Each case in a switch statement is, technically speaking, a label. For some obscure and old reasons, you are not allowed to have a variable declaration as the first line after a label. By commenting out the assignment

ptr = (struct sockaddr_in *) res->ai_addr;

the line

struct sockaddr_in *sockAddrIn = (struct sockaddr_in *) res->ai_addr;

becomes the first line after the label AF_INET: which, like I said, is illegal in C.

The solution is to wrap all of your case statements in curly brackets like so:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int main (void) {

  struct addrinfo hints; 
  memset (&hints, 0, sizeof hints);

  hints.ai_family = AF_UNSPEC; 
  hints.ai_socktype = SOCK_DGRAM;  
  hints.ai_flags = AI_CANONNAME;   

  struct addrinfo *res;

  getaddrinfo ("example.com", "http", &hints, &res);
  printf ("Host: %s\n", "example.com");

  void *ptr;

  while (res != NULL) {
    printf("AI Family for current addrinfo: %i\n", res->ai_family);
    switch (res->ai_family) {
      case AF_INET:
      {
        ptr = (struct sockaddr_in *) res->ai_addr;
        struct sockaddr_in *sockAddrIn = (struct sockaddr_in *) res->ai_addr;
        break;
      }
    }
    res = res->ai_next;
  }
  return 0;
}

Anyway, I think this is better coding practice.

As a complement to the accepted answer, you can declare your variables before the case labels.

switch(a) {
    int b; //can't initialize variable here
    case 0:
    ...
}

Or just use an empty statement.