Understanding the relationship between addrinfo object and sockaddr structure in Winsock

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Understanding the relationship between addrinfo object and sockaddr structure in Winsock

Gflockay-9238 0

I came across an article on Winsock programming (https://learn.microsoft.com/en-us/windows/win32/winsock/creating-a-socket-for-the-client) and I am having trouble understanding the relationship between the addrinfo object and the sockaddr structure. Specifically, the article mentions that the addrinfo object contains a sockaddr structure, but I'm not sure what that means. Can someone help clarify this relationship and how it works in Winsock programming?

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Answer 1

Xiaopo Yang - MSFT 12,731 Microsoft External Staff

//
//  Structure used in getaddrinfo() call
//

typedef struct addrinfo
{
    int                 ai_flags;       // AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST
    int                 ai_family;      // PF_xxx
    int                 ai_socktype;    // SOCK_xxx
    int                 ai_protocol;    // 0 or IPPROTO_xxx for IPv4 and IPv6
    size_t              ai_addrlen;     // Length of ai_addr
    char *              ai_canonname;   // Canonical name for nodename
    _Field_size_bytes_(ai_addrlen) struct sockaddr *   ai_addr;        // Binary address
    struct addrinfo *   ai_next;        // Next structure in linked list
}
ADDRINFOA, *PADDRINFOA;

INT WSAAPI getaddrinfo(
  [in, optional] PCSTR           pNodeName,
  [in, optional] PCSTR           pServiceName,
  [in, optional] const ADDRINFOA *pHints,
  [out]          PADDRINFOA      *ppResult
);

addrinfo structure is used in getaddrinfo() call as input and output which contains response information about the host.

While The sockaddr structure contains information about the host and varies depending on the protocol selected. It then is used in the bind function , the connect function, etc.

Gflockay-9238 0 Reputation points

2023-07-26T10:47:07.2333333+00:00

While The sockaddr structure contains information about the host and varies depending on the protocol selected, isn't things stored in addrinfo? And i still don't know what they mean by addrinfo contains sockaddr structure

Xiaopo Yang - MSFT 12,731 Microsoft External Staff

addrinfo structure is only used in getaddrinfo() call and Its fields is used as input. Its ai_addr is used to provide protocol-independent translation from an ANSI host name to an address.

#define DEFAULT_PORT "27015"

struct addrinfo *result = NULL, *ptr = NULL, hints;

ZeroMemory(&hints, sizeof (hints));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
hints.ai_flags = AI_PASSIVE;

// Resolve the local address and port to be used by the server
iResult = getaddrinfo(NULL, DEFAULT_PORT, &hints, &result);
if (iResult != 0) {
    printf("getaddrinfo failed: %d\n", iResult);
    WSACleanup();
    return 1;
}

While The sockaddr structure contains information about the host and varies depending on the protocol selected. It then is used in the bind function , the connect function, etc.

IPv4:

//
// IPv4 Socket address, Internet style
//

typedef struct sockaddr_in {

#if(_WIN32_WINNT < 0x0600)
    short   sin_family;
#else //(_WIN32_WINNT < 0x0600)
    ADDRESS_FAMILY sin_family;
#endif //(_WIN32_WINNT < 0x0600)

    USHORT sin_port;
    IN_ADDR sin_addr;
    CHAR sin_zero[8];
} SOCKADDR_IN, *PSOCKADDR_IN;

A direct sockaddr example without getaddrinfo():

// Declare variables
SOCKET ListenSocket;
struct sockaddr_in saServer;
hostent* localHost;
char* localIP;
// Create a listening socket
ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
// Get the local host information
localHost = gethostbyname("");
localIP = inet_ntoa (*(struct in_addr *)*localHost->h_addr_list);
// Set up the sockaddr structure
saServer.sin_family = AF_INET;
saServer.sin_addr.s_addr = inet_addr(localIP);
saServer.sin_port = htons(5150);
// Bind the listening socket using the
// information in the sockaddr structure
bind( ListenSocket,(SOCKADDR*) &saServer, sizeof(saServer) );

Gflockay-9238 0 Reputation points

2023-07-27T20:14:50.05+00:00

I still don't understand how addrinfo contains sockaddr structure
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2023-07-28T00:26:01.8033333+00:00

addrinfo contains a pointer to a sockaddr structure. The ai_addr member in each returned addrinfo structure points to a filled-in socket address structure. The length, in bytes, of each returned addrinfo structure is specified in the ai_addrlen member.
Gflockay-9238 0 Reputation points

2023-07-28T09:49:23.45+00:00

Thank you

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Understanding the relationship between addrinfo object and sockaddr structure in Winsock

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