add socket state callback example

Author: bradh352

docs.md

@@ -5,7 +5,7 @@ layout: page
 ---
 
 - [Manpages](#manpages)
-- [Example](#example)
+- [Examples](#examples)
 
 c-ares provides a set of library functions, datatypes, and enumerations
 which integrators will use for their implementations.  When you install c-ares,
@@ -19,10 +19,11 @@ you get man pages which describe their use and meanings.
 - [{{ my_page.path | replace: "docs/", "" | replace: ".html", "" }}](/{{ my_page.path }})
 {% endfor -%}
 
-## Example
-A simple name resolve might look like:
+## Examples
 
-`example.c`:
+### Event Thread example (recommended)
+
+`example_ev.c`:
 {% highlight C %}
 #include <stdio.h>
 #include <string.h>
@@ -110,8 +111,190 @@ int main(int argc, char **argv)
 
 Compilation:
 {% highlight bash %}
-cc -I/usr/local/include -o example example.c -Wl,-rpath /usr/local/lib -lcares
+cc -I/usr/local/include -o example_ev example_ev.c -Wl,-rpath /usr/local/lib -lcares
 {% endhighlight %}
 
+### Sock State Callback example
+
+`example_ss.c`:
+{% highlight C %}
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <poll.h>
+#include <ares.h>
+
+typedef struct {
+  ares_channel_t *channel;
+  struct pollfd  *fds;
+  size_t          nfds;
+} dnsstate_t;
+
+void sock_state_cb(void *data, ares_socket_t socket_fd, int readable, int writable)
+{
+  dnsstate_t *state = data;
+  size_t      idx;
+
+  /* Find match */
+  for (idx=0; idx<state->nfds; idx++) {
+    if (state->fds[idx].fd == socket_fd) {
+      break;
+    }
+  }
+
+  /* Not found */
+  if (idx >= state->nfds) {
+    /* Do nothing */
+    if (!readable && !writable) {
+      return;
+    }
+
+    /* Add */
+    state->nfds++;
+    state->fds = realloc(state->fds, sizeof(*state->fds) * state->nfds);
+  } else {
+    /* Remove */
+    if (!readable && !writable) {
+      memmove(&state->fds[idx], &state->fds[idx+1],
+        sizeof(*state->fds) * (state->nfds - idx - 1));
+      state->nfds--;
+      return;
+    }
+  }
+
+  state->fds[idx].fd     = socket_fd;
+  state->fds[idx].events = 0;
+  if (readable) {
+    state->fds[idx].events |= POLL_IN;
+  }
+  if (writable) {
+    state->fds[idx].events |= POLLOUT;
+  }
+}
+
+void process(dnsstate_t *state)
+{
+  struct timeval tv;
+
+  while (1) {
+    int    rv;
+    int    timeout;
+    size_t i;
+
+    /* Since we don't have any other program state to wait on, we'll just
+     * stop looping when we know there are no remaining queries, which is
+     * easily indicated by ares_timeout() returning NULL when maxtv is NULL */
+    if (ares_timeout(state->channel, NULL, &tv) == NULL) {
+      break;
+    }
+
+    timeout = tv.tv_sec * 1000 + tv.tv_usec / 1000;
+
+    rv = poll(state->fds, state->nfds, timeout);
+    if (rv < 0) {
+      continue;
+    } else if (rv == 0) {
+      /* Process timeouts */
+      ares_process_fd(state->channel, ARES_SOCKET_BAD, ARES_SOCKET_BAD);
+      continue;
+    }
+
+    for (i=0; i<state->nfds; i++) {
+      if (state->fds[i].revents == 0) {
+        continue;
+      }
+
+      /* Notify about read/write events per FD */
+      ares_process_fd(state->channel,
+        (state->fds[i].revents & (POLLERR|POLLHUP|POLLIN))?
+          state->fds[i].fd:ARES_SOCKET_BAD,
+        (state->fds[i].revents & POLLOUT)?
+          state->fds[i].fd:ARES_SOCKET_BAD);
+    }
+  }
+}
+
+/* Callback that is called when DNS query is finished */
+static void addrinfo_cb(void *arg, int status, int timeouts,
+                        struct ares_addrinfo *result)
+{
+  (void)arg; /* Example does not use user context */
+  printf("Result: %s, timeouts: %d\n", ares_strerror(status), timeouts);
+
+  if (result) {
+    struct ares_addrinfo_node *node;
+    for (node = result->nodes; node != NULL; node = node->ai_next) {
+      char        addr_buf[64] = "";
+      const void *ptr          = NULL;
+      if (node->ai_family == AF_INET) {
+        const struct sockaddr_in *in_addr =
+          (const struct sockaddr_in *)((void *)node->ai_addr);
+        ptr = &in_addr->sin_addr;
+      } else if (node->ai_family == AF_INET6) {
+        const struct sockaddr_in6 *in_addr =
+          (const struct sockaddr_in6 *)((void *)node->ai_addr);
+        ptr = &in_addr->sin6_addr;
+      } else {
+        continue;
+      }
+      ares_inet_ntop(node->ai_family, ptr, addr_buf, sizeof(addr_buf));
+      printf("Addr: %s\n", addr_buf);
+    }
+  }
+  ares_freeaddrinfo(result);
+}
+
+int main(int argc, char **argv)
+{
+  dnsstate_t                 state;
+  struct ares_options        options;
+  int                        optmask = 0;
+  struct ares_addrinfo_hints hints;
+
+  memset(&state, 0, sizeof(state));
 
+  if (argc != 2) {
+    printf("Usage: %s domain\n", argv[0]);
+    return 1;
+  }
+
+  /* Initialize library */
+  ares_library_init(ARES_LIB_INIT_ALL);
 
+  /* Enable sock state callbacks, we should not use ares_fds() or ares_getsock()
+   * in modern implementations. */
+  memset(&options, 0, sizeof(options));
+  optmask                   |= ARES_OPT_SOCK_STATE_CB;
+  options.sock_state_cb      = sock_state_cb;
+  options.sock_state_cb_data = &state;
+
+  /* Initialize channel to run queries, a single channel can accept unlimited
+   * queries */
+  if (ares_init_options(&state.channel, &options, optmask) != ARES_SUCCESS) {
+    printf("c-ares initialization issue\n");
+    return 1;
+  }
+
+  /* Perform an IPv4 and IPv6 request for the provided domain name */
+  memset(&hints, 0, sizeof(hints));
+  hints.ai_family = AF_UNSPEC;
+  hints.ai_flags  = ARES_AI_CANONNAME;
+  ares_getaddrinfo(state.channel, argv[1], NULL, &hints, addrinfo_cb,
+                   NULL /* user context not specified */);
+
+  /* Wait until no more requests are left to be processed */
+  process(&state);
+
+  /* Cleanup */
+  ares_destroy(state.channel);
+  free(state.fds);
+
+  ares_library_cleanup();
+  return 0;
+}
+{% endhighlight %}
+
+Compilation:
+{% highlight bash %}
+cc -I/usr/local/include -o example_ss example_ss.c -Wl,-rpath /usr/local/lib -lcares
+{% endhighlight %}