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

Question

Here is my code:

#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; } 

which compiles fine.

However when I comment out this line:

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

I will get:

$ gcc ex4.c ex4.c:30:9: error: expected expression         struct sockaddr_in *sockAddrIn = (struct sockaddr_in *) res->ai_addr;         ^ 1 error generated. 

What am I missing?

Answer 1

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.