Declaring variable with same name but different type

Question

I am trying to make something with supports IPV4 or IPV6. The code is similar for these two so I tried something like this. Since the only difference in code later on depends on this sin variable how can I compile just one of them. Lets suppose "ipv" variable in IF statement is true or false depends on user input

//FOR IPV4 //struct sockaddr_in sin; //FOR IPV6 //struct sockaddr_in6 sin; //IPV4 // IPV4 --------------------------------------- if (ipv){ struct sockaddr_in sin; if ( (s = socket(AF_INET, SOCK_STREAM, 0 ) ) < 0) { perror("Chyba pri vytvareni socketu"); return -1; } sin.sin_family = AF_INET; sin.sin_port = htons(port_number); sin.sin_addr.s_addr = INADDR_ANY; } // IPV6 --------------------------------------- else{ struct sockaddr_in6 sin; if ( (s = socket(AF_INET6, SOCK_STREAM, 0 ) ) < 0) { perror("Chyba pri vytvareni socketu"); return -1; } sin.sin6_family = AF_INET6; sin.sin6_port = htons(port_number); sin.sin6_addr = in6addr_any; sin.sin6_flowinfo = 0; } if (bind(s, (struct sockaddr *)&sin, sizeof(sin) ) < 0 ) { printf("error on bind\n"); return -1; } if (listen(s, 5)) { printf ("error on listen\n"); return -1; } sinlen = sizeof(sin); pid_t pid; while (1) { /* accepting new connection request from client, socket id for the new connection is returned in t */ if ( (t = accept(s, (struct sockaddr *) &sin, &sinlen) ) < 0 ) { printf("error on accept\n"); /* accept error */ return -1; } continues .... not important

this code will give me:

server.cpp: In function ‘int main(int, char**)’: server.cpp:132:35: error: ‘sin’ was not declared in this scope if (bind(s, (struct sockaddr *)&sin, sizeof(sin) ) < 0 ) { ^ server.cpp:145:19: error: ‘sin’ was not declared in this scope sinlen = sizeof(sin); ^ make: *** [all] Error 1

So question is how can I make this done without writing the same code twice with different "sin";

Change your variable name. The name sin conflicts with the sin (trigonometry) function in the math library. — Thomas Matthews
– Thomas Matthews, Commented Oct 12, 2015 at 15:48
Make variable sockaddr sin before if's and initialize it in the enc of each if with your local sin. — user2487382
– user2487382, Commented Oct 12, 2015 at 15:48
@Status well its is sockaddr_in or sockaddr_in6 which is difference ... please post a code since I dont really understand what you mean :) — Denis
– Denis, Commented Oct 12, 2015 at 15:55
If you use the getaddrinfo()-API in POSIX, you don't have to worry about this detail, because it will automatically allocate the right address. — user3185968
– user3185968, Commented Oct 12, 2015 at 15:57

Denis · Accepted Answer · 2015-10-13 12:48:06Z

I did it like this by using sockaddr_storage and its working well

struct sockaddr_storage sin; struct sockaddr_in *sin4; struct sockaddr_in *sin6; // IPV4 --------------------------------------- if (ipv == true){ sin4 = (struct sockaddr_in*)&sin; if ( (s = socket(AF_INET, SOCK_STREAM, 0 ) ) < 0) { perror("Chyba pri vytvareni socketu"); return -1; } sin4->sin_family = AF_INET; sin4->sin_port = htons(port_number); sin4->sin_addr.s_addr = INADDR_ANY; } // IPV6 --------------------------------------- else{ sin6 = (struct sockaddr_in6*)&sin; if ( (s = socket(AF_INET6, SOCK_STREAM, 0 ) ) < 0) { perror("Chyba pri vytvareni socketu"); return -1; } sin6->sin6_family = AF_INET6; sin6->sin6_port = htons(port_number); sin6->sin6_addr = in6addr_any; sin6->sin6_flowinfo = 0; } if (bind(s, (struct sockaddr *)&sin, sizeof(sin) ) < 0 ) { printf("error on bind\n"); return -1; } if (listen(s, 5)) { printf ("error on listen\n"); return -1; } sinlen = sizeof(sin); pid_t pid; while (1) { /* accepting new connection request from client, socket id for the new connection is returned in t */ if ( (t = accept(s, (struct sockaddr *) &sin, &sinlen) ) < 0 ) { printf("error on accept\n"); /* accept error */ return -1; } continues .... not important

AndreyS Scherbakov · Accepted Answer · 2015-10-13 12:53:59Z

It's a typical case when you might use a union.

typedef union { struct sockaddr_in v4; struct sockaddr_in6 v6; } sockaddr_union; sockaddr_union sin;

Then, when you know you're working with IPV4, use sin.v4 in place of sin, and when you know you're working with IPV6, use sin.v6

Don't call it sockaddr_un. That's the name of the struct used for UNIX domain sockets.

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