How to cast sockaddr_storage and avoid breaking st

问题:

I'm using Beej's Guide to Networking and came across an aliasing issue. He proposes a function to return either the IPv4 or IPv6 address of a particular struct:

1  void *get_in_addr( struct sockaddr *sa )
2  {
3      if (sa->sa_family == AF_INET)
4        return &(((struct sockaddr_in*)sa)->sin_addr);
5      else
6        return &(((struct sockaddr_in6*)sa)->sin6_addr);
7  }

This causes GCC to spit out a strict-aliasing error for sa on line 3. As I understand it, it is because I call this function like so:

struct sockaddr_storage their_addr;
...
inet_ntop(their_addr.ss_family,
          get_in_addr((struct sockaddr *)&their_addr),
          connection_name,
          sizeof connection_name);

I'm guessing the aliasing has to do with the fact that the their_addr variable is of type sockaddr_storage and another pointer of a differing type points to the same memory.

Is the best way to get around this sticking sockaddr_storage, sockaddr_in, and sockaddr_in6 into a union? It seems like this should be well worn territory in networking, I just can't find any good examples with best practices.

Also, if anyone can explain exactly where the aliasing issue takes place, I'd much appreciate it.

回答1:

I tend to do this to get GCC do the right thing with type-punning, which is explicitly allowed with unions:

/*! Multi-family socket end-point address. */
typedef union address
{
    struct sockaddr sa;
    struct sockaddr_in sa_in;
    struct sockaddr_in6 sa_in6;
    struct sockaddr_storage sa_stor;
}
address_t;

回答2:

I tend to do this to get GCC do the right thing with type-punning, which is explicitly allowed with unions

I am pretty sure this (mis)use of union will not work (or only by accident) with GCC:

short type_pun2 (int i, int *pi, short *ps) {
    *pi = i;
    return *ps;
}

union U {
    int i;
    short s;
};

short type_pun (int i) {
    U u;
    return type_pun2 (i, &u.i, &u.s);
}

The correct way to do that is with memcpy, not union.

回答3:

I recently had a similar alias warning on HPUX system when trying to write code to get the MAC address of the machine

The &(((struct sockaddr_in *)addr)->sin_addr) complains about strict-aliasing rules

This is the code in some context

 char ip[INET6_ADDRSTRLEN] = {0};
 strucut sockaddr *addr

 ...
 get addr from ioctl(socket,SOCGIFCONF...) call
 ...
 inet_ntop(AF_INET, &(((struct sockaddr_in *)addr)->sin_addr),ip,sizeof ip);

I overcame the aliasing warning by doing the following

struct sockaddr_in sin;
memcpy(&sin,addr,sizeof(struct sockaddr));
inet_ntop(AF_INET, &sin.sin_addr,ip,sizeof ip);

And whilst this is potentially dangerous I added the following lines before it

 static_assert(sizeof(sockaddr)==sizeof(sockaddr_in));

I'm not sure if that is something would be considered bad practice, but it worked and was cross platform to other *Nix flavors and compilers

回答4:

The issue has nothing to do with the call to the function. Rather, it's with ((struct sockaddr_in*)sa)->sin_addr. The problem is that sa is a pointer of one type, but you're casting it to a pointer of a different type and then dereferencing it. This breaks a rule called "strict aliasing", which says that variables of different types can never alias. In your case, aliasing to a different type is exactly what you want to do.

The simple solution is to turn off this optimization, which allows aliasing in this manner. On GCC, the flag is -fno-strict-aliasing.

The better solution is to use a union, as mentioned by Nikolai.

void *get_in_addr(struct sockaddr *sa)
{
    union {
        struct sockaddr     *sa;
        struct sockaddr_in  *sa_in;
        struct sockaddr_in6 *sa_in6;
    } u;
    u.sa = sa;
    if (sa->sa_family == AF_INET)
        return &(u.sa_in->sin_addr);
    else
        return &(u.sa_in6->sin6_addr);
}

That said, I can't actually get GCC to give me a warning when using your original code, so I'm not sure if this buys you anything.