Converting IP addresses from char * to uin32_t

Question

I was inspired by Conversion of IP address to integer

My code looks like this:

    uint32_r parseIPV4string(char * ipAddress){
    char ipbytes[4];
    sscanf(ipAddress, "%uhh.%uhh.%uhh.%uhh", &ipbytes[3], &ipbytes[2], &ipbytes[1], &ipbytes[0]);
    return ipbytes[0] | ipbytes[1] << 8 | ipbytes[2] << 16 | ipbytes[3] << 24;
}

Virtually an exact copy, my problem is however, that my ip addresses aren't coming out properly. I am watching in shock and awe as "129.173.118.0" and "129.173.31.187" both return 2164260864

Can someone explain what is going on?

Perhaps I am using the parser incorrectly, I am unsure exactly how it works, namely the "%uhh." are new to me and I have no idea as to whatever is going on in that return statement.

Answer 1

Your sscanf is failing. When this happens, it leaves the arguments in an indeterminate state. So your return statement returns garbage.

%uhh means to read into an unsigned int , and then match the letter 'h' twice. If your input string did not actually contain h after the first number, then matching fails and sscanf will return 1 (or 0 if there wasn't even a first number).

You probably meant %hhu , to read an integer and store in an unsigned char , however you also need to make ipbytes be unsigned char to match the format specifier. The specifier for plain or signed char is %hhd .

This array should be unsigned anyway, otherwise your | later on is going to mess up (remember that negative numbers have a lot of 1 bits set, in 2's complement).

A further problem is that if your system has 32-bit int, then even when using unsigned char , the expression ipbytes[3] << 24 causes undefined behaviour if the high bit of ipbytes[3] is set, due to signed integer overflow: the integer promotions unfortunately promote unsigned char to a signed int .

Finally, if the system has 16-bit int then the << 16 and << 24 fail entirely.

A robust way to write the function would be to avoid any undesirable promotions:

uint32_t parseIPV4string(char const * ipAddress)
{
    unsigned int ip[4];

    if ( 4 != sscanf(ipAddress, "%u.%u.%u.%u", &ip[0], &ip[1], &ip[2], &ip[3]) )
         return 0;   // or some other indicator or error

    return ipbytes[3] + ipbytes[2] * 0x100 + ipbytes[1] * 0x10000ul + ipbytes[0] * 0x1000000ul;
}

If you really want to use | and << then you either have to use some ugly casts; or change ip to have the type uint32_t in which case "%u" must be replaced by "%" SCNu32 .