Closing a Socket

The Winsock API provides closesocket to close an existing socket and free associated resources. It should be called after all pending I/O operations have completed or been cancelled.

Syntax

int closesocket(
    SOCKET s
);

Parameters

s – A descriptor identifying the socket to be closed.

Return Value

Returns 0 on success. On failure, -1 is returned and WSAGetLastError provides the error code.

Remarks

Calling closesocket does not automatically release any associated Winsock resources. Call WSACleanup after all sockets are closed.
If the socket is bound to a port, the port becomes available for reuse after the socket is closed.
Pending overlapped I/O operations are canceled. Completion routines will receive WSA_OPERATION_ABORTED.

Example

#include <winsock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "ws2_32.lib")

int main() {
    WSADATA wsaData;
    SOCKET ListenSocket = INVALID_SOCKET;
    int iResult;

    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed: %d\n", iResult);
        return 1;
    }

    ListenSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ListenSocket == INVALID_SOCKET) {
        printf("Error at socket(): %ld\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    // ... bind, listen, accept ...

    iResult = closesocket(ListenSocket);
    if (iResult == SOCKET_ERROR) {
        printf("closesocket failed: %d\n", WSAGetLastError());
    }

    WSACleanup();
    return 0;
}