libevent源码分析：http-server例子

http-server例子是libevent提供的一个简单web服务器，实现了对静态网页的处理功能。

/*
* gcc -g -o http-server http-server.c -levent
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <signal.h>
#include <fcntl.h>
#include <unistd.h>
#include <dirent.h>
#include <errno.h>

#include <event2/event.h>
#include <event2/http.h>
#include <event2/buffer.h>
#include <event2/util.h>
#include <event2/keyvalq_struct.h>

#include <netinet/in.h>
#include <arpa/inet.h>

char uri_root[512];

static const struct table_entry {
    const char *extension;
    const char *content_type;
} content_type_table[] = {
    { "txt", "text/plain" }, 
    { "c", "text/plain" }, 
    { "h", "text/plain" }, 
    { "html", "text/html" }, 
    { "htm", "text/html" },
    { "css", "text/css" },
    { "gif", "image/gif" }, 
    { "jpg", "image/jpg" }, 
    { "jpeg", "image/jpeg" }, 
    { "png", "image/png" }, 
    { "pdf", "application/pdf" }, 
    { "ps", "application/postscript" }, 
    { NULL, NULL },
};

/* Try to guess a good content-type for 'path' */
const char* guess_content_type(const char *path)
{
    const char *last_period, *extension;
    const struct table_entry *ent;
    last_period = strrchr(path, '.');
    if (!last_period || strchr(last_period, '/'))
    {
        goto not_found;
    }

    extension = last_period + 1;
    for (ent = &content_type_table[0]; ent->extension; ++ent)
    {
        if (!evutil_ascii_strcasecmp(ent->extension, extension))
        {
            return ent->content_type;
        }
    }

not_found:
    return "application/misc";
}

/* Callbase used for the /dump URI, and for every non-get request:
** dumps all information to stdout and gives base a trivial 200 ok */
void dump_request_cb(struct evhttp_request *req, void *arg)
{
    const char *cmdtype;
    struct evkeyvalq *headers;
    struct evkeyval *header;
    struct evbuffer *buf;

    switch (evhttp_request_get_command(req))
    {
    case EVHTTP_REQ_GET:
        cmdtype = "GET";
        break;
    case EVHTTP_REQ_POST:
        cmdtype = "POST";
        break;
    case EVHTTP_REQ_HEAD:
        cmdtype = "HEAD";
        break;
    case EVHTTP_REQ_PUT:
        cmdtype = "PUT";
        break;
    case EVHTTP_REQ_DELETE:
        cmdtype = "DELETE";
        break;
    case EVHTTP_REQ_OPTIONS:
        cmdtype = "OPTIONS";
        break;
    case EVHTTP_REQ_TRACE:
        cmdtype = "TRACE";
        break;
    case EVHTTP_REQ_CONNECT:
        break;
    case EVHTTP_REQ_PATCH:
        cmdtype = "PATCH";
        break;
    default:
        cmdtype = "unknown";
        break;
    }

    printf("Received a %s request for %s
Header:
", cmdtype, evhttp_request_get_uri(req));

    headers = evhttp_request_get_input_headers(req);
    for (header = headers->tqh_first; header; header = header->next.tqe_next)
    {
        printf(" %s: %s
", header->key, header->value);
    }

    buf = evhttp_request_get_input_buffer(req);
    puts("Input data: <<<<");
    while (evbuffer_get_length(buf))
    {
        int n;
        char cbuf[128];
        n = evbuffer_remove(buf, cbuf, sizeof(cbuf));
        if (n > 0)
        {
            (void)fwrite(cbuf, 1, n, stdout);
        }
    }
    puts(">>>");

    evhttp_send_reply(req, 200, "ok", NULL);
}

/* This callback gets invoked when we get and http request than doesn't match 
* any other callback. Like any evhttp server callback, it has a simple job:
* it must eventually call evhttp_send_error() or evhttp_send_reply(). 
*/
void send_document_cb(struct evhttp_request *req, void *arg)
{
    struct evbuffer *evb = NULL;
    const char *docroot = arg;
    const char *uri = evhttp_request_get_uri(req);
    struct evhttp_uri *decoded = NULL;
    const char *path;
    char *decoded_path;
    char *whole_path = NULL;
    size_t len;
    int fd = -1;
    struct stat st;

    if (evhttp_request_get_command(req) != EVHTTP_REQ_GET)
    {
        dump_request_cb(req, arg);
        return;
    }

    printf("Got a GET request for <%s>
", uri);

    /* Decode the URI */
    decoded = evhttp_uri_parse(uri);
    if (!decoded)
    {
        printf("It's not a good URI, Sneding BADREQUEST
");
        evhttp_send_error(req, HTTP_BADREQUEST, 0);
        return;
    }

    /* Let's see what path the user asked for. */
    path = evhttp_uri_get_path(decoded);
    if (!path)
    {
        path = "/";
    }

    /* We need to decode it, to see what path the user really wanted */
    decoded_path = evhttp_uridecode(path, 0, NULL);
    if (decoded_path == NULL)
    {
        goto err;
    }

    /* Don't allow any ".."'s in the path, to avoid exposing stuff outside 
    * of the docroot. This test is both overzealous and underzealous:
    * it forbids aceptable paths like "/this/one..here", but it doesn't
    * do anything to prevent symlink following. */
    if (strstr(decoded_path, ".."))
    {
        goto err;
    }

    len = strlen(decoded_path) + strlen(docroot) + 2;
    if (!(whole_path = malloc(len)))
    {
        perror("malloc");
        goto err;
    }
    evutil_snprintf(whole_path, len, "%s/%s", docroot, decoded_path);

    if (stat(whole_path, &st) < 0)
    {
        goto err;
    }

    /* This holds the content we're sending */
    evb = evbuffer_new();

    if (S_ISDIR(st.st_mode))
    {
        /* If it's a directory, read the comments and make a little index page */
        DIR *d;
        struct dirent *ent;
        const char *trailing_slash = "";

        if (!strlen(path) || path[strlen(path) - 1] != '/')
        {
            trailing_slash = "/";
        }

        if (!(d = opendir(whole_path)))
        {
            goto err;
        }

        evbuffer_add_printf(evb, 
            "<!DOCTYPE html>
"
            "<html>
  "
            "  <head>
"
            "    <meta charset='utf-8'>
"
            "    <title>%s</title>
"
            "    <base href='%s%s'>
"
            "  </head>
"
            "  <body>
"
            "    <h1>%s</h1>
"
            "    <ul>
", 
            decoded_path, /* xxx html-escape this. */
            path, /* xxx html-escape this? */
            trailing_slash, 
            decoded_path /* xx html-escape this */
            );

        while ((ent = readdir(d)))
        {
            const char *name = ent->d_name;
            evbuffer_add_printf(evb,
                "    <li><a href="%s">%s</a>
", name, name);
        }

        evbuffer_add_printf(evb, "</ul></body></html>
");
        closedir(d);
        evhttp_add_header(evhttp_request_get_output_headers(req),
            "Content-Type", "text/html");
    }
    else
    {
        /* Otherwise it's a file; and it to the buffer to get send via sendfile */
        const char *type = guess_content_type(decoded_path);
        if ((fd = open(whole_path, O_RDONLY)) < 0)
        {
            perror("open");
            goto err;
        }

        if (fstat(fd, &st) < 0)
        {
            /* Make sure the length still matches, now that we opened the file :/ */
            perror("fstat");
            goto err;
        }
        evhttp_add_header(evhttp_request_get_output_headers(req),
            "Content-Type", type);
        evbuffer_add_file(evb, fd, 0, st.st_size);
    }

    evhttp_send_reply(req, 200, "OK", evb);
    goto done;

err:
    evhttp_send_error(req, 404, "Document was not found");
    if (fd >= 0)
    {
        close(fd);
    }

done:
    if (decoded)
    {
        evhttp_uri_free(decoded);
    }

    if (decoded_path)
    {
        free(decoded_path);
    }

    if (whole_path)
    {
        free(whole_path);
    }

    if (evb)
    {
        evbuffer_free(evb);
    }
}

void syntax(void)
{
    fprintf(stdout, "Syntax: http-server <docroot>
");
}

int main(int argc, char **argv)
{
    struct event_base *base;
    struct evhttp *http;
    struct evhttp_bound_socket *handle;
    int port = 0;

    if (signal(SIGPIPE, SIG_IGN) == SIG_ERR)
    {
        printf("signal error, errno[%d], error[%s]", errno, strerror(errno));
        return -1;
    }

    if (argc < 2)
    {
        syntax();
        return -1;
    }

    base = event_base_new();
    if (!base)
    {
        printf("Couldn't create an event_base:exiting
");
        return -1;
    }

    /* Create a new http oject to handle request */
    http = evhttp_new(base);
    if (!http)
    {
        printf("Couldn't create evhttp.Exiting
");
        return -1;
    }

    /* The /dump URI will dump all requests to stdout and say 200 ok */
    evhttp_set_cb(http, "/dump", dump_request_cb, NULL);

    /* We want to accept arbitrary requests, so we need to set a "generic" cb
    * We can also add callbacks for specific paths */
    evhttp_set_gencb(http, send_document_cb, argv[1]);

    /* Now we teel the evhttp what port to listen on */
    handle = evhttp_bind_socket_with_handle(http, "0.0.0.0", port);
    if (!handle)
    {
        printf("Couldn't bind to port[%d], exiting
", port);
        return -1;
    }

    {
        /* Extract and display the address we're listening on. */
        struct sockaddr_storage ss;
        evutil_socket_t fd;
        ev_socklen_t socklen = sizeof(ss);
        char addrbuf[128];
        void *inaddr;
        const char *addr;
        int got_port = -1;
        fd = evhttp_bound_socket_get_fd(handle);
        memset(&ss, 0, sizeof(ss));
        if (getsockname(fd, (struct sockaddr *)&ss, &socklen))
        {
            perror("getsockname() failed");
            return -1;
        }

        if (ss.ss_family == AF_INET)
        {
            got_port = ntohs(((struct sockaddr_in*)&ss)->sin_port);
            inaddr = &((struct sockaddr_in*)&ss)->sin_addr;
        }
        else if (ss.ss_family == AF_INET6)
        {
            got_port = ntohs(((struct sockaddr_in6*)&ss)->sin6_port);
            inaddr = &((struct sockaddr_in6*)&ss)->sin6_addr;
        }
        else
        {
            printf("Weird address family
");
            return 1;
        }

        addr = evutil_inet_ntop(ss.ss_family, inaddr, addrbuf, sizeof(addrbuf));
        if (addr)
        {
            printf("Listening on %s:%d
", addr, got_port);
            evutil_snprintf(uri_root, sizeof(uri_root), "http://%s:%d", addr, got_port);
        }
        else
        {
            printf("evutil_inet_ntop failed
");
            return -1;
        }
    }
    event_base_dispatch(base);

    return 0;
}

下面就通过分析这个例子来理解evhttp对象的使用与实现：

1、首先介绍一个这段代码里面的几个函数及其作用：

1）guess_content_type：传入请求的路径，返回文件类型（根据请求资源的后缀名返回响应的MIME类型）

2）dump_requese_cb：这个函数是当uri为/dump时的回调，操作是打印全部的请求信息。

3）send_document_cb：这个函数是通用uri的回调函数，就是将请求的资源发送给客户端（浏览器）。

4）syntax：打印用法的函数

5）main：主函数

2、调用event_base_new函数得到一个event base对象。

3、调用evhttp_new函数得到一个evhttp对象。

4、调用evhttp_set_cb、evthttp_set_gencb设置回调函数和通用回调函数。

5、调用evhttp_bind_socket_with_handle函数设置监听端口。

6、打印监听端口信息。

7、调用event_base_dispatch进入事件循环。

这里使用了一个新的类evhttp，这个也是对基本函数更高层次的封装，方便编写http相关的程序，关于这个类会在后面详细的分析，这里略过。

到这里就分析完http-server了，可以发现使用libevent提供的函数来编写一个http-server服务器是多么的简单。