Windows Sockets API Reference

Networking Documentation

WSAGetLastError Function

The WSAGetLastError function retrieves the extended error code for the calling thread. This function is used to obtain detailed error information for Windows Sockets API functions.

Syntax

int WSAGetLastError(void);

Return Value

Return value
The return value is an integer that represents the extended error code. If the last Winsock function call was successful, the return value is zero. Otherwise, it is one of the Winsock error codes.

Remarks

Most Windows Sockets API functions set an extended error code when they fail. The WSAGetLastError function is used to retrieve this code. It is important to call WSAGetLastError immediately after a Winsock function fails, as subsequent Winsock calls might overwrite the error code.

The extended error codes are defined in the winsock2.h header file as constants starting with WSAE. A comprehensive list of these error codes can be found in the Winsock documentation.

Note: The WSAGetLastError function is a synonym for WSAGetLastError. You can use either name.

Example

Retrieving an Error Code


#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

#pragma comment(lib, "Ws2_32.lib")

int main() {
    WSADATA wsaData;
    int iResult;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        std::cerr << "WSAStartup failed: " << iResult << std::endl;
        return 1;
    }

    // Attempt a connection that will likely fail
    SOCKET ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ConnectSocket == INVALID_SOCKET) {
        int errorCode = WSAGetLastError(); // Or WSAGetLastError()
        std::cerr << "Socket creation failed. Error code: " << errorCode << std::endl;
        // You can then use a lookup mechanism or switch statement to interpret errorCode
        WSACleanup();
        return 1;
    }

    // Example of a failing connect call (assuming 127.0.0.1:1 is not listening)
    sockaddr_in clientService;
    clientService.sin_family = AF_INET;
    clientService.sin_addr.s_addr = inet_addr("127.0.0.1");
    clientService.sin_port = htons(1);

    iResult = connect(ConnectSocket, (SOCKADDR*)&clientService, sizeof(clientService));
    if (iResult == SOCKET_ERROR) {
        int errorCode = WSAGetLastError(); // Get the specific error code
        std::cerr << "Connection failed. Error code: " << errorCode << std::endl;
        // Example: WSAECONNREFUSED (10061) is a common error for failed connections
        if (errorCode == WSAECONNREFUSED) {
            std::cerr << "Error detail: Connection refused." << std::endl;
        }
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // If connection was successful (unlikely in this example)
    std::cout << "Connection successful." << std::endl;
    closesocket(ConnectSocket);
    WSACleanup();
    return 0;
}

See Also