【网络库学习】libevent篇_libevent库 udp

(一）linux编译

./configure --prefix=xxx --disable-openssl # 针对libevent 2.1.12版本 我这里不需要ssl
make
make install
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(二） udp编程

#include <sys/types.h>
#include <sys/socket.h>
#include <string.h>
#include <netinet/in.h>

#include <event.h>
#include <event2/event.h>

void udp_read_cb(int fd, short ev, void *p)
{
    char buff[1024] = "";
    struct sockaddr_in sa;
    int sock_len = sizeof(sockaddr_in);
    memset(&sa,0,sock_len);

   ssize_t len = recvfrom(fd,buff,1023,0,(struct sockaddr*)&sa,(socklen_t*)&sock_len);
   if (len >= 0)
   {
        printf("recv:%s",buff);
   } else {
       evutil_closesocket(fd);
   }
}

int udp_handle_loop()
{
    /* 1.Creating an event_base */
    struct event_base* base = event_base_new();
    if (NULL == base)
    {
        perror("event_base_new failed");
        return -1;
    }
    printf("event base create\n");
    /* Working with events
     * Libevent’s basic unit of operation is the event. Every event represents a set of conditions, including:
     *   A file descriptor being ready to read from or write to.
     *   A file descriptor becoming ready to read from or write to (Edge-triggered IO only).
     *   A timeout expiring.
     *   A signal occurring.
     *   A user-triggered event.
    */
    struct event* timer_ev = evtimer_new(base,timer_cb,NULL);
    event_add(timer_ev,NULL);

    /* udp socket */
    int udpSocket = socket(AF_INET,SOCK_DGRAM,0);
    if (-1 == udpSocket)
    {
        perror("socket failed");
        return -1;
    }
    struct sockaddr_in sa;
    sa.sin_family = AF_INET;
    sa.sin_port = htons(8080);
    sa.sin_addr.s_addr = INADDR_ANY;

    int opt = 1;
    setsockopt(udpSocket,SOL_SOCKET,SO_REUSEADDR,(const void*)&opt,sizeof(opt));

    int ret = ::bind(udpSocket,(struct sockaddr*)&sa,sizeof(sa));
    if (ret < 0)
    {
        perror("bind error");
        return -1;
    }
    struct event* udp_ev = event_new(base,udpSocket,EV_READ | EV_PERSIST,udp_read_cb,NULL);

    struct timeval val;
    val.tv_usec = 150;

    event_add(udp_ev,NULL);
    /* 2.Working with an event loop */
    event_base_dispatch(base);

    event_base_free(base);
    evutil_closesocket(udpSocket);
}
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(三） tcp编程

#include <event2/listener.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>

#include <arpa/inet.h>

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

static void
echo_read_cb(struct bufferevent *bev, void *ctx)
{
        /* This callback is invoked when there is data to read on bev. */
        struct evbuffer *input = bufferevent_get_input(bev);
        struct evbuffer *output = bufferevent_get_output(bev);

        /* Copy all the data from the input buffer to the output buffer. */
        evbuffer_add_buffer(output, input);
}

static void
echo_event_cb(struct bufferevent *bev, short events, void *ctx)
{
        if (events & BEV_EVENT_ERROR)
                perror("Error from bufferevent");
        if (events & (BEV_EVENT_EOF | BEV_EVENT_ERROR)) {
                bufferevent_free(bev);
        }
}

static void
accept_conn_cb(struct evconnlistener *listener,
    evutil_socket_t fd, struct sockaddr *address, int socklen,
    void *ctx)
{
        /* We got a new connection! Set up a bufferevent for it. */
        struct event_base *base = evconnlistener_get_base(listener);
        struct bufferevent *bev = bufferevent_socket_new(
                base, fd, BEV_OPT_CLOSE_ON_FREE);

        bufferevent_setcb(bev, echo_read_cb, NULL, echo_event_cb, NULL);

        bufferevent_enable(bev, EV_READ|EV_WRITE);
}

static void
accept_error_cb(struct evconnlistener *listener, void *ctx)
{
        struct event_base *base = evconnlistener_get_base(listener);
        int err = EVUTIL_SOCKET_ERROR();
        fprintf(stderr, "Got an error %d (%s) on the listener. "
                "Shutting down.\n", err, evutil_socket_error_to_string(err));

        event_base_loopexit(base, NULL);
}

int
main(int argc, char **argv)
{
        struct event_base *base;
        struct evconnlistener *listener;
        struct sockaddr_in sin;

        int port = 9876;

        if (argc > 1) {
                port = atoi(argv[1]);
        }
        if (port<=0 || port>65535) {
                puts("Invalid port");
                return 1;
        }

        base = event_base_new();
        if (!base) {
                puts("Couldn't open event base");
                return 1;
        }

        /* Clear the sockaddr before using it, in case there are extra
         * platform-specific fields that can mess us up. */
        memset(&sin, 0, sizeof(sin));
        /* This is an INET address */
        sin.sin_family = AF_INET;
        /* Listen on 0.0.0.0 */
        sin.sin_addr.s_addr = htonl(0);
        /* Listen on the given port. */
        sin.sin_port = htons(port);

        listener = evconnlistener_new_bind(base, accept_conn_cb, NULL,
            LEV_OPT_CLOSE_ON_FREE|LEV_OPT_REUSEABLE, -1,
            (struct sockaddr*)&sin, sizeof(sin));
        if (!listener) {
                perror("Couldn't create listener");
                return 1;
        }
        evconnlistener_set_error_cb(listener, accept_error_cb);

        event_base_dispatch(base);
        return 0;
}
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来源：https://libevent.org/libevent-book/Ref8_listener.html

(四） http编程

/*
  A trivial static http webserver using Libevent's evhttp.

  This is not the best code in the world, and it does some fairly stupid stuff
  that you would never want to do in a production webserver. Caveat hackor!

 */

/* Compatibility for possible missing IPv6 declarations */
#include "../util-internal.h"

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/stat.h>

#ifdef _WIN32
#include <winsock2.h>
#include <ws2tcpip.h>
#include <windows.h>
#include <getopt.h>
#include <io.h>
#include <fcntl.h>
#ifndef S_ISDIR
#define S_ISDIR(x) (((x) & S_IFMT) == S_IFDIR)
#endif
#else /* !_WIN32 */
#include <sys/stat.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <unistd.h>
#include <dirent.h>
#endif /* _WIN32 */
#include <signal.h>

#ifdef EVENT__HAVE_SYS_UN_H
#include <sys/un.h>
#endif
#ifdef EVENT__HAVE_AFUNIX_H
#include <afunix.h>
#endif

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

#ifdef _WIN32
#include <event2/thread.h>
#endif /* _WIN32 */

#ifdef EVENT__HAVE_NETINET_IN_H
#include <netinet/in.h>
# ifdef _XOPEN_SOURCE_EXTENDED
#  include <arpa/inet.h>
# endif
#endif

#ifdef _WIN32
#ifndef stat
#define stat _stat
#endif
#ifndef fstat
#define fstat _fstat
#endif
#ifndef open
#define open _open
#endif
#ifndef close
#define close _close
#endif
#ifndef O_RDONLY
#define O_RDONLY _O_RDONLY
#endif
#endif /* _WIN32 */

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/htm" },
	{ "css", "text/css" },
	{ "gif", "image/gif" },
	{ "jpg", "image/jpeg" },
	{ "jpeg", "image/jpeg" },
	{ "png", "image/png" },
	{ "pdf", "application/pdf" },
	{ "ps", "application/postscript" },
	{ NULL, NULL },
};

struct options
{
	int port;
	int iocp;
	int verbose;

	int unlink;
	const char *unixsock;
};

/* Try to guess a good content-type for 'path' */
static 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; /* no exension */
	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";
}

/* Callback used for the /dump URI, and for every non-GET request:
 * dumps all information to stdout and gives back a trivial 200 ok */
static 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: cmdtype = "CONNECT"; break;
	case EVHTTP_REQ_PATCH: cmdtype = "PATCH"; break;
	default: cmdtype = "unknown"; break;
	}

	printf("Received a %s request for %s\nHeaders:\n",
	    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\n", 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 any http request that 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().
 */
static 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>\n",  uri);

	/* Decode the URI */
	decoded = evhttp_uri_parse(uri);
	if (!decoded) {
		printf("It's not a good URI. Sending BADREQUEST\n");
		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 */
#ifdef _WIN32
		HANDLE d;
		WIN32_FIND_DATAA ent;
		char *pattern;
		size_t dirlen;
#else
		DIR *d;
		struct dirent *ent;
#endif
		const char *trailing_slash = "";

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

#ifdef _WIN32
		dirlen = strlen(whole_path);
		pattern = malloc(dirlen+3);
		memcpy(pattern, whole_path, dirlen);
		pattern[dirlen] = '\\';
		pattern[dirlen+1] = '*';
		pattern[dirlen+2] = '\0';
		d = FindFirstFileA(pattern, &ent);
		free(pattern);
		if (d == INVALID_HANDLE_VALUE)
			goto err;
#else
		if (!(d = opendir(whole_path)))
			goto err;
#endif

		evbuffer_add_printf(evb,
                    "<!DOCTYPE html>\n"
                    "<html>\n <head>\n"
                    "  <meta charset='utf-8'>\n"
		    "  <title>%s</title>\n"
		    "  <base href='%s%s'>\n"
		    " </head>\n"
		    " <body>\n"
		    "  <h1>%s</h1>\n"
		    "  <ul>\n",
		    decoded_path, /* XXX html-escape this. */
		    path, /* XXX html-escape this? */
		    trailing_slash,
		    decoded_path /* XXX html-escape this */);
#ifdef _WIN32
		do {
			const char *name = ent.cFileName;
#else
		while ((ent = readdir(d))) {
			const char *name = ent->d_name;
#endif
			evbuffer_add_printf(evb,
			    "    <li><a href=\"%s\">%s</a>\n",
			    name, name);/* XXX escape this */
#ifdef _WIN32
		} while (FindNextFileA(d, &ent));
#else
		}
#endif
		evbuffer_add_printf(evb, "</ul></body></html>\n");
#ifdef _WIN32
		FindClose(d);
#else
		closedir(d);
#endif
		evhttp_add_header(evhttp_request_get_output_headers(req),
		    "Content-Type", "text/html");
	} else {
		/* Otherwise it's a file; add it to the buffer to get
		 * sent 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);
}

static void
print_usage(FILE *out, const char *prog, int exit_code)
{
	fprintf(out, "Syntax: [ OPTS ] %s <docroot>\n", prog);
	fprintf(out, " -p      - port\n");
	fprintf(out, " -U      - bind to unix socket\n");
	fprintf(out, " -u      - unlink unix socket before bind\n");
	fprintf(out, " -I      - IOCP\n");
	fprintf(out, " -v      - verbosity, enables libevent debug logging too\n");
	exit(exit_code);
}
static struct options
parse_opts(int argc, char **argv)
{
	struct options o;
	int opt;

	memset(&o, 0, sizeof(o));

	while ((opt = getopt(argc, argv, "hp:U:uIv")) != -1) {
		switch (opt) {
			case 'p': o.port = atoi(optarg); break;
			case 'U': o.unixsock = optarg; break;
			case 'u': o.unlink = 1; break;
			case 'I': o.iocp = 1; break;
			case 'v': ++o.verbose; break;
			case 'h': print_usage(stdout, argv[0], 0); break;
			default : fprintf(stderr, "Unknown option %c\n", opt); break;
		}
	}

	if (optind >= argc || (argc-optind) > 1) {
		print_usage(stdout, argv[0], 1);
	}

	return o;
}

static void
do_term(int sig, short events, void *arg)
{
	struct event_base *base = arg;
	event_base_loopbreak(base);
	fprintf(stderr, "Got %i, Terminating\n", sig);
}

static int
display_listen_sock(struct evhttp_bound_socket *handle)
{
	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;
	}
#ifdef EVENT__HAVE_STRUCT_SOCKADDR_UN
	else if (ss.ss_family == AF_UNIX) {
		printf("Listening on <%s>\n", ((struct sockaddr_un*)&ss)->sun_path);
		return 0;
	}
#endif
	else {
		fprintf(stderr, "Weird address family %d\n",
		    ss.ss_family);
		return 1;
	}

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

	return 0;
}

int
main(int argc, char **argv)
{
	struct event_config *cfg = NULL;
	struct event_base *base = NULL;
	struct evhttp *http = NULL;
	struct evhttp_bound_socket *handle = NULL;
	struct evconnlistener *lev = NULL;
	struct event *term = NULL;
	struct options o = parse_opts(argc, argv);
	int ret = 0;

#ifdef _WIN32
	{
		WORD wVersionRequested;
		WSADATA wsaData;
		wVersionRequested = MAKEWORD(2, 2);
		WSAStartup(wVersionRequested, &wsaData);
	}
#else
	if (signal(SIGPIPE, SIG_IGN) == SIG_ERR) {
		ret = 1;
		goto err;
	}
#endif

	setbuf(stdout, NULL);
	setbuf(stderr, NULL);

	/** Read env like in regress */
	if (o.verbose || getenv("EVENT_DEBUG_LOGGING_ALL"))
		event_enable_debug_logging(EVENT_DBG_ALL);

	cfg = event_config_new();
#ifdef _WIN32
	if (o.iocp) {
#ifdef EVTHREAD_USE_WINDOWS_THREADS_IMPLEMENTED
		evthread_use_windows_threads();
		event_config_set_num_cpus_hint(cfg, 8);
#endif
		event_config_set_flag(cfg, EVENT_BASE_FLAG_STARTUP_IOCP);
	}
#endif

	base = event_base_new_with_config(cfg);
	if (!base) {
		fprintf(stderr, "Couldn't create an event_base: exiting\n");
		ret = 1;
	}
	event_config_free(cfg);
	cfg = NULL;

	/* Create a new evhttp object to handle requests. */
	http = evhttp_new(base);
	if (!http) {
		fprintf(stderr, "couldn't create evhttp. Exiting.\n");
		ret = 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]);

	if (o.unixsock) {
#ifdef EVENT__HAVE_STRUCT_SOCKADDR_UN
		struct sockaddr_un addr;

		if (o.unlink && (unlink(o.unixsock) && errno != ENOENT)) {
			perror(o.unixsock);
			ret = 1;
			goto err;
		}

		addr.sun_family = AF_UNIX;
		strcpy(addr.sun_path, o.unixsock);

		lev = evconnlistener_new_bind(base, NULL, NULL,
			LEV_OPT_CLOSE_ON_FREE, -1,
			(struct sockaddr *)&addr, sizeof(addr));
		if (!lev) {
			perror("Cannot create listener");
			ret = 1;
			goto err;
		}

		handle = evhttp_bind_listener(http, lev);
		if (!handle) {
			fprintf(stderr, "couldn't bind to %s. Exiting.\n", o.unixsock);
			ret = 1;
			goto err;
		}
#else /* !EVENT__HAVE_STRUCT_SOCKADDR_UN */
		fprintf(stderr, "-U is not supported on this platform. Exiting.\n");
		ret = 1;
		goto err;
#endif /* EVENT__HAVE_STRUCT_SOCKADDR_UN */
	}
	else {
		handle = evhttp_bind_socket_with_handle(http, "0.0.0.0", o.port);
		if (!handle) {
			fprintf(stderr, "couldn't bind to port %d. Exiting.\n", o.port);
			ret = 1;
			goto err;
		}
	}

	if (display_listen_sock(handle)) {
		ret = 1;
		goto err;
	}

	term = evsignal_new(base, SIGINT, do_term, base);
	if (!term)
		goto err;
	if (event_add(term, NULL))
		goto err;

	event_base_dispatch(base);

#ifdef _WIN32
	WSACleanup();
#endif

err:
	if (cfg)
		event_config_free(cfg);
	if (http)
		evhttp_free(http);
	if (term)
		event_free(term);
	if (base)
		event_base_free(base);

	return ret;
}

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来源：libevent-2.1.10-stable\sample\http-server.c

(五） 原理

1） 事件源
Linux上是文件描述符，Windows上就是Socket或者Handle了，这里统一称为“句柄集”；程序在指定的句柄上注册关心的事件，比如I/O事件。
2） event demultiplexer——事件多路分发机制
由操作系统提供的I/O多路复用机制，比如select和epoll。
程序首先将其关心的句柄（事件源）及其事件注册到event demultiplexer上；
当有事件到达时，event demultiplexer会发出通知“在已经注册的句柄集中，一个或多个句柄的事件已经就绪”；
程序收到通知后，就可以在非阻塞的情况下对事件进行处理了。
对应到libevent中，依然是select、poll、epoll等，但是libevent使用结构体eventop进行了封装，以统一的接口来支持这些I/O多路复用机制，达到了对外隐藏底层系统机制的目的。
3） Reactor——反应器
Reactor，是事件管理的接口，内部使用event demultiplexer注册、注销事件；并运行事件循环，当有事件进入“就绪”状态时，调用注册事件的回调函数处理事件。
对应到libevent中，就是event_base结构体。
源码里最终还是调用io复用的接口，例如select、poll、epoll等
(以上转载自https://zhuanlan.zhihu.com/p/371223583)

struct event_base *
event_base_new(void)
{
	struct event_base *base = NULL;
	struct event_config *cfg = event_config_new();
	if (cfg) {
		base = event_base_new_with_config(cfg);
		event_config_free(cfg);
	}
	return base;
}
struct event_base *
event_base_new_with_config(const struct event_config *cfg)
{
...
	for (i = 0; eventops[i] && !base->evbase; i++) {
		if (cfg != NULL) {
			/* determine if this backend should be avoided */
			if (event_config_is_avoided_method(cfg,
				eventops[i]->name))
				continue;
			if ((eventops[i]->features & cfg->require_features)
			    != cfg->require_features)
				continue;
		}
		/* also obey the environment variables */
		if (should_check_environment &&
		    event_is_method_disabled(eventops[i]->name))
			continue;
		base->evsel = eventops[i];// eventops
		base->evbase = base->evsel->init(base);
	}
...
}
/* Array of backends in order of preference. */
static const struct eventop *eventops[] = {
#ifdef EVENT__HAVE_EVENT_PORTS
	&evportops,
#endif
#ifdef EVENT__HAVE_WORKING_KQUEUE
	&kqops,
#endif
#ifdef EVENT__HAVE_EPOLL
	&epollops,
#endif
#ifdef EVENT__HAVE_DEVPOLL
	&devpollops,
#endif
#ifdef EVENT__HAVE_POLL
	&pollops,
#endif
#ifdef EVENT__HAVE_SELECT
	&selectops,
#endif
#ifdef _WIN32
	&win32ops,
#endif
	NULL
};

const struct eventop selectops = {
	"select",
	select_init,
	select_add,
	select_del,
	select_dispatch,
	select_dealloc,
	0, /* doesn't need reinit. */
	EV_FEATURE_FDS,
	0,
};

static int
select_dispatch(struct event_base *base, struct timeval *tv)
{
...
res = select(nfds, sop->event_readset_out,
	    sop->event_writeset_out, NULL, tv);
...
}
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(六） 参考资料

https://libevent.org/doc/
http://www.wangafu.net/~nickm/libevent-book/