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:language: C
Connection listeners: accepting TCP connections
The evconnlistener mechanism gives you a way to listen for and accept
incoming TCP connections.
All the functions and types in this section are declared in
event2/listener.h. They first appeared in Libevent 2.0.2-alpha, unless
otherwise noted.
Creating or freeing an evconnlistener
struct evconnlistener *evconnlistener_new(struct event_base *base,
evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
evutil_socket_t fd);
struct evconnlistener *evconnlistener_new_bind(struct event_base *base,
evconnlistener_cb cb, void *ptr, unsigned flags, int backlog,
const struct sockaddr *sa, int socklen);
void evconnlistener_free(struct evconnlistener *lev);
The two evconnlistener_new*() functions both allocate and return a new
connection listener object. A connection listener uses an event_base to
note when there is a new TCP connection on a given listener socket.
When a new connection arrives, it invokes the callback function you give it.
In both functions, the 'base' parameter is an event_base that the listener
should use to listen for connections. The 'cb' function is a callback to
invoke when a new connection is received; if 'cb' is NULL, the listener is
treated as disabled until a callback is set. The 'ptr' pointer will be passed
to the callback. The 'flags' argument controls the behavior of the listener
-- more on this below. The 'backlog' parameter controls the maximum number
of pending connections that the network stack should allow to wait in a
not-yet-accepted state at any time; see documentation for your system's
listen() function for more details. If 'backlog' is negative, Libevent tries
to pick a good value for the backlog; if it is zero, Libevent assumes that
you have already called listen() on the socket you are providing it.
The functions differ in how they set up their listener socket. The
evconnlistener_new() function assumes that you have already bound a socket to
the port you want to listen on, and that you're passing the socket in as
'fd'. If you want Libevent to allocate and bind to a socket on its own,
call evconnlistener_new_bind(), and pass in the sockaddr you want to bind to,
and its length.
TIP: [When using evconnlistener_new, make sure your listening socket is in
non-blocking mode by using evutil_make_socket_nonblocking or by manually
setting the correct socket option. When the listening socket is left in
blocking mode, undefined behavior might occur.]
To free a connection listener, pass it to evconnlistener_free().
Recognized flags
These are the flags you can pass to the 'flags' argument of the
evconnlistener_new() function. You can give any number of these, OR'd
By default, when the connection listener accepts a new incoming
socket, it sets it up to be nonblocking so that you can use it
with the rest of Libevent. Set this flag if you do not want this
If this option is set, the connection listener closes its
underlying socket when you free it.
If this option is set, the connection listener sets the
close-on-exec flag on the underlying listener socket. See your
platform documentation for fcntl and FD_CLOEXEC for more
By default on some platforms, once a listener socket is closed,
no other socket can bind to the same port until a while has
passed. Setting this option makes Libevent mark the socket as
reusable, so that once it is closed, another socket can be
opened to listen on the same port.
Allocate locks for the listener, so that it's safe to use it from
multiple threads. New in Libevent 2.0.8-rc.
Initialize the listener to be disabled, not enabled. You can
turn it on manually with evconnlistener_enable(). New in Libevent
If possible, tell the kernel to not announce sockets as having been
accepted until some data has been received on them, and they are ready
for reading. Do not use this option if your protocol
_doesn't_ start out with the client transmitting data, since in that case
this option will sometimes cause the kernel to never tell you about the
connection. Not all operating systems support this option: on ones that
don't, this option has no effect. New in Libevent 2.1.1-alpha.
The connection listener callback
typedef void (*evconnlistener_cb)(struct evconnlistener *listener,
evutil_socket_t sock, struct sockaddr *addr, int len, void *ptr);
When a new connection is received, the provided callback function is
invoked. The 'listener' argument is the connection listener that
received the connection. The 'sock' argument is the new socket
itself. The 'addr' and 'len' arguments are the address from which
the connection was received and the length of that address
respectively. The 'ptr' argument is the user-supplied pointer that
was passed to evconnlistener_new().
Enabling and disabling an evconnlistener
int evconnlistener_disable(struct evconnlistener *lev);
int evconnlistener_enable(struct evconnlistener *lev);
These functions temporarily disable or reenable listening for new connections.
Adjusting an evconnlistener's callback
void evconnlistener_set_cb(struct evconnlistener *lev,
evconnlistener_cb cb, void *arg);
This function adjusts the callback and callback argument of an existing
evconnlistener. It was introduced in 2.0.9-rc.
Inspecting an evconnlistener
evutil_socket_t evconnlistener_get_fd(struct evconnlistener *lev);
struct event_base *evconnlistener_get_base(struct evconnlistener *lev);
These functions return a listener's associated socket and event_base
The evconnlistener_get_fd() function first appeared in Libevent 2.0.3-alpha.
Detecting errors
You can set an error callback that gets informed whenever an accept() call
fails on the listener. This can be important to do if you're facing an error
condition that would lock the process unless you addressed it.
typedef void (*evconnlistener_errorcb)(struct evconnlistener *lis, void *ptr);
void evconnlistener_set_error_cb(struct evconnlistener *lev,
evconnlistener_errorcb errorcb);
If you use evconnlistener_set_error_cb() to set an error callback on a
listener, the callback will be invoked every time that an error occurs on the
listener. It will receive the listener as its first argument, and the
argument passed as 'ptr' to evconnlistener_new() as its second argument.
This function was introduced in Libevent 2.0.8-rc.
Example code: an echo server.