c++ - 使用带有仿函数结构的 libuv 而不是函数回调

Question

有谁知道为什么在使用仿函数结构而不是实际函数作为回调时 libuv 会出现段错误。在下面的示例中，我创建了结构 CB 来代替 connection_cb。在 operator() 中发生段错误。

#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

粘贴的代码将不起作用，因为 libuv 需要 uv_listen 中的函数指针 - 您提供的是指向结构的指针。这个结构碰巧有一个operator()，但这并不能使你的结构地址成为处理器可以跳转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() 来调用它使代码更具可读性。此外，由于 operator() 是非静态的，它需要一个对 this 的隐式引用，而 libuv 不会提供，因为它不需要提供。

要实现您正在尝试做的事情，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:

代替:

    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 句柄上的 .data 字段正是为此目的而提供的。