使用函数结构的libuv代替函数回调

Question

有人知道为什么在使用函子结构而不是作为回调的实际函数时libuv分段错误。在下面的示例中，我创建了CB结构来代替connection_cb。在运算符()中发生分段错误。

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


uv_buf_t alloc_buffer(uv_handle_t * handle, size_t size);
void connection_cb(uv_stream_t * server, int status);
void read_cb(uv_stream_t * stream, ssize_t nread, uv_buf_t buf);

struct CB {
    State *state_;
    CB(State *state) : state_(state) {}
    void operator()(uv_stream_t *server, int status) {
        uv_tcp_t * client = (uv_tcp_t *)malloc(sizeof(uv_tcp_t));

        /* if status not zero there was an error */
        if (status == -1) {
            printf("error 2\n");
        }

        /* initialize the new client */
        uv_tcp_init(loop, client);

        /* now let bind the client to the server to be used for incomings */
        if (uv_accept(server, (uv_stream_t *) client) == 0) {
            /* start reading from stream */
            int r = uv_read_start((uv_stream_t *) client, (uv_alloc_cb)alloc_buffer, read_cb);

            if (r) {
                printf("error 3\n");
            }
            state_->onConnect();
        } else {
            /* close client stream on error */
            uv_close((uv_handle_t *) client, NULL);
        }
    }
};

CB cb;
uv_tcp_t server;
uv_loop_t * loop;

int main() {
    loop = uv_default_loop();

    /* convert a humanreadable ip address to a c struct */
    struct sockaddr_in addr;
    uv_ip4_addr("127.0.0.1", 3005, &addr);

    /* initialize the server */
    uv_tcp_init(loop, &server);
    /* bind the server to the address above */
    uv_tcp_bind(&server, (const struct sockaddr *)&addr, 0);

    /* let the server listen on the address for new connections */
    int r = uv_listen((uv_stream_t *) &server, 128, (uv_connection_cb)&cb);

    if (r) {
        printf("error 1\n");
        return -1;
    }

    /* execute all tasks in queue */
    return uv_run(loop, UV_RUN_DEFAULT);
}


/**
 * Callback which is executed on each new connection.
 */
void connection_cb(uv_stream_t * server, int status) {
    /* dynamically allocate a new client stream object on conn */
    uv_tcp_t * client = (uv_tcp_t *)malloc(sizeof(uv_tcp_t));

    /* if status not zero there was an error */
    if (status == -1) {
        printf("error 2\n");
    }

    /* initialize the new client */
    uv_tcp_init(loop, client);

    /* now let bind the client to the server to be used for incomings */
    if (uv_accept(server, (uv_stream_t *) client) == 0) {
        /* start reading from stream */
        //int r = uv_read_start((uv_stream_t *) client, (uv_alloc_cb)alloc_buffer, read_cb);
        int r = 0;

        if (r) {
            printf("error 3\n");
        }
    } else {
        /* close client stream on error */
        uv_close((uv_handle_t *) client, NULL);
    }
}

/**
 * Callback which is executed on each readable state.
 */
void read_cb(uv_stream_t * stream, ssize_t nread, uv_buf_t buf) {
    /* dynamically allocate memory for a new write task */
    uv_write_t * req = (uv_write_t *) malloc(sizeof(uv_write_t));

    /* if read bytes counter -1 there is an error or EOF */
    if (nread == -1) {
        printf("error 4\n");
        uv_close((uv_handle_t *) stream, NULL);
    }

    /* write sync the incoming buffer to the socket */
    int r = uv_write(req, stream, &buf, 1, NULL);

    if (r) {
        printf("error 5\n");
    }
}

/**
 * Allocates a buffer which we can use for reading.
 */
uv_buf_t alloc_buffer(uv_handle_t * handle, size_t size) {
        return uv_buf_init((char *) malloc(size), size);
}

Answer 1

粘贴的代码将无法工作，因为libuv需要uv_listen中的一个函数指针--您提供的是指向结构的指针。这种结构碰巧有一个operator()，但这并不使您的结构的地址成为处理器可以跳到并执行代码的地址。operator()就像您的结构的任何其他方法一样，但是您可以使用简单的()或.operator()来调用它，以提高代码的可读性。此外，由于operator()是非静态的，因此它期望有一个对this的隐式引用，而libuv不会提供这个引用，因为它不希望这样做。

为了实现您要做的事情，您应该提供一个普通的C函数回调，并将额外的上下文数据存储在句柄的.data字段中：

而不是：

    int r = uv_listen((uv_stream_t *) &server, 128, (uv_connection_cb)&cb);

使用：

    server.data = &cb;
    int r = uv_listen((uv_stream_t *) &server, 128, [](uv_stream_t *server, int status){
        (*(CB*)server->data)(server, status);
    });

uv手柄上的.data字段正是为此目的而提供的。