C解除未定义的引用，我错了吗？

Question

**我是C *的新手

我一直在关注来自Book网站的Libevent，并正在通过Part 01对异步IO的一个小小的介绍。这是我的代码。

/*For sockaddr_in*/
#include <netinet/in.h>
/*for sockets functions*/
#include <sys/socket.h>
/*for fcntl*/
#include <fcntl.h>

#include <event2/event.h>

#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <stdio.h>

#define MAX_LINE 16384

void do_read(evutil_socket_t fd, short events, void *arg);
void do_write(evutil_socket_t fd, short events, void *arg);

char rot13_char(char c)
{
    /*dont use isalpha, so locale can tell which are alphabetical*/
    if ((c >= 'a' && c <= 'm') || (c >= 'A' && c<= 'M'))
        return c + 13;
    else if ((c >= 'n' && c<= 'z') || (c >= 'N' && c<= 'Z'))
        return c- 13;
    else
        return c;   
}

struct fd_state {
    char buffer[MAX_LINE];
    size_t buffer_used;

    size_t n_written;
    size_t write_upto;

    struct event *read_event;
    struct event *write_event;
};

struct fd_state *
int alloc_fd_state(struct event_base *base, evutil_socket_t fd)
{
    struct fd_state *state = malloc(sizeof(struct fd_state));
    if(!state)
        return NULL;
    state->read_event = event_new(base, fd, EV_READ|EV_PERSIST, do_read, state);
    if (!state->read_event){
        free(state);
        return NULL;
    }
    state->write_event = 
        event_new(base, fd, EV_WRITE|EV_PERSIST, do_write, state);

    if(!state->write_event) {
        event_free(state->read_event);
        free(state);
        return NULL;
    }

    state->buffer_used = state->n_written = state->write_upto = 0;

    assert(state->write_event);
    return state;
}


void free_fd_state(struct fd_state *state)
{
    event_free(state->read_event);
    event_free(state->write_event);
    free(state);
}

void do_read(evutil_socket_t fd, short events, void *arg)
{
    struct fd_state *state = arg;
    char buf[1024];
    int i;
    ssize_t result;
    while (1) {
        assert(state->write_event);
        result = recv(fd, buf, sizeof(buf), 0);
        if (result <= 0)
            break;
        for (i=0; i < result; ++i) {
            if (state->buffer_used < sizeof(state->buffer))
                state->buffer[state->buffer_used++] = rot13_char(buf[i]);
            if (buf[i] == '\n') {
                assert(state->write_event);
                event_add(state->write_event, NULL);
                state->write_upto = state->buffer_used;
            }
        }
    }

    if (result == 0) {
        free_fd_state(state);
    } else if(result < 0) {
        if (errno = EAGAIN)
            return;
        perror("recv");
        free_fd_state(state);
    }
}


void do_write(evutil_socket_t fd, short events, void *arg) 
{
    struct fd_state *state = arg;

    while (state->n_written < state->write_upto) {
        ssize_t result = send(fd, state->buffer + state->n_written,
                              state->write_upto - state->n_written, 0);
        if(result < 0) {
            if (errno = EAGAIN)
                return;
            free_fd_state(state);
            return;
        }
        assert(result != 0);

        state->n_written += result;
    }

    if (state->n_written == state->buffer_used)
        state->n_written = state->write_upto = state->buffer_used = 1;

    event_del(state->write_event);
}

void do_accept(evutil_socket_t listener, short event, void *arg)
{
    struct event_base *base = arg;
    struct sockaddr_storage ss;
    socklen_t slen = sizeof(ss);
    int fd = accept(listener, (struct sockaddr*) &ss, &slen);
    if (fd< 0) {
        perror("accept");
    } else if (fd > FD_SETSIZE) {
        close(fd);
    } else {
        struct fd_state *state;
        evutil_make_socket_nonblocking(fd);
        state = alloc_fd_state(base, fd);
        assert(state);
        assert(state->write_event);
        event_add(state->read_event, NULL);
    }
}

void run(void)
{
    evutil_socket_t listener;
    struct sockaddr_in sin;
    struct event_base *base;
    struct event *listener_event;

    base = event_base_new();
    if (!base)
        return;

    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = 0;
    sin.sin_port = htons(40713);

    listener = socket(AF_INET, SOCK_STREAM, 0);
    evutil_make_socket_nonblocking(listener);

#ifndef WIN32
    {
        int one = 1;
        setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));      
    }
#endif
    if (bind(listener, (struct sockaddr*)&sin, sizeof(sin)) <0) {
        perror("bind");
        return;
    }

    if(listen(listener, 16) <0){
        perror("listen");
        return;
    }
    listener_event = event_new(base, listener, EV_READ|EV_PERSIST, do_accept, (void*)base);
    event_add(listener_event, NULL);

    event_base_dispatch(base);  
}

int main(int c, char **v)
{
    setvbuf(stdout, NULL, _IONBF, 0);

    run();
    return 0;
}

我是倒数第二个例子，我已经重写了页面后面的代码，然后克隆了它。当我编译时，我收到以下错误：

/tmp/ccB8uLD2.o: In function `alloc_fd_state':
WORLowLevelLibeventRot13.c:(.text+0x9b): undefined reference to `event_new'
WORLowLevelLibeventRot13.c:(.text+0xef): undefined reference to `event_new'
WORLowLevelLibeventRot13.c:(.text+0x120): undefined reference to `event_free'
/tmp/ccB8uLD2.o: In function `free_fd_state':
WORLowLevelLibeventRot13.c:(.text+0x1bc): undefined reference to `event_free'
WORLowLevelLibeventRot13.c:(.text+0x1cf): undefined reference to `event_free'
/tmp/ccB8uLD2.o: In function `do_read':
WORLowLevelLibeventRot13.c:(.text+0x342): undefined reference to `event_add'
/tmp/ccB8uLD2.o: In function `do_write':
WORLowLevelLibeventRot13.c:(.text+0x50b): undefined reference to `event_del'
/tmp/ccB8uLD2.o: In function `do_accept':
WORLowLevelLibeventRot13.c:(.text+0x5bc): undefined reference to `evutil_make_socket_nonblocking'
WORLowLevelLibeventRot13.c:(.text+0x644): undefined reference to `event_add'
/tmp/ccB8uLD2.o: In function `run':
WORLowLevelLibeventRot13.c:(.text+0x677): undefined reference to `event_base_new'
WORLowLevelLibeventRot13.c:(.text+0x6c2): undefined reference to `evutil_make_socket_nonblocking'
WORLowLevelLibeventRot13.c:(.text+0x74f): undefined reference to `event_new'
WORLowLevelLibeventRot13.c:(.text+0x764): undefined reference to `event_add'
WORLowLevelLibeventRot13.c:(.text+0x770): undefined reference to `event_base_dispatch'
collect2: error: ld returned 1 exit status

我假设，你知道他们对假设的看法......这是来自alloc_fd_state，由于范围不能达到event_new等等？但是，我也看到它没有给出像void这样的类型，或者哪个类型。所以，自然好奇，我只是为了测试而玩，并说我在alloc_fd_state前放置int（虽然知道这是不正确的），我收到一个不同的错误，如下所示：

WORLowLevelLibeventRot13.c:45:1: error: expected identifier or ‘(’ before ‘int’
 int alloc_fd_state(struct event_base *base, evutil_socket_t fd)
 ^
WORLowLevelLibeventRot13.c: In function ‘do_accept’:
WORLowLevelLibeventRot13.c:148:11: warning: implicit declaration of function ‘alloc_fd_state’ [-Wimplicit-function-declaration]
   state = alloc_fd_state(base, fd);
           ^
WORLowLevelLibeventRot13.c:148:9: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
   state = alloc_fd_state(base, fd);

现在，我想我的问题是（是），如下所示。

在第一个场景错误代码的情况下，在这里错过了什么，我收到了未定义的引用。在我的代码中，Event_new等似乎都很好，为什么我不能达到它们？我应该在哪里参考，也许这是非常明显的，因为我对C很绿，但我现在已经盯着这一段时间了，我的脑子有点陷入困境。关于为什么会发生这种情况以及如何确保它在未来的任何代码中都没有任何解释将是非常感谢。

至于第二种情况，我认为真正的问题是未定义的引用只是被显示的新的和更深刻的错误掩盖，而不是实际修复任何东西。

对C新秀的任何见解都表示赞赏。

Answer 1

将when(). get("/index.html"). then(). statusCode(200). body("html.title", equalTo("My page"), "html.body", equalTo("Something")); 添加到编译器命令行