Why can you compare sa_family, an unsigned integer, a struct defined by socket.h be compared to AF_INET

Question

I'm reading a book about network programming in C. One of the sample programs (here: https://github.com/codeplea/Hands-On-Network-Programming-with-C/blob/master/chap01/unix_list.c) compares a sa_family type (which is an unsigned integer) to what I'm guessing is an enumerator, AF_INET and AF_INET6. How does this work?

I've been looking at socket.h and ifaddrs.h, but don't see any enums in there.

int main() {

    struct ifaddrs *addresses;

    if (getifaddrs(&addresses) == -1) {
        printf("getifaddrs call failed\n");
        return -1;
    }

    struct ifaddrs *address = addresses;
    while(address) {
        int family = address->ifa_addr->sa_family;
        if (family == AF_INET || family == AF_INET6) {

            printf("%s\t", address->ifa_name);
            printf("%s\t", family == AF_INET ? "IPv4" : "IPv6");

            char ap[100];
            const int family_size = family == AF_INET ?
                sizeof(struct sockaddr_in) : sizeof(struct sockaddr_in6);
            getnameinfo(address->ifa_addr,
                    family_size, ap, sizeof(ap), 0, 0, NI_NUMERICHOST);
            printf("\t%s\n", ap);

        }
        address = address->ifa_next;
    }


    freeifaddrs(addresses);
    return 0;
}

How does line 13 work? if (family == AF_INET || family == AF_INET6)

Thanks!

And what is unclear? There is used the equality operator with integers. — Vlad from Moscow, Aug 22 '19 at 17:05
Do the answers in this question help you at all? What is AF_INET, and why do I need it? https://stackoverflow.com/questions/1593946/what-is-af-inet-and-why-do-i-need-it — cmwt, Aug 22 '19 at 18:07
Well this is a *double* misunderstanding. They're *not* enumerators and if they *were* then it would be totally OK in C. — Antti Haapala -- Слава Україні, Aug 22 '19 at 21:20

landru27 · Answer 1 · 2019-08-22T22:02:18.717

2

The reason you don't find any enumeration is because AF_INET and AF_INET6 are not "an enumerator".

But the header file you want to look at is sys/socket.h. In there, you will find:

#define AF_INET         2               /* internetwork: UDP, TCP, etc. */
...
#define AF_INET6        30              /* IPv6 */

In other words, AF_INET and AF_INET6 are numbers -- in fact, integers, just like the sa_family type.

Knowing that they are integers, I'm guessing you can now see exactly how

if (family == AF_INET || family == AF_INET6)

works.

edited Aug 22 '19 at 22:02

answered Aug 22 '19 at 20:49

landru27

1,654
12
20

those constants are signed integers... ;) – Antti Haapala -- Слава Україні Aug 22 '19 at 21:19

score 2 · Accepted Answer · answered Aug 22 '19 at 20:55

Define

Posix actually specifies that AF_INET is a macro, so on a particular platform, AF_INET could be defined as e.g.

#define AF_INET 16

So code like

 if (family == AF_INET)

Will be converted after the preprocessor stage to literally the code if (family == 16)

Enum

It would not be much, if any, of a difference if it was an enum though.

An enum is just an integer, so you could compare that with a variable of integer type.

Suppose there's

enum SA_FAMILY {
   AF_INET,
   AF_INET6
   AF_UNIX
};

Here the the first enum constant will have the integer value 0, the next has the value 1, and so on.

This means you can compare one of the enumeration constans to a variable of integer type (And where the so called "usual arithemtic conversion" rules dictate how the values are converted in the various cases of whether you're using a short int, an unsigned int and so on.)

So if you have

unsigned int family = 0;

if (family == AF_INET) {
...
}

You're also just doing if (family == 0).

score 0 · Answer 3 · answered Aug 22 '19 at 20:35

0

if (family == AF_INET || family == AF_INET6)

means "if family is equal to AF_INET or if family is equal to AF_INET6". The equality operator has higher priority than the logical or operator.

answered Aug 22 '19 at 20:35

the busybee

10,755
3
13
30

Why can you compare sa_family, an unsigned integer, a struct defined by socket.h be compared to AF_INET

3 Answers3

Define

Enum