Tcl_PrintDouble(interp, value, dst)
Tcl_PrintDouble generates a string that represents the value of value and stores it in memory at the location given by dst. It uses %g format to generate the string, with one special twist: the string is guaranteed to contain either a “.” or an “e” so that it does not look like an integer. Where %g would generate an integer with no decimal point, Tcl_PrintDouble adds “.0”
If the tcl_precision value is non-zero, the result will have precisely that many digits of significance. If the value is zero (the default), the result will have the fewest digits needed to represent the number in such a way that Tcl_NewDoubleObj will generate the same number when presented with the given string. IEEE semantics of rounding to even apply to the conversion.