c-strtold module contains a string to number (‘long double’)
conversion function operating on single-byte character strings, that operates
as if the locale encoding was ASCII.
(The "C" locale on many systems has the locale encoding "ASCII".)
The function is:
extern long double c_strtold (const char *string, char **endp);
In particular, only a period ‘.’ is accepted as decimal point, even when the current locale's notion of decimal point is a comma ‘,’.