From f72fba0af15802be8a4da4168189e35ada8abc59 Mon Sep 17 00:00:00 2001
From: Dov Murik <dov.murik@gmail.com>
Date: Mon, 26 Oct 2015 16:12:42 -0400
Subject: [PATCH] udp-echo-server: send the echo response when socket is
 writable

Instead of calling sendto() directly from the udp_cb callback (which is
called when a new packet is received), prepare a response and tell libev
to call our udp_response_cb callback when the socket is writable.

This also removes all the global variables.
---
 src/udp-echo-server.c | 52 ++++++++++++++++++++++++++++++++++---------
 1 file changed, 42 insertions(+), 10 deletions(-)

diff --git a/src/udp-echo-server.c b/src/udp-echo-server.c
index 02f1e13..0f69a63 100644
--- a/src/udp-echo-server.c
+++ b/src/udp-echo-server.c
@@ -13,29 +13,61 @@
 #define DEFAULT_PORT    3333
 #define BUF_SIZE        4096
 
-// Lots of globals, what's the best way to get rid of these?
-int sd; // socket descriptor
-struct sockaddr_in addr;
-int addr_len = sizeof(addr);
-char buffer[BUF_SIZE];
+typedef struct {
+    ev_io watcher;
+    struct sockaddr addr;
+    socklen_t addr_len;
+    char* buffer;
+    size_t buffer_len;
+} response_details;
+
+// This callback is called when data is writable on the UDP socket, with
+// w->data pointing to a response_details instance.
+static void udp_response_cb(EV_P_ ev_io *w, int revents) {
+    response_details *r = w->data;
+
+    // Echo the buffer back
+    sendto(w->fd, r->buffer, r->buffer_len, 0, &r->addr, r->addr_len);
+
+    // Tell libev not to call this watcher again, because we're freeing it from memory
+    ev_io_stop(EV_A_ w);
+
+    free(r->buffer);
+    free(r);
+}
 
 // This callback is called when data is readable on the UDP socket.
 static void udp_cb(EV_P_ ev_io *w, int revents) {
+    struct sockaddr_in addr;
+    int addr_len = sizeof(addr);
+    char buffer[BUF_SIZE];
+    response_details *r;
+
     puts("udp socket has become readable");
-    socklen_t bytes = recvfrom(sd, buffer, sizeof(buffer) - 1, 0, (struct sockaddr*) &addr, (socklen_t *) &addr_len);
+    socklen_t bytes = recvfrom(w->fd, buffer, sizeof(buffer) - 1, 0, (struct sockaddr*) &addr, (socklen_t *) &addr_len);
 
     // add a null to terminate the input, as we're going to use it as a string
     buffer[bytes] = '\0';
 
     printf("udp client said: %s", buffer);
 
-    // Echo it back.
-    // WARNING: this is probably not the right way to do it with libev.
-    // Question: should we be setting a callback on sd becomming writable here instead?
-    sendto(sd, buffer, bytes, 0, (struct sockaddr*) &addr, sizeof(addr));
+    // Prepare a response_details struct to be passed as w->data to the
+    // udp_response_cb callback (that will be called when the socket is writable).
+    r = calloc(1, sizeof(response_details));
+    memcpy(&r->addr, &addr, sizeof(addr));
+    r->addr_len = addr_len;
+    r->buffer = calloc(bytes, sizeof(char));
+    memcpy(r->buffer, buffer, bytes);
+    r->buffer_len = bytes;
+
+    ev_io_init(&r->watcher, udp_response_cb, w->fd, EV_WRITE);
+    r->watcher.data = r;
+    ev_io_start(EV_A_ &r->watcher);
 }
 
 int main(void) {
+    struct sockaddr_in addr;
+    int sd; // socket descriptor
     int port = DEFAULT_PORT;
     puts("udp_echo server started...");