Output with std::string

Question

I have the following code for listing the ip address of a host.

// Loop through all the linked list of address and get the ip addresses
for(p = res; p != NULL; p = p->ai_next){
    void *addr;
    string ipver;

    if(p->ai_family == AF_INET){
        // IPv4 address
        sockaddr_in *ipv4 = (sockaddr_in*)p->ai_addr;
        addr = &(ipv4->sin_addr);
        ipver = "IPv4";
    }
    else{
        // IPv6 address
        sockaddr_in6 *ipv6 = (sockaddr_in6*)p->ai_addr;
        addr = &(ipv6->sin6_addr);
        ipver = "IPv6";
    }

    // use INET6_ADDRSTRLEN becuase we need it to big enough
    // convert the ip from network(bits) to presentation(eg. 127.0.0.1)
    inet_ntop(p->ai_family, addr, const_cast<char*>(ipstr.c_str()), INET6_ADDRSTRLEN);
    // print it out
    cout << ipver <<" " << ipstr << endl;
}

The problem is with:

cout << ipver << " " << ipstr << endl;

When this line gets executed i don't get any output. But when i call ipstr.c_str(), like this:

cout << ipver <<" " << ipstr.c_str() << endl;

I do get output.

I am using GCC 4.8

const_cast<char*>(ipstr.c_str()) -- yikes. When you find yourself writing this (using const_cast), stop and think hard of what you're doing. There's a very good chance it's the wrong thing. — Dan Mašek
– Dan Mašek, Commented Apr 9, 2016 at 2:46
@timrau its declared as a std::string but i did not include it — user4603522
– user4603522, Commented Apr 9, 2016 at 2:49
@DanMašek yeah i looked at Slava's answer and that solved it. Thanks for the advice. — user4603522
– user4603522, Commented Apr 9, 2016 at 2:52

Slava · Accepted Answer · 2016-04-09 02:43:41Z

1

You cannot modify buffer, returned by std::string::c_str() and there is a reason it returns const char * so this code:

 inet_ntop(p->ai_family, addr, const_cast<char*>(ipstr.c_str()), INET6_ADDRSTRLEN);

leads to UB. create a buffer and use it:

 char buffer[INET6_ADDRSTRLEN];
 ipstr = inet_ntop(p->ai_family, addr, buffer, INET6_ADDRSTRLEN);

answered Apr 9, 2016 at 2:43

Slava

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