An example on how to use libuv with libcurl's multi interface Should be equally fast on Unixes (uses epoll/kqueue/etc like libev/libevent) but MUCH faster on Windows due to libuv's usage of IO completion ports. Could come in handy if you have to manage several hundreds or thousands of connections!

curl_libuv_example.c

#include <stdio.h>
#include <stdlib.h>
#include <uv.h>
#include <curl/curl.h>

uv_loop_t *loop;
CURLM *curl_handle;
uv_timer_t timeout;

typedef struct curl_context_s {
  uv_poll_t poll_handle;
  curl_socket_t sockfd;
} curl_context_t;

curl_context_t* create_curl_context(curl_socket_t sockfd) {
  int r;
  curl_context_t* context;

  context = (curl_context_t*) malloc(sizeof *context);

  context->sockfd = sockfd;

  r = uv_poll_init_socket(loop, &context->poll_handle, sockfd);
  context->poll_handle.data = context;

  return context;
}

void curl_close_cb(uv_handle_t* handle) {
  curl_context_t* context = (curl_context_t*) handle->data;
  free(context);
}

void destroy_curl_context(curl_context_t* context) {
  uv_close((uv_handle_t*) &context->poll_handle, curl_close_cb);
}


void add_download(const char *url, int num) {
    char filename[50];
    sprintf(filename, "%d.download", num);
    FILE *file;

    file = fopen(filename, "w");
    if (file == NULL) {
        fprintf(stderr, "Error opening %s\n", filename);
        return;
    }

    CURL *handle = curl_easy_init();
    curl_easy_setopt(handle, CURLOPT_WRITEDATA, file);
    curl_easy_setopt(handle, CURLOPT_URL, url);
    curl_multi_add_handle(curl_handle, handle);
    fprintf(stderr, "Added download %s -> %s\n", url, filename);
}

void curl_perform(uv_poll_t *req, int status, int events) {
    uv_timer_stop(&timeout);
    int running_handles;
    int flags = 0;
    if (events & UV_READABLE) flags |= CURL_CSELECT_IN;
    if (events & UV_WRITABLE) flags |= CURL_CSELECT_OUT;

    curl_context_t *context;

    context = (curl_context_t*)req;

    curl_multi_socket_action(curl_handle, context->sockfd, flags, &running_handles);

    char *done_url;

    CURLMsg *message;
    int pending;
    while ((message = curl_multi_info_read(curl_handle, &pending))) {
        switch (message->msg) {
        case CURLMSG_DONE:
            curl_easy_getinfo(message->easy_handle, CURLINFO_EFFECTIVE_URL, &done_url);
            printf("%s DONE\n", done_url);

            curl_multi_remove_handle(curl_handle, message->easy_handle);
            curl_easy_cleanup(message->easy_handle);

            break;
        default:
            fprintf(stderr, "CURLMSG default\n");
            abort();
        }
    }
}

void on_timeout(uv_timer_t *req, int status) {
    int running_handles;
    curl_multi_socket_action(curl_handle, CURL_SOCKET_TIMEOUT, 0, &running_handles);
}

void start_timeout(CURLM *multi, long timeout_ms, void *userp) {
    if (timeout_ms <= 0)
        timeout_ms = 1; /* 0 means directly call socket_action, but we'll do it in a bit */
    uv_timer_start(&timeout, on_timeout, timeout_ms, 0);
}

int handle_socket(CURL *easy, curl_socket_t s, int action, void *userp, void *socketp) {
    curl_context_t *curl_context;
    if (action == CURL_POLL_IN || action == CURL_POLL_OUT) {
        if (socketp) {
            curl_context = (curl_context_t*) socketp;
        }
        else {
            curl_context = create_curl_context(s);
        }
        curl_multi_assign(curl_handle, s, (void *) curl_context);
    }

    switch (action) {
        case CURL_POLL_IN:
            uv_poll_start(&curl_context->poll_handle, UV_READABLE, curl_perform);
            break;
        case CURL_POLL_OUT:
            uv_poll_start(&curl_context->poll_handle, UV_WRITABLE, curl_perform);
            break;
        case CURL_POLL_REMOVE:
            if (socketp) {
                uv_poll_stop(&((curl_context_t*)socketp)->poll_handle);
                destroy_curl_context((curl_context_t*) socketp);                
                curl_multi_assign(curl_handle, s, NULL);
            }
            break;
        default:
            abort();
    }

    return 0;
}

int main(int argc, char **argv) {
    loop = uv_default_loop();

    if (argc <= 1)
        return 0;

    if (curl_global_init(CURL_GLOBAL_ALL)) {
        fprintf(stderr, "Could not init cURL\n");
        return 1;
    }

    uv_timer_init(loop, &timeout);

    curl_handle = curl_multi_init();
    curl_multi_setopt(curl_handle, CURLMOPT_SOCKETFUNCTION, handle_socket);
    curl_multi_setopt(curl_handle, CURLMOPT_TIMERFUNCTION, start_timeout);

    while (argc-- > 1) {
        add_download(argv[argc], argc);
    }

    uv_run(loop, UV_RUN_DEFAULT);
    curl_multi_cleanup(curl_handle);
    return 0;
}

at https://gist.github.com/clemensg/5248927#file-curl_libuv_example-c-L40

If I use POSTFIELDS and a buffer allocated the stack(like in line 40), It doesn't send anything it all (since the pointer points to zero data).

How do I associate some data with a CURL handle and free it up later?