Tribblix: manual page: CURLMOPT

CURLMOPT_SOCKETFUNCTION(3) Introduction to Library Functions

NAME

CURLMOPT_SOCKETFUNCTION - callback informed about what to wait for

SYNOPSIS

#include <curl/curl.h>

int socket_callback(CURL *easy,
curl_socket_t socket,
int what,
void *clientp,
void *socketp);

CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_SOCKETFUNCTION,
socket_callback);

DESCRIPTION

Pass a pointer to your callback function, which should match the
prototype shown above.

When the curl_multi_socket_action(3) function is called, it uses this
callback to inform the application about updates in the socket (file
descriptor) status by doing none, one, or multiple calls to the
socket_callback. The callback function gets status updates with
changes since the previous time the callback was called. If the given
callback pointer is set to NULL, no callback is called.

libcurl then expects the application to monitor the sockets for the
specific activities and tell libcurl again when something happens on
one of them. Tell libcurl by calling curl_multi_socket_action(3).

This callback may get invoked at any time when interacting with
libcurl. This may even happen after all transfers are done and is
likely to happen during a call to curl_multi_cleanup(3) when cached
connections are shut down.

libcurl may use a number of internal file descriptors for name
resolving, Happy Eyeballs racing, internal communication and more, in
addition to the main sockets used for network transfers. All of those
file descriptors might get passed to this callback as "sockets".

CALLBACK ARGUMENTS

easy identifies the specific transfer for which this update is
related. Since this callback manages a whole multi handle, an
application should not make assumptions about which particular handle
that is passed here. It might even be an internal easy handle that
the application did not add itself.

socket is the specific socket this function invocation concerns. If
the what argument is not CURL_POLL_REMOVE then it holds information
about what activity on this socket the application is supposed to
monitor. Subsequent calls to this callback might update the what bits
for a socket that is already monitored.

The socket callback should return 0 on success, and -1 on error. If
this callback returns error, all transfers currently in progress in
this multi handle are aborted and made to fail.

clientp is set with CURLMOPT_SOCKETDATA(3).

socketp is set with curl_multi_assign(3) or NULL.

The what parameter informs the callback on the status of the given
socket. It can hold one of these values:

CURL_POLL_IN
Wait for incoming data. For the socket to become readable.

CURL_POLL_OUT
Wait for outgoing data. For the socket to become writable.

CURL_POLL_INOUT
Wait for incoming and outgoing data. For the socket to become
readable or writable.

CURL_POLL_REMOVE
The specified socket/file descriptor is no longer used by
libcurl for any active transfer. It might soon be added again.

When a socket is given a CURL_POLL_REMOVE value, it might be
because libcurl is going to close it, but it might also mean
that it does not need any more monitoring for the moment. An
application cannot assume either. The same socket might appear
soon in a call asking for monitoring again.

A socket that is removed like this loses its assigned pointer
as set with curl_multi_assign(3).

DEFAULT

NULL (no callback)

PROTOCOLS

This functionality affects all supported protocols

EXAMPLE

struct priv {
void *ours;
};

static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
{
struct priv *p = cbp;
printf("our ptr: %p\n", p->ours);

if(what == CURL_POLL_REMOVE) {
/* remove the socket from our collection */
}
if(what & CURL_POLL_IN) {
/* wait for read on this socket */
}
if(what & CURL_POLL_OUT) {
/* wait for write on this socket */
}

return 0;
}

int main(void)
{
struct priv setup;
CURLM *multi = curl_multi_init();
/* ... use socket callback and custom pointer */
curl_multi_setopt(multi, CURLMOPT_SOCKETFUNCTION, sock_cb);
curl_multi_setopt(multi, CURLMOPT_SOCKETDATA, &setup);
}

AVAILABILITY

Added in curl 7.15.4

RETURN VALUE

Returns CURLM_OK.