Question

I am still learning C and had a question related to something I see fairly often. Please correct me if I’m wrong, is statement 1 the equivalent of statement 2?

1. (struct sockaddr *) &echoServAddr
2. struct sockaddr echoServAddr

If I understand this correctly, we are casting &echoServAddr to a struct framed the same as sockaddr.

---

So is the following code passing a struct by address?

/* Bind to the local address */
if (bind(servSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0) {
    perror("bind() failed");
    exit(1);
} 

Answer 1

// This declares a variable of type "struct sockaddr"
struct sockaddr echoServAddr;

// This merely takes a pointer to your structure,
// It (redundantly) casts that pointer to "struct sockaddr *" 
struct sockaddr *myPtr = (struct sockaddr *) &echoServAddr;

// This calls the function "bind()" and passes it a pointer to your structure
if (bind(servSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0) {
    perror("bind() failed");
    exit(1);
} 

PS:
Yes, you can cast one a pointer of one struct type to a pointer of a different struct type.

And unless the underlying struct’s are in fact compatible, doing so could make you very Sad 🙂