Winsock Data Structures

Winsock (Windows Sockets API) defines several key data structures that are used to represent network information, addresses, and socket options. Understanding these structures is crucial for developing network applications with Winsock.

Commonly Used Structures

`SOCKADDR` and `SOCKADDR_IN`

The generic SOCKADDR structure is used by various network protocols. For TCP/IP, the more specific SOCKADDR_IN structure is used. This structure contains information about the Internet address family, IP address, and port number.


struct sockaddr {
    u_short sa_family; // Address family
    char    sa_data[14]; // Protocol-specific address information
};

struct sockaddr_in {
    short int           sin_family; // AF_INET (Address Family: Internet)
    unsigned short int  sin_port;   // Port number
    struct in_addr      sin_addr;   // IP address
    char                sin_zero[8]; // Not used
};

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;
};

sin_family: short int Specifies the address family, which must be AF_INET for TCP/IP.

sin_port: unsigned short int Specifies the port number. Must be in network byte order.

sin_addr: struct in_addr Contains the IP address of the host. Must be in network byte order.

`WSADATA`

The WSADATA structure is used by the WSAStartup function to return information about the Windows Sockets implementation.


typedef struct WSAData {
    WORD                    wVersion;       // Requested version of Winsock
    WORD                    wHighVersion;   // Highest version of Winsock supported
    char                    szDescription[WSADESCRIPTION_LEN+1]; // Description of Winsock
    char                    szSystemStatus[WSASYS_STATUS_LEN+1]; // Status of Winsock
    unsigned short          iMaxSockets;    // Max sockets supported
    unsigned short          iMaxUdpDg;      // Max datagrams supported
    char FAR *              lpVendorInfo;   // Vendor-specific info
} WSADATA;

wVersion: WORD The version of the Winsock DLL that the calling application expects to use.

wHighVersion: WORD The highest version of Winsock supported by the Winsock DLL.

szDescription: char[] A null-terminated string that describes the Winsock implementation.

`TIMEVAL`

The TIMEVAL structure is used to specify a timeout value for socket operations, particularly for functions like select.


struct timeval {
    long tv_sec;  // Seconds
    long tv_usec; // Microseconds
};

tv_sec: long Number of seconds.

tv_usec: long Number of microseconds.

`FD_SET`

The FD_SET structure is used with the select function to represent a set of socket file descriptors that you want to monitor for readiness.


typedef struct fd_set {
    u_int fd_count;               // how many sockets are in the set
    SOCKET  fd_array[FD_SETSIZE]; // the sockets
} fd_set;

fd_count: u_int The number of sockets currently in the set.

fd_array: SOCKET[] An array of socket descriptors.

Other Important Structures

LINGER: Controls socket behavior when closing.
HOSTENT: Represents host information (e.g., from DNS lookups).
PROTOENT: Represents protocol information.
SERVENT: Represents service information.

For detailed information on each structure and its members, please refer to the individual structure documentation.