到目前为止，我发现的所有例子要么只能读或写，要么是10000行的野兽，我甚至不知道从哪里开始理解它们是如何工作的。
为了测试我的代码，我将浏览器指向我的服务器，并发送了一个简单的http请求。结果令人困惑。
例如，在某个时刻，GetQueuedCompletionStatus返回，WSARecv说它读取了我发送的http响应的字节数，尽管这个响应应该（并且确实）在客户端结束，而recvbuffer甚至没有填充这些字节。
另外，我不明白当其他浏览器关闭连接时，什么时候释放缓冲区，因为GetQueuedCompletionStatus在我调用closesocket后会返回几次。
此外，一旦GetQueuedCompletionStatus返回，我就不知道什么时候有数据要读或什么时候有数据要写。我可以两个都试一下，看看哪个失败了，但这似乎很粗鲁。
为了揭示我可能对IOCP的任何误解，我写了一些伪代码来表达我认为我的代码所做的事情：

main {
    create server socket
    create io completion port
    while true {
        accept client socket
        create completion port for client socket
        create recv buffer and send buffer for client
        call WSARecv once with 0 bytes for whatever reason
    }
}

worker thread {
    while true {
        wait until GetQueuedCompletionStatus returns
        do something if that failed, not quite sure what (free buffers?)
        if no bytes were transferred, close socket
        try to recv data
        try to send data
    }
}

实际代码：

#include <stdio.h>
#include <stdlib.h>
#include <winsock2.h>

#define BUFFER_SIZE 1024

typedef struct {
    WSAOVERLAPPED overlapped;
    SOCKET socket;
    WSABUF sendbuf;
    WSABUF recvbuf;
    char sendbuffer[BUFFER_SIZE];
    char recvbuffer[BUFFER_SIZE];
} client;

DWORD WINAPI worker_thread(HANDLE iocp){
    DWORD flags = 0, n = 0;
    ULONG unused;
    client *c;

    while (1){
        int ret = GetQueuedCompletionStatus(iocp, &n, &unused, (LPOVERLAPPED*)&c, INFINITE);
        printf("%3d triggered\n", c->socket);

        if (ret == FALSE){
            printf("%3d GetQueuedCompletionStatus error %i\n", c->socket, WSAGetLastError());
            continue;
        }

        if (c->socket == INVALID_SOCKET){
            printf("error: socket already closed\n");
            continue;
        }

        if (n == 0) {
            printf("%3d disconnected\n", c->socket);
            closesocket(c->socket);
            c->socket = INVALID_SOCKET;
            continue;
        }

        /* how do I know if there is data to read or data to write? */

        WSARecv(c->socket, &(c->recvbuf), 1, &n, &flags, &(c->overlapped), NULL);
        printf("%3d WSARecv %ld bytes\n", c->socket, n);

        WSASend(c->socket, &(c->sendbuf), 1, &n, flags, &(c->overlapped), NULL);
        printf("%3d WSASend %ld bytes\n", c->socket, n);

        /* TODO handle partial sends */
        c->sendbuf.len = 0;
    }

    return 0;
}

SOCKET make_server(int port){
    int yes = 1;
    struct sockaddr_in addr;
    SOCKET sock;

    WSADATA wsaData;
    WSAStartup(MAKEWORD(2, 2), &wsaData);

    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(port);

    sock = WSASocket(AF_INET, SOCK_STREAM, 0, NULL, 0, WSA_FLAG_OVERLAPPED);

    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (const char*)yes, sizeof(yes));

    bind(sock, (struct sockaddr*)&addr, sizeof(addr));

    listen(sock, SOMAXCONN);

    return sock;
}

int main(){
    const char *text =
        "HTTP/1.0 200 OK\r\n"
        "Content-Length: 13\r\n"
        "Content-Type: text/html\r\n"
        "Connection: Close\r\n"
        "\r\n"
        "Hello, World!";

    SOCKET server_socket = make_server(8080);

    HANDLE iocp = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);

    CreateThread(NULL, 0, worker_thread, iocp, 0, NULL);

    while (1){
        DWORD flags = 0, n = 0;
        client *c;
        struct sockaddr_in addr;
        int addrlen = sizeof(addr);

        SOCKET client_socket = WSAAccept(server_socket, (struct sockaddr*)&addr, &addrlen, NULL, 0);

        printf("%3d connected\n", client_socket);

        CreateIoCompletionPort((HANDLE)client_socket, iocp, 0, 0);

        c = (client*)calloc(1, sizeof(*c));

        c->socket = client_socket;
        c->sendbuf.len = strlen(text);
        c->recvbuf.len = BUFFER_SIZE;
        c->sendbuf.buf = c->sendbuffer;
        c->recvbuf.buf = c->recvbuffer;
        strcpy(c->sendbuf.buf, text);

        /* for some reason I have to receive 0 bytes once */
        WSARecv(c->socket, &(c->recvbuf), 1, &n, &flags, &(c->overlapped), NULL);
    }
}

示例输出：

/* Browser makes two tcp connections on socket 124 and 128. */
124 connected
128 connected

/* GetQueuedCompletionStatus returned for socket 124. */
124 triggered

/* We received the browser's http request. */
124 WSARecv 375 bytes

/* Send http response to browser. */
124 WSASend 96 bytes

/* GetQueuedCompletionStatus returned again. */
124 triggered

/* This is wrong, we should not receive our response to the browser. */
/* Also we didn't even receive data here. */
/* recvbuffer still contains the http request. */
124 WSARecv 96 bytes

/* this is ok */
124 WSASend 0 bytes
124 triggered
124 disconnected

/* Why does GetQueuedCompletionStatus still return? the socket is closed! */
/* Also how can I tell when I can safely free the buffers */
/* if GetQueuedCompletionStatus keeps returning? */
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236

/* same again for second http request */
128 triggered
128 WSARecv 375 bytes
128 WSASend 96 bytes
128 triggered
128 WSARecv 96 bytes
128 WSASend 0 bytes
128 triggered
128 disconnected
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
128 connected
128 triggered
128 WSARecv 375 bytes
128 WSASend 96 bytes
128 triggered
128 WSARecv 96 bytes
128 WSASend 0 bytes
128 triggered
128 disconnected
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
128 connected
128 triggered
128 WSARecv 289 bytes
128 WSASend 96 bytes
128 triggered
128 WSARecv 96 bytes
128 WSASend 0 bytes
128 triggered
128 disconnected
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236
 -1 triggered
 -1 GetQueuedCompletionStatus error 1236