size_t fmt_double(char *dest,double d,size_t maxlen,size_t prec);
fmt_double writes an ASCII representation ('0' to '9', base 10) of d to dest and returns the number of bytes written. No more than maxlen bytes will be written. prec digits will be written, using scientific notation if necessary.
fmt_double does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_double returns the number of bytes it would have written.