diff --git a/clients/c/Makefile b/clients/c/Makefile
index 3d8522f..1d11246 100644
--- a/clients/c/Makefile
+++ b/clients/c/Makefile
@@ -1,13 +1,25 @@
 CC ?= gcc
-CFLAGS ?= -Wall -Werror -I/usr/local/include -fpic
-LDFLAGS ?= -Wall -L/usr/local/lib -L.
+CFLAGS ?= -Wall -Werror -I/usr/local/include -fpic -O
+LDFLAGS ?= -Wall -L/usr/local/lib -L. -O
 LIBS ?= -lcurl -ljansson
 
+all: libcosmopolite.so
+
 test: test.o libcosmopolite.so
 	$(CC) $(LDFLAGS) -o test test.o -lcosmopolite $(LIBS)
+	./test
 
 libcosmopolite.so: cosmopolite.o
 	$(CC) -shared $(LDFLAGS) -o libcosmopolite.so cosmopolite.o $(LIBS)
 
-%.o: %.c
+%.o: %.c *.h
 	$(CC) -c $(CFLAGS) $< -o $@
+
+install: libcosmopolite.so cosmopolite.h
+	cp libcosmopolite.so /usr/local/lib
+	cp cosmopolite.h /usr/local/include
+	chown root /usr/local/lib/libcosmopolite.so /usr/local/include/cosmopolite.h
+	chmod 0644 /usr/local/lib/libcosmopolite.so /usr/local/include/cosmopolite.h
+
+clean:
+	rm -f test libcosmopolite.so *.o
diff --git a/clients/c/cosmopolite.c b/clients/c/cosmopolite.c
index 5b9850d..b9739a0 100644
--- a/clients/c/cosmopolite.c
+++ b/clients/c/cosmopolite.c
@@ -180,7 +180,7 @@ static void cosmo_handle_message(cosmo *instance, json_t *event) {
   assert(!pthread_mutex_unlock(&instance->lock));
 
   if (instance->callbacks.message) {
-    instance->callbacks.message(event);
+    instance->callbacks.message(event, instance->passthrough);
   }
 }
 
@@ -435,7 +435,7 @@ json_t *cosmo_get_last_message(cosmo *instance, json_t *subject) {
   return ret;
 }
 
-cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_callbacks *callbacks) {
+cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_callbacks *callbacks, void *passthrough) {
   curl_global_init(CURL_GLOBAL_DEFAULT);
   srandomdev();
 
@@ -446,6 +446,7 @@ cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_cal
   cosmo_uuid(instance->instance_id);
 
   memcpy(&instance->callbacks, callbacks, sizeof(instance->callbacks));
+  instance->passthrough = passthrough;
 
   assert(!pthread_mutex_init(&instance->lock, NULL));
   assert(!pthread_cond_init(&instance->cond, NULL));
diff --git a/clients/c/cosmopolite.h b/clients/c/cosmopolite.h
index 5086089..5d1c830 100644
--- a/clients/c/cosmopolite.h
+++ b/clients/c/cosmopolite.h
@@ -9,13 +9,14 @@
 #define COSMO_UUID_SIZE 37
 
 typedef struct {
-  void (*message)(const json_t *);
+  void (*message)(const json_t *, void *);
 } cosmo_callbacks;
 
 typedef struct {
   char client_id[COSMO_UUID_SIZE];
   char instance_id[COSMO_UUID_SIZE];
   cosmo_callbacks callbacks;
+  void *passthrough;
 
   pthread_mutex_t lock;
   pthread_cond_t cond;
@@ -31,7 +32,7 @@ typedef struct {
 
 void cosmo_uuid(char *uuid);
 
-cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_callbacks *callbacks);
+cosmo *cosmo_create(const char *base_url, const char *client_id, const cosmo_callbacks *callbacks, void *passthrough);
 void cosmo_shutdown(cosmo *instance);
 
 const char *cosmo_current_profile(cosmo *instance);
diff --git a/clients/c/test.c b/clients/c/test.c
index ca40b08..8ba3405 100644
--- a/clients/c/test.c
+++ b/clients/c/test.c
@@ -2,7 +2,7 @@
 
 #include "cosmopolite.h"
 
-void on_message(const json_t *message) {
+void on_message(const json_t *message, void *passthrough) {
   printf("new message: %lld\n", json_integer_value(json_object_get(message, "id")));
 }
 
@@ -14,7 +14,7 @@ int main(int argc, char *argv[]) {
     .message = on_message
   };
 
-  cosmo *instance = cosmo_create("https://playground.cosmopolite.org/cosmopolite", client_id, &callbacks);
+  cosmo *instance = cosmo_create("https://playground.cosmopolite.org/cosmopolite", client_id, &callbacks, NULL);
   json_t *subject = cosmo_subject("foobar", NULL, NULL);
   cosmo_subscribe(instance, subject, -1, 0);
   json_decref(subject);