvt - convert a floating-point number to a string
char *ecvt(double number, int ndigits, int *decpt, int *sign);
DESCRIPTIONThe ecvt() function converts number to a null-terminated string of ndigits digits (where ndigits is reduced to an system-specific limit determined by the precision of a double), and returns a pointer to the string. The high-order digit is nonzero, unless number is zero. The low order digit is rounded. The string itself does not contain a decimal point; however, the position of the decimal point relative to the start of the string is stored in *decpt. A negative value for *decpt means that the decimal point is to the left of the start of the string. If the sign of number is negative, *sign is set to a non-zero value, otherwise it's set to 0. If number is zero, it is unspecified whether *decpt is 0 or 1.
RETURN VALUEBoth the ecvt() and fcvt() functions return a pointer to a static string containing the ASCII representation of number. The static string is overwritten by each call to ecvt() or fcvt().
NOTESThese functions are obsolete. Instead, sprintf() is recommended. Linux libc4 and libc5 specified the type of ndigits as size_t. Not all locales use a point as the radix character (`decimal point').
CONFORMING TOSysVR2, XPG2
SEE ALSOecvt_r(3), gcvt(3), qecvt(3), setlocale(3), sprintf(3)