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'll give the "don't want two problems" solution:

#include 



int isIp_v4( char* ip){
        int num;
        int flag = 1;
        int counter=0;
        char* p = strtok(ip,".");

        while (p && flag ){
                num = atoi(p);

                if (num>=0 && num<=255 && (counter++<4)){
                        flag=1;
                        p=strtok(NULL,".");

                }
                else{
                        flag=0;
                        break;
                }
        }

        return flag && (counter==3);

}

EDIT: strtok may not be thread safe (credits to Adam Rosenfield)