_CP_xtd is equivalent to the result = xtd(string) BASIC statement.
int _CP_xtd(double* result, CPSTR* string)
This function returns -1 if an error occurs. The error code is contained in _CP_errno.
/* Prints 15. */
CPSTR * s = _CP_mkstr("F");
double f;
_CP_xtd(&f,s);
_CP_unix_env();
printf("%f\n",f);