Switch-case won't compile after commenting out an unused line

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.