diff --git a/src/algo_naive.c b/src/algo_naive.c
index 9a05e73..b391634 100644
--- a/src/algo_naive.c
+++ b/src/algo_naive.c
@@ -9,10 +9,10 @@ struct naive_ctx {
   GHashTable* write_waiting; // Structure to track packets waiting to be written
 };
 
-void on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
-void on_tcp_write(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
-void on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
-void on_udp_write (struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
+int on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
+int on_tcp_write(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
+int on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
+int on_udp_write (struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo);
 
 void free_nothing(void* app_ctx) {}
 void free_naive(void* app_ctx) {
@@ -25,7 +25,7 @@ void free_naive(void* app_ctx) {
   free(ctx);
 }
 
-void on_tcp_co(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_tcp_co(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   int conn_sock1, conn_sock2;
   struct sockaddr_in addr;
   socklen_t in_len;
@@ -55,7 +55,7 @@ void on_tcp_co(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   sprintf(to_fdinfo.url, "tcp:write:127.0.0.1:%s", port);
   evt_core_add_fd (ctx, &to_fdinfo);
 
-  return;
+  return 1;
 
 co_error:
   perror("Failed to handle new connection");
@@ -148,7 +148,7 @@ void notify_read(struct evt_core_ctx* ctx, struct naive_ctx* app_ctx) {
   }
 }
 
-void on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   struct buffer_packet* bp;
   struct evt_core_fdinfo *to_fdinfo = NULL;
   struct naive_ctx* app_ctx = fdinfo->cat->app_ctx;
@@ -156,14 +156,14 @@ void on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   char url[255];
 
   // 1. Get current read buffer OR a new read buffer OR subscribe to be notified later
-  if ((bp = get_read_buffer(app_ctx, fdinfo)) == NULL) return;
+  if ((bp = get_read_buffer(app_ctx, fdinfo)) == NULL) return 1;
 
   // 2. Try to read a whole packet in the buffer
-  while (read_res != FDS_AGAIN && bp->mode == BP_READING) {
+  while (bp->mode == BP_READING) {
     read_res = read_packet_from_tcp (fdinfo->fd, bp);
     if (read_res == FDS_ERR) goto co_error;
+    if (read_res == FDS_AGAIN) return 1;
   }
-  if (bp->mode != BP_WRITING) return;
 
   // 3. A whole packet has been read, we will find someone to write it
   sprintf(url, "udp:write:127.0.0.1:%d", bp->ip.ap.str.port);
@@ -171,7 +171,7 @@ void on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   if (to_fdinfo == NULL) {
     fprintf(stderr, "No fd for URL %s in tcp-read. Dropping packet :( \n", url);
     mv_buffer_wtor (app_ctx, fdinfo, bp);
-    return;
+    return 1;
   }
   //printf("Pass packet from %s to %s\n", fdinfo->url, url);
 
@@ -179,39 +179,39 @@ void on_tcp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   mv_buffer_rtow (app_ctx, fdinfo, to_fdinfo, bp);
   on_udp_write(ctx, to_fdinfo);
 
-  return;
+  return 0;
 co_error:
   perror("Failed to TCP read");
   exit(EXIT_FAILURE);
 }
 
-void on_tcp_write(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_tcp_write(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   struct buffer_packet* bp;
   struct naive_ctx* app_ctx = fdinfo->cat->app_ctx;
   int write_res = FDS_READY;
 
   // 1. Get current write buffer OR a buffer from the waiting queue OR leave
-  if ((bp = get_write_buffer(app_ctx, fdinfo)) == NULL) return;
+  if ((bp = get_write_buffer(app_ctx, fdinfo)) == NULL) return 1;
 
   // 2. Write data from the buffer to the socket
-  while (write_res != FDS_AGAIN && bp->mode == BP_WRITING) {
+  while (bp->mode == BP_WRITING) {
     write_res = write_packet_to_tcp(fdinfo->fd, bp);
     if (write_res == FDS_ERR) goto co_error;
+    if (write_res == FDS_AGAIN) return 1;
   }
-  if (bp->mode != BP_READING) return;
 
   // 3. A whole packet has been written
   // Release the buffer and notify
   mv_buffer_wtor(app_ctx, fdinfo, bp);
   notify_read(ctx, app_ctx);
 
-  return;
+  return 0;
 co_error:
   perror("Failed to TCP write");
   exit(EXIT_FAILURE);
 }
 
-void on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   struct buffer_packet* bp;
   struct evt_core_fdinfo *to_fdinfo;
   struct naive_ctx* app_ctx = fdinfo->cat->app_ctx;
@@ -219,13 +219,13 @@ void on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   char url[255];
 
   // 1. Get current read buffer OR a new read buffer OR subscribe to be notified later
-  if ((bp = get_read_buffer(app_ctx, fdinfo)) == NULL) return;
+  if ((bp = get_read_buffer(app_ctx, fdinfo)) == NULL) return 1;
 
   // 2. Read packet from socket
   bp->ip.ap.str.port = url_get_port_int (fdinfo->url);
   read_res = read_packet_from_udp (fdinfo->fd, bp, fdinfo->other);
   if (read_res == FDS_ERR) goto co_error;
-  if (bp->mode != BP_WRITING) return;
+  if (read_res == FDS_AGAIN) return 1;
 
   // 3. A whole packet has been read, we will find someone to write it
   sprintf(url, "tcp:write:127.0.0.1:7500");
@@ -233,7 +233,7 @@ void on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   if (to_fdinfo == NULL) {
     fprintf(stderr, "No fd for URL %s in udp-read. Dropping packet :( \n", url);
     mv_buffer_wtor (app_ctx, fdinfo, bp);
-    return;
+    return 1;
   }
   //printf("Pass packet from %s to %s\n", fdinfo->url, url);
 
@@ -241,32 +241,32 @@ void on_udp_read(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   mv_buffer_rtow (app_ctx, fdinfo, to_fdinfo, bp);
   on_tcp_write(ctx, to_fdinfo);
 
-  return;
+  return 0;
 
 co_error:
   perror("Failed to UDP read");
   exit(EXIT_FAILURE);
 }
 
-void on_udp_write (struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_udp_write (struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   struct buffer_packet* bp;
   struct naive_ctx* app_ctx = fdinfo->cat->app_ctx;
   int write_res = FDS_READY;
 
   // 1. Get current write buffer OR a buffer from the waiting queue OR leave
-  if ((bp = get_write_buffer(app_ctx, fdinfo)) == NULL) return;
+  if ((bp = get_write_buffer(app_ctx, fdinfo)) == NULL) return 1;
 
   // 2. Write buffer
   write_res = write_packet_to_udp(fdinfo->fd, bp, fdinfo->other);
   if (write_res == FDS_ERR) goto co_error;
-  if (bp->mode != BP_READING) return;
+  if (write_res == FDS_AGAIN) return 1;
 
   // 3. A whole packet has been written
   // Release the buffer and notify
   mv_buffer_wtor(app_ctx, fdinfo, bp);
   notify_read(ctx, app_ctx);
 
-  return;
+  return 0;
 co_error:
   perror("Failed to UDP write");
   exit(EXIT_FAILURE);
@@ -277,7 +277,7 @@ void naive_free_simple(void* v) {
   free(v);
 }
 
-void on_err(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_err(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   struct naive_ctx* app_ctx = fdinfo->cat->app_ctx;
   struct buffer_packet* bp;
 
@@ -299,6 +299,8 @@ void on_err(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
 
   // 3. If appears in the read waiting queue, remove it
   g_queue_remove_all (app_ctx->read_waiting, &(fdinfo->fd));
+
+  return 1;
 }
 
 void algo_naive(struct algo_skel* as) {
diff --git a/src/evt_core.c b/src/evt_core.c
index 29c9324..1de3786 100644
--- a/src/evt_core.c
+++ b/src/evt_core.c
@@ -163,7 +163,9 @@ void evt_core_loop(struct evt_core_ctx* ctx) {
         fdinfo = evt_core_get_from_fd(ctx, err_fd);
         if (fdinfo != NULL) {
           fprintf(stderr, "Clearing fd=%d on cat=%s\n", err_fd, fdinfo->cat->name);
-          if (fdinfo->cat->err_cb != NULL) fdinfo->cat->err_cb(ctx, fdinfo);
+          if (fdinfo->cat->err_cb != NULL) {
+            while (fdinfo->cat->err_cb(ctx, fdinfo) == 0);
+          }
           evt_core_rm_fd (ctx, err_fd);
         } else {
           fprintf(stderr, "The file descriptor is not registered in a category, this is probably a logic error\n");
@@ -178,7 +180,7 @@ void evt_core_loop(struct evt_core_ctx* ctx) {
         fprintf(stderr, "Ignoring file descriptor %d as it is not registered. This is a bug.\n", events[n].data.fd);
         continue;
       }
-      fdinfo->cat->cb(ctx, fdinfo);
+      while(fdinfo->cat->cb(ctx, fdinfo) == 0);
 
     }
   }
diff --git a/src/evt_core.h b/src/evt_core.h
index cce94cd..aadadff 100644
--- a/src/evt_core.h
+++ b/src/evt_core.h
@@ -15,7 +15,7 @@ struct evt_core_cat;
 struct evt_core_fdinfo;
 
 typedef void (*evt_core_free_app_ctx)(void*);
-typedef void (*evt_core_cb)(struct evt_core_ctx*, struct evt_core_fdinfo*);
+typedef int (*evt_core_cb)(struct evt_core_ctx*, struct evt_core_fdinfo*);
 
 struct evt_core_cat {
   void* app_ctx;
diff --git a/src/meas_lat.c b/src/meas_lat.c
index 1465d36..fec0e1b 100644
--- a/src/meas_lat.c
+++ b/src/meas_lat.c
@@ -3,6 +3,7 @@
 #include <sys/timerfd.h>
 #include <time.h>
 #include <unistd.h>
+#include <errno.h>
 #include "evt_core.h"
 #include "net_tools.h"
 
@@ -44,13 +45,15 @@ struct measure_conf* create_measure_conf(int udp_sock, char* max_mes, char* plsi
   return mc;
 }
 
-void on_udp(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_udp(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   ssize_t res;
   int secs, nsecs;
   uint64_t micro_sec;
   struct timespec curr;
   struct measure_conf* mc = fdinfo->other;
   res = read(fdinfo->fd, mc->payload, mc->payload_size);
+  if (res == -1 && errno == EAGAIN) return 1;
+
   if (res != mc->payload_size) {
     perror("read error");
     exit(EXIT_FAILURE);
@@ -70,9 +73,10 @@ void on_udp(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
     printf("Measurement done\n");
     exit(EXIT_SUCCESS);
   }
+  return 0;
 }
 
-void on_timer(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
+int on_timer(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
   ssize_t s;
   uint64_t ticks;
   struct measure_conf* mc = fdinfo->other;
@@ -100,6 +104,7 @@ void on_timer(struct evt_core_ctx* ctx, struct evt_core_fdinfo* fdinfo) {
     perror("Send error");
     exit(EXIT_FAILURE);
   }
+  return 1;
 }
 
 void free_timer_conf(void* v) {
diff --git a/udp-echo.py b/udp-echo.py
index dab96d3..08aa9d9 100755
--- a/udp-echo.py
+++ b/udp-echo.py
@@ -25,6 +25,7 @@ def receive_next(sock):
 def receive_and_send_one(sock):
     "Waits for a single datagram over the socket and echoes it back."
     input_data, addr = receive_next(sock)
+    print(input_data)
     #message = input_data.decode()
     #logger.info("Received message from %s: %s (%s bytes).", addr, message, len(input_data))
     output_len = sock.sendto(input_data, addr)