_CP_num is equivalent to the result = num(string) BASIC function.
int _CP_num(int* result, CPSTR* string)
This function returns -1 if an error occurs. The error code is contained in _CP_errno.
/* is "1" alphabetic? */
CPSTR * s = _CP_mkstr("1");
int i;
_CP_num(&i,s);
_CP_unix_env();
printf("%d\n", i);