I can't find this segmentation fault

Question

So I'm doing this codewars kata and I'm succeding in every test but in the end the result fails because my code has a segmentation fault and I don't think I know enough about the language to find it! Can someone please halp?

int is_valid_ip(const char *addr) 
{
    char set[] = "1234567890";
    int current;
    int octet_counter = 0;
    char *octet = 0;
    octet = strtok(addr, ".");
    while (octet)
    {
        if (strspn(octet, set) != strlen(octet)) return 0; // checks for spaces
        if (strlen(octet) > 1 && (octet[0]) == '0') return 0; // checks for preceding zeros
        sscanf(octet, "%d", &current);
        if (current < 0 || current > 255) return 0; // checks for range
        octet = strtok(0, ".");
        ++octet_counter;
    }
    if (octet_counter == 4) return 1; // checks for number of octets
    return 0;
};

My code was kind of cleaner but after so much messing around trying to solve this problems it's become this...

Answer 1

As strtok() modifies the string to be tokenized and addr is defined as const char * (I assume this is a requirement) you may make a copy of the input string *addr:

char ip[16]; // enought to hold nnn.nnn.nnn.nnn
if(strlen(addr)>15) return 0;
strcpy(ip, addr);

subsequently operate on ip instead of addr

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Or... avoid using strtok and parse/scan the string without modifying it.