Learn to use lwip

Okay... I went through every line of code in the enetEcho example and read the lwip rawapi documentation. I think I now understand a bit more. So I tried to replace the echo_init() function in the example with an own udp_service_init() function, that I wrote by myself, based on the information I got from the different sources. It compiles, but won't work. This is my code, that should receive an udp packet and send it back. It gets an IP Adress after booting, but then, nothing happens. Anyone here who might give a comment on that?

/*
 * udpServices.c
 *
 *  Created on: 03.08.2016
 *      Author: Janos Buttgereit
 */

#include "udpServices.h"
#include "lwip/udp.h"

struct ip_addr janosMacIP;

struct udp_pcb *toMatlabUDP;

#define MAX_SIZE                           4096

/*Global copy buffer for echoing data back to the sender */
unsigned char udpdata[MAX_SIZE];

err_t incommingUDP(void *arg, struct udp_pcb *upcb, struct pbuf *p, struct ip_addr *addr, u16_t port){
	struct pbuf *first_p = p;
	char *data;
	unsigned int cnt = 0, i;
	unsigned int len, tot_len;
	  tot_len = p->tot_len;

	  //pbuf is a linked list
	  do {
	        data = (char*)p->payload;
	        len  = p->len;
	        for(i = 0; i < len; i++, cnt++)
	        {
	            udpdata[cnt] = data[i];
	        }
	        //now process the next pbuf until the next pointer points to NULL
	        p = p->next;

	    }while(p!=NULL);


	//first test: send data right back
	err_t err;

	err = udp_send(toMatlabUDP, first_p);

	pbuf_free(first_p);

	return ERR_OK;
};

void udp_service_init(){

	IP4_ADDR(&janosMacIP, 192, 167, 1, 102);
	//which IP Adress & port should be listened to?
	udp_bind(toMatlabUDP, &janosMacIP, 5050);

	//to which IP Adress & port should the data be sent?
	udp_connect(toMatlabUDP, &janosMacIP, 5051);

	//which function ist called if new data arrives?
	udp_recv(toMatlabUDP, (udp_recv_fn)incommingUDP, NULL);
};