CODEWITHSUNDEEP
clinuxsocketsnetwork-programmingip
Kiril Kirov
Kiril Kirov

Reputation: 38163

What does the abbreviation "s_", "ai_", "sin_", "in" (if such) in the IP structures mean?

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
};

Upvotes: 20

Views: 2140

Answers (3)

Steve-o
Steve-o

Reputation: 12866

The Microsoft definition of in_addr could imply that the "S_" prefix means struct as per @R..'s answer and seeing "un" for union, however they use a capital "S" unlike POSIX land.

typedef struct in_addr {
  union {
    struct {
      u_char s_b1,s_b2,s_b3,s_b4;
    } S_un_b;
    struct {
      u_short s_w1,s_w2;
    } S_un_w;
    u_long S_addr;
  } S_un;
} IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR;

For the set of socket structures "s" usually means "sock" short for "socket", so there does not appear to be a definition reason.

For curiosity the MSDN page on IPv6 Support shows struct in6_addr containing the name s6_addr implying the IPv6 version of an IPv4 "socket" structure rather than just a "IPv4 structure".

Upvotes: 0

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215221

larsman is mostly right, but it's not merely a matter of legacy single-namespace considerations. All the structs defined in standard headers use names of this form to avoid stepping on the application's namespace for macros. If struct members were not prefixed with ai_, sin_, etc., then whatever member names were included in the struct (including extensions not even specified in the C or POSIX standards) would clash and result in errors if an application defined the same name as a preprocessor macro. By using these "struct-local namespaces" that can be reserved by simple pattern rules in the standards (for instance, netdb.h reserves ai_*) there is a clear distinction between names reserved for use by the implementation and names reserved for use by the application, and new extensions or new revisions of the standard will not result in clashes.

Upvotes: 12

Fred Foo
Fred Foo

Reputation: 363547

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.

Upvotes: 36

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