Question

Pretty simple questions. And yes, maybe not (that) important, but I’m really curious what do they mean and I couldn’t find their meanings.

---

// ipv4
struct sockaddr_in {
    short int          sin_family;  // Address family, AF_INET
    unsigned short int sin_port;    // Port number
    struct in_addr     sin_addr;    // Internet address
    unsigned char      sin_zero[8]; // Same size as struct sockaddr
};

// ipv4    
struct in_addr {
    uint32_t s_addr; // that's a 32-bit int (4 bytes)
};

// ipv6
struct addrinfo {
    int              ai_flags;     // AI_PASSIVE, AI_CANONNAME, etc.
    int              ai_family;    // AF_INET, AF_INET6, AF_UNSPEC
    int              ai_socktype;  // SOCK_STREAM, SOCK_DGRAM
    int              ai_protocol;  // use 0 for "any"
    size_t           ai_addrlen;   // size of ai_addr in bytes
    struct sockaddr *ai_addr;      // struct sockaddr_in or _in6
    char            *ai_canonname; // full canonical hostname

    struct addrinfo *ai_next;      // linked list, next node
};

// ipv6    
struct sockaddr {
    unsigned short    sa_family;    // address family, AF_xxx
    char              sa_data[14];  // 14 bytes of protocol address
}; 

Answer 1

sin_ means sockaddr_in, ai_ means addrinfo, sa_ means sockaddr. I’m not sure about the s_ in in_addr. The sockets API was designed with pre-standard early 1980s C compilers in mind, which might have had a single namespace for all struct members.