#include <curl/curl.h> CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_SOCKETDATA, void *pointer);
This pointer will not be touched by libcurl but will only be passed in to the socket callbacks's userp argument.
static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp) { GlobalInfo *g = (GlobalInfo*) cbp; SockInfo *fdp = (SockInfo*) sockp; if(what == CURL_POLL_REMOVE) { remsock(fdp); } else { if(!fdp) { addsock(s, e, what, g); } else { setsock(fdp, s, e, what, g); } } return 0; } main() { GlobalInfo setup; /* ... use socket callback and custom pointer */ curl_multi_setopt(multi, CURLMOPT_SOCKETFUNCTION, sock_cb); curl_multi_setopt(multi, CURLMOPT_SOCKETDATA, &setup); }