diff --git a/src/execute_packet_callback.c b/src/execute_packet_callback.c
index 89131f5..0019b6a 100644
--- a/src/execute_packet_callback.c
+++ b/src/execute_packet_callback.c
@@ -36,7 +36,7 @@ static struct PacketCallbackQueueNode* const wait_for_next_packet_callback(struc
     return node_to_process;
 }
 
-void execute_packet_callback(struct PacketCallbackQueue* const queue, void (* const _Atomic * const packet_handler) (void* const), const enum ConnectionType connection_type, struct SwiftNetMemoryAllocator* const pending_message_memory_allocator, _Atomic bool* closing, void* const connection, struct SwiftNetVector* const pending_messages) {
+void execute_packet_callback(struct PacketCallbackQueue* const queue, void (* const _Atomic * const packet_handler) (void* const, void* const), const enum ConnectionType connection_type, struct SwiftNetMemoryAllocator* const pending_message_memory_allocator, _Atomic bool* closing, void* const connection, struct SwiftNetVector* const pending_messages, _Atomic(void*)* user_data) {
     while (1) {
         if (atomic_load_explicit(closing, memory_order_acquire) == true) {
             break;
@@ -67,7 +67,7 @@ void execute_packet_callback(struct PacketCallbackQueue* const queue, void (* co
             vector_unlock(pending_messages);
         }
 
-        void (*const packet_handler_loaded)(void*) = atomic_load(packet_handler);
+        void (*const packet_handler_loaded)(void* const, void* const) = atomic_load(packet_handler);
         if (unlikely(packet_handler_loaded == NULL)) {
             if (connection_type == CONNECTION_TYPE_CLIENT) {
                 swiftnet_client_destroy_packet_data(node->packet_data, connection);
@@ -80,7 +80,7 @@ void execute_packet_callback(struct PacketCallbackQueue* const queue, void (* co
             continue;
         }
 
-        (*packet_handler_loaded)(node->packet_data);
+        (*packet_handler_loaded)(node->packet_data, atomic_load_explicit(user_data, memory_order_acquire));
 
         allocator_free(&packet_callback_queue_node_memory_allocator, (void*)node);
     }
@@ -89,7 +89,7 @@ void execute_packet_callback(struct PacketCallbackQueue* const queue, void (* co
 void* execute_packet_callback_client(void* const void_client) {
     struct SwiftNetClientConnection* const client = void_client;
 
-    execute_packet_callback(&client->packet_callback_queue, (void*)&client->packet_handler, CONNECTION_TYPE_CLIENT, &client->pending_messages_memory_allocator, &client->closing, void_client, &client->pending_messages);
+    execute_packet_callback(&client->packet_callback_queue, (void*)&client->packet_handler, CONNECTION_TYPE_CLIENT, &client->pending_messages_memory_allocator, &client->closing, void_client, &client->pending_messages, &client->packet_handler_user_arg);
 
     return NULL;
 }
@@ -97,7 +97,7 @@ void* execute_packet_callback_client(void* const void_client) {
 void* execute_packet_callback_server(void* const void_server) {
     struct SwiftNetServer* const server = void_server;
 
-    execute_packet_callback(&server->packet_callback_queue, (void*)&server->packet_handler, CONNECTION_TYPE_SERVER, &server->pending_messages_memory_allocator, &server->closing, void_server, &server->pending_messages);
+    execute_packet_callback(&server->packet_callback_queue, (void*)&server->packet_handler, CONNECTION_TYPE_SERVER, &server->pending_messages_memory_allocator, &server->closing, void_server, &server->pending_messages, &server->packet_handler_user_arg);
 
     return NULL;
 }
diff --git a/src/generic_functions.c b/src/generic_functions.c
index 478fe70..0940204 100644
--- a/src/generic_functions.c
+++ b/src/generic_functions.c
@@ -1,5 +1,6 @@
 #include "internal/internal.h"
 #include "swift_net.h"
+#include <stdatomic.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -15,16 +16,18 @@ static inline void swiftnet_validate_new_handler(const void* const new_handler,
     #endif
 }
 
-void swiftnet_client_set_message_handler(struct SwiftNetClientConnection* const client, void (* const new_handler)(struct SwiftNetClientPacketData* const)) {
+void swiftnet_client_set_message_handler(struct SwiftNetClientConnection* const client, void (* const new_handler)(struct SwiftNetClientPacketData* const, void* const), void* const user_arg) {
     swiftnet_validate_new_handler(new_handler, __func__);
 
-    atomic_store(&client->packet_handler, new_handler);
+    atomic_store_explicit(&client->packet_handler, new_handler, memory_order_release);
+    atomic_store_explicit(&client->packet_handler_user_arg, user_arg, memory_order_release);
 }
 
-void swiftnet_server_set_message_handler(struct SwiftNetServer* const server, void (* const new_handler)(struct SwiftNetServerPacketData* const)) {
+void swiftnet_server_set_message_handler(struct SwiftNetServer* const server, void (* const new_handler)(struct SwiftNetServerPacketData* const, void* const), void* const user_arg) {
     swiftnet_validate_new_handler(new_handler, __func__);
 
-    atomic_store(&server->packet_handler, new_handler);
+    atomic_store_explicit(&server->packet_handler, new_handler, memory_order_release);
+    atomic_store_explicit(&server->packet_handler_user_arg, user_arg, memory_order_release);
 }
 
 // Read packet data into buffers
diff --git a/src/initialize_client_socket.c b/src/initialize_client_socket.c
index f054d3a..6422f37 100644
--- a/src/initialize_client_socket.c
+++ b/src/initialize_client_socket.c
@@ -106,6 +106,8 @@ struct SwiftNetClientConnection* swiftnet_create_client(const char* const ip_add
 
     new_connection->server_addr.s_addr = inet_addr(ip_address);
 
+    atomic_store_explicit(&new_connection->packet_handler_user_arg, NULL, memory_order_release);
+
     // Request the server information, and proccess it
     const struct SwiftNetPacketInfo request_server_information_packet_info = construct_packet_info(
         0x00,
diff --git a/src/initialize_server_socket.c b/src/initialize_server_socket.c
index b432126..52e8847 100644
--- a/src/initialize_server_socket.c
+++ b/src/initialize_server_socket.c
@@ -54,9 +54,10 @@ struct SwiftNetServer* swiftnet_create_server(const uint16_t port, const bool lo
     atomic_store(&new_server->packet_queue.owner, PACKET_QUEUE_OWNER_NONE);
 
     memset(&new_server->packet_callback_queue, 0x00, sizeof(struct PacketCallbackQueue));
-    atomic_store(&new_server->packet_callback_queue.owner, PACKET_CALLBACK_QUEUE_OWNER_NONE);
+    atomic_store_explicit(&new_server->packet_callback_queue.owner, PACKET_CALLBACK_QUEUE_OWNER_NONE, memory_order_release);
 
-    atomic_store(&new_server->packet_handler, NULL);
+    atomic_store_explicit(&new_server->packet_handler, NULL, memory_order_release);
+    atomic_store_explicit(&new_server->packet_handler_user_arg, NULL, memory_order_release);
 
     new_server->pending_messages_memory_allocator = allocator_create(sizeof(struct SwiftNetPendingMessage), 100);
     new_server->pending_messages = vector_create(100);
diff --git a/src/swift_net.h b/src/swift_net.h
index 402d158..3fb381b 100644
--- a/src/swift_net.h
+++ b/src/swift_net.h
@@ -217,7 +217,8 @@ struct SwiftNetClientConnection {
     struct SwiftNetPortInfo port_info;
     struct in_addr server_addr;
     socklen_t server_addr_len;
-    _Atomic(void (*)(struct SwiftNetClientPacketData* const)) packet_handler;
+    _Atomic(void (*)(struct SwiftNetClientPacketData* const, void* const user)) packet_handler;
+    _Atomic(void*) packet_handler_user_arg;
     _Atomic bool closing;
     _Atomic bool initialized;
     uint16_t addr_type;
@@ -240,7 +241,8 @@ struct SwiftNetServer {
     pcap_t* pcap;
     struct ether_header eth_header;
     uint16_t server_port;
-    _Atomic(void (*)(struct SwiftNetServerPacketData* const)) packet_handler;
+    _Atomic(void (*)(struct SwiftNetServerPacketData* const, void* const user)) packet_handler;
+    _Atomic(void*) packet_handler_user_arg;
     _Atomic bool closing;
     uint16_t addr_type;
     bool loopback;
@@ -265,7 +267,8 @@ struct SwiftNetServer {
  */
 extern void swiftnet_server_set_message_handler(
     struct SwiftNetServer* const server,
-    void (* const new_handler)(struct SwiftNetServerPacketData* const)
+    void (* const new_handler)(struct SwiftNetServerPacketData* const, void* const),
+    void* const user_arg
 );
 
 /**
@@ -275,7 +278,8 @@ extern void swiftnet_server_set_message_handler(
  */
 extern void swiftnet_client_set_message_handler(
     struct SwiftNetClientConnection* const client,
-    void (* const new_handler)(struct SwiftNetClientPacketData* const)
+    void (* const new_handler)(struct SwiftNetClientPacketData* const, void* const),
+    void* const user_arg
 );
 
 /**
diff --git a/tests/src/making_request.c b/tests/src/making_request.c
index 0d6f5a7..0ac1fb8 100644
--- a/tests/src/making_request.c
+++ b/tests/src/making_request.c
@@ -53,7 +53,7 @@ static void reset_test_state() {
     atomic_store_explicit(&g_test_result, INT_MAX, memory_order_release);
 }
 
-static void on_client_packet(struct SwiftNetClientPacketData* packet) {
+static void on_client_packet(struct SwiftNetClientPacketData* packet, void* const user) {
     struct SwiftNetClientConnection* const client_conn = atomic_load_explicit(&g_client_conn, memory_order_acquire);
 
     if (packet->metadata.data_length != atomic_load_explicit(&g_request_data_len, memory_order_acquire)) {
@@ -99,7 +99,7 @@ static void on_client_packet(struct SwiftNetClientPacketData* packet) {
     atomic_store_explicit(&g_sent_response, true, memory_order_release);
 }
 
-static void on_server_packet(struct SwiftNetServerPacketData* packet) {
+static void on_server_packet(struct SwiftNetServerPacketData* packet, void* const user) {
     struct SwiftNetServer* const server = atomic_load_explicit(&g_server, memory_order_acquire);
 
     if (packet->metadata.expecting_response) {
@@ -241,7 +241,7 @@ int test_making_request(const union Args* args_ptr) {
         return -1;
     }
 
-    swiftnet_server_set_message_handler(server, on_server_packet);
+    swiftnet_server_set_message_handler(server, on_server_packet, NULL);
 
     struct SwiftNetClientConnection* const client_conn = swiftnet_create_client(args.ip_address, 8080, 1000);
     if (client_conn == NULL) {
@@ -249,7 +249,7 @@ int test_making_request(const union Args* args_ptr) {
         return -1;
     }
 
-    swiftnet_client_set_message_handler(client_conn, on_client_packet);
+    swiftnet_client_set_message_handler(client_conn, on_client_packet, NULL);
 
     atomic_store_explicit(&g_client_conn, client_conn, memory_order_release);
     atomic_store_explicit(&g_server, server, memory_order_release);
diff --git a/tests/src/sending_packet.c b/tests/src/sending_packet.c
index ccfaf54..a277282 100644
--- a/tests/src/sending_packet.c
+++ b/tests/src/sending_packet.c
@@ -46,7 +46,7 @@ static void reset_test_state() {
     atomic_store_explicit(&g_test_result, INT_MAX, memory_order_release);
 }
 
-static void on_client_packet(struct SwiftNetClientPacketData* packet) {
+static void on_client_packet(struct SwiftNetClientPacketData* packet, void* const user) {
     struct SwiftNetClientConnection* const client_conn = atomic_load_explicit(&g_client_conn, memory_order_acquire);
 
     while (!atomic_load_explicit(&g_client_send_done, memory_order_acquire)) usleep(1000);
@@ -84,7 +84,7 @@ static void on_client_packet(struct SwiftNetClientPacketData* packet) {
     swiftnet_client_destroy_packet_data(packet, client_conn);
 }
 
-static void on_server_packet(struct SwiftNetServerPacketData* packet) {
+static void on_server_packet(struct SwiftNetServerPacketData* packet, void* const user) {
     struct SwiftNetServer* const server = atomic_load_explicit(&g_server, memory_order_acquire);
 
     while (!atomic_load_explicit(&g_server_send_done, memory_order_acquire)) usleep(1000);
@@ -143,7 +143,7 @@ int test_sending_packet(const union Args* args_ptr) {
         return -1;
     }
 
-    swiftnet_server_set_message_handler(server, on_server_packet);
+    swiftnet_server_set_message_handler(server, on_server_packet, NULL);
 
     struct SwiftNetClientConnection* const client_conn = swiftnet_create_client(args.ip_address, 8080, 1000);
     if (client_conn == NULL) {
@@ -151,7 +151,7 @@ int test_sending_packet(const union Args* args_ptr) {
         return -1;
     }
 
-    swiftnet_client_set_message_handler(client_conn, on_client_packet);
+    swiftnet_client_set_message_handler(client_conn, on_client_packet, NULL);
 
     atomic_store_explicit(&g_client_conn, client_conn, memory_order_release);
     atomic_store_explicit(&g_server, server, memory_order_release);