return value of strtod() if string equals to zero

Question

As per MSDN:

• strtod returns 0 if no conversion can be performed or an underflow occurs.

What if my string equals to zero (i.e., 0.0000)

Answer 1

Use the str_end parameter of the function. For example:

const char* str = "123junk";
char* str_end;
double d = strtod(str, &str_end);
// Here:
//   d will be 123
//   str_end will point to (str + 3) (the 'j')

// You can tell the string has some junk data if *str_end != '\0'
if (*str_end != '\0') {
    printf("Found bad data '%s' at end of string\n", str_end);
}

If conversion totally fails, str will equal str_end:

const char* str = "junk";
char* str_end;
double d = strtod(str, &str_end);
// Here:
//   d will be 0 (conversion failed)
//   str_end will equal str

if (str == str_end) {
    printf("The string doesn't start with a number!\n");
}

You can combine these two methods to make sure the string was (completely) successfully converted (that is, by checking str != str_end && *str_end == '\0')