C Libevent undefined reference ，我错了吗？

Question

**我是C新手*

我一直在关注 Book来自 Libevent网站，正在处理 Part 01对异步 IO 的简要介绍。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，我已经重写了页面后面的代码，但是我已经克隆了它。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，由于范围原因无法到达 event_new 等？但是，我还看到它没有被赋予诸如 void 之类的类型。所以，自然地好奇，我只是为了测试而玩弄并说我将 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，我想我的问题是(是)，如下所示。

在第一个场景错误代码的情况下，这里遗漏了什么我收到 undefined reference 。 Event_new 等在我的代码中似乎都很好，为什么我无法访问它们？我应该在哪里引用，也许这是非常明显的，因为我对 C 还很陌生，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它不会出现在任何 future 的代码中，我们将不胜感激。

对于第二种情况，我假设 undefined reference 的真正问题只是被显示的新的和更严重的错误所掩盖，而不是实际修复任何东西。

对于 C 菜鸟的任何见解表示赞赏。

Answer 1

在编译命令行添加-levent_core