Re: [Libevent-users] Line-based evbuffer?

[Azat Khuzhin <a3at.mail@xxxxxxxxx>]

On Tue, Feb 2, 2016 at 4:32 PM, Ãystein SchÃnning-Johansen
<oysteijo@xxxxxxxxx> wrote:
> It is probably a misconception on my side. I'm pretty good at
> misconceptions. :-)
>
> To illustrate, I set up a really naive server in python:
>>>> import socket
>>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>>> s.bind(('', 8888))
>>>> s.listen(10)
>>>> while 1:
> ...    conn, addr = s.accept()
> ...    conn.send(b"Line 1\nLine 2\nLine 3\nLast line\n")
> ...    conn.close()

I use this:
$ nc -lp 8888 <<<$'line 1\nline 2'

> OK, then I code a simple client. (There are som short-cuts to avoid a lot of
> code, but this compiles and runs with -std=gnu99)
>
> #include <event2/dns.h>
> #include <event2/bufferevent.h>
> #include <event2/buffer.h>
> #include <event2/util.h>
> #include <event2/event.h>
>
> #include <stdlib.h>
> #include <stdio.h>
>
> void buffercb(struct evbuffer *input, const struct evbuffer_cb_info *info,
> void *ptr)
> {
> #if 1
>     char *request_line;
>     size_t len;
>
>     request_line = evbuffer_readln( input, &len, EVBUFFER_EOL_CRLF );

Gotcha, buffercb() is a readcb of evbuffer, IOW it is invoked *every
time* you read from/to this *evbuffer*, see?
IOW when libevent calls buffercb(), buffercb() will call
evbuffer_readln(), and evbuffer_readln() also will call buffercb(),
and because of this recursion you have latest-first behavior.

>     if ( request_line ){
>         printf("%s\n", request_line ); fflush( stdout );
>         free( request_line );
>     }
> }
>
> int main(int argc, char **argv)
> {
>     struct event_base *base;
>     struct evdns_base *dns_base;
>     struct bufferevent *bev;
>
>     if( argc != 3 ){
>         printf("Usage: %s host port", argv[0]);
>         return -1;
>     }
>
>     base = event_base_new();
>     dns_base = evdns_base_new(base, 1);
>
>     bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
>     bufferevent_setcb(bev, NULL, NULL, NULL, base);
>     bufferevent_enable(bev, EV_READ|EV_WRITE);
>     bufferevent_socket_connect_hostname(
>         bev, dns_base, AF_UNSPEC, argv[1], atoi(argv[2]));
>
>     evbuffer_add_cb( bufferevent_get_input(bev), buffercb, NULL );

That's why it is better to use "readcb" over bufferevent that
installed via bufferevent_setcb() instead of callbacks over evbuffer.

Of course you can remove that callback in buffercb(), and translate
that evbuffer_readln() into loop, but this is not how things should
work.

>
>     event_base_dispatch(base);
>     return 0;
> }
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