Determine if a string is a valid IPv4 address in C

Question

What would be a good way to determine if a string contains an IPv4 address? Should I use isdigit()?

Answer 1

I needed to figure out if incoming string "contains" a valid IP address, and to return a pointer to the portion of the incoming string that is the valid IP address, if so. If not, returns a null pointer.

Here is code that seems to work, although not well tested yet, I just wrote it and gave it a quick try. I didn't add a check yet for limiting the numbers to one-byte values, but do check to ensure that they are limited to three digit numbers.

int IsDigit(char ch)
{
   int is_digit = 0;
   if ( ch >= '0' && ch <= '9' )
   {
      is_digit = 1;
   }
   return is_digit;
}

#define FIND_IP_START         0
#define FIND_IP_DIGIT         1
#define FIND_IP_DIG_OR_DEC    2
#define FIND_IP_DECIMAL       3
#define FIND_IP_DIG_OR_END    4
#define FIND_IP_END           5
#define FIND_IP_DONE          6

char * StringContainsValidIpAddress(char * input_buf_pointer)
{
   char * pos       = input_buf_pointer;
   int    octets    = 0; 
   int    digits    = 0;
   int    state     = FIND_IP_START;
   char * ip_string = 0;

   char   ch        = *pos; 

   while ( (ch != NULL) && (state != FIND_IP_DONE) )
   {
      switch ( state )
      {
      case FIND_IP_START:
         if ( IsDigit(ch) )
         {
            ip_string = pos;  //potential start of ip string
            digits = 1;   // first digit
            octets = 1;   // of first octet
            state = FIND_IP_DIG_OR_DEC;
         }
         break;
      case FIND_IP_DIGIT:
         if ( IsDigit(ch) )
         {
            digits = 1;    // first digit
            octets++;      // of next octet
            if ( octets == 4 )
            {
               state = FIND_IP_DIG_OR_END;
            }
            else
            {
                   state = FIND_IP_DIG_OR_DEC;
            }
         }
         else
         {
            // Start over
            state = FIND_IP_START;
         }
         break;
      case FIND_IP_DIG_OR_DEC:
         // Here we are looking for another digit 
         // of the same octet or the decimal between 
         // octets.
         if (ch == '.')
         {
               state = FIND_IP_DIGIT;
         }
         else if ( IsDigit(ch) )
         {
            digits++;      // next digit
            if ( digits == 3 )
            {
               state = FIND_IP_DECIMAL;
            }
         }
         else
         {
            // Start over
            state = FIND_IP_START;
         }
         break;
      case FIND_IP_DECIMAL:
         if (ch == '.')
         {
               state = FIND_IP_DIGIT;
         }
         break;
      case FIND_IP_DIG_OR_END:
         // Here we are looking for another digit 
         // of the same octet or the end (which could
         // be a space or CR or LF or really any 
         // non-digit).
         if ( IsDigit(ch) )
         {
            digits++;      // next digit
            if ( digits == 3 )
            {
               state = FIND_IP_END;
            }
         }
         else
         {  
            *pos = 0;  // Null terminate the IP address string
            state = FIND_IP_DONE;
         }
         break;
      case FIND_IP_END:
         if ( !IsDigit(ch) )
         {
            *pos = 0;  // Null terminate the IP address string
            state = FIND_IP_DONE;
         }
         break;
      case FIND_IP_DONE:
         break;
      default:
         break;
      }

      // Fetch the next character
      ch = *++pos; 
   } 

   if (state == FIND_IP_DONE) 
   {
      return ip_string; 
   }
   else
   {
      return 0;
   }
}