C Libevent未定义引用，我错了吗

Question

**I是C*的新手

我一直在关注Libevent网站上的这本书，并正在阅读第01部分——异步IO的一个小介绍。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;
}

我在倒数第二个例子中，我重写了页面后面的代码，并克隆了它。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

我在假设，你知道他们怎么说假设。。。。这是来自alloc_fd_state，由于作用域而无法到达event_new等？然而，我也看到它没有被赋予一种类型，比如void，或者其他类型。所以，自然很好奇，我只是为了测试而玩，说我把int放在alloc_fd_state前面（虽然知道这是不正确的），但我收到了一个不同的错误，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，我想我的问题如下。

在第一个场景错误代码的情况下，这里遗漏了接收未定义引用的内容。Event_new等在我的代码中似乎都很好，为什么我不能访问它们？我应该参考哪里，也许这是非常明显的，因为我对C很熟悉，但我已经盯着这个看了一段时间了，我的大脑有点陷入困境。任何关于为什么会发生这种情况的解释，以及我如何保证在未来的任何代码中都不会发生这种情况，都将不胜感激。

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、更深刻的错误所掩盖，而不是真正修复任何东西。

对于C级新秀来说，任何见解都值得赞赏。

Answer 1

将-levent_core添加到编译器命令行