Problems with pointers to a struct sockaddr_in in C

Asked Jun 09 '22 at 20:12

Active Jun 09 '22 at 20:12

Viewed 73 times

I came across some problems when accessing a struct sockaddr_in* (socket C lib type).

Considering the following function:

void f(struct sockaddr_in* in){
  /* Some networking operations. */
  in->sin_addr.s_addr = htonl(INADDR_ANY);
  in->sin_port = htons(PORT);
  in->sin_family = AF_INET;
}

This code doesn't work (segmentation fault):

int main(){
  struct sockaddr_in* s;
  f(s);
}

While this one works:

int main(){
  struct sockaddr_in s;
  f(&s);

  // Likewise does:
  struct sockaddr_in s;
  struct sockaddr_in* p;
  p = &s;
  f(p);
}

Why does this happens?

asked Jun 09 '22 at 20:12

FueledByPizza

1

In the first case, what does `s` point to? – dbush Jun 09 '22 at 20:14
There's no reason to use `malloc()` here, and several good reasons not to (heap allocations are relatively expensive, and invite memory leaks). Declaring a `struct sockaddr_in` on the stack and passing in a pointer to it is preferred. – Jeremy Friesner Jun 09 '22 at 20:33
@dbush To nothing, I had a completely irrational thought... – FueledByPizza Jun 09 '22 at 20:46
@SupportUkraine That's just right. In fact, it's just normal it can't refer to a "non-existing" variable. It works with the malloc() instead. – FueledByPizza Jun 09 '22 at 20:47
And the reason is explained well by @JeremyFriesner, surely you all agree with your answers, they're linked. Great, thank you! – FueledByPizza Jun 09 '22 at 20:47
No need to talk about sockaddr_in - you could have the exact same problem with int – user253751 Jun 10 '22 at 10:43
@user253751 Yes, it is. – FueledByPizza Jun 10 '22 at 17:46

Problems with pointers to a struct sockaddr_in in C

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