C :: UDO Socket Programming

I am trying to test my client-server socket program wherein the client connects to the server,client sends a message, server receives it and echo back to the client.So far, in the program server receives the message from the client, prints it BUT when it tries to send the message back to the client it shows an error.

sendto(): Invalid argument.I am new to socket programming.

Code:

//Server
#include<stdio.h> //printf
#include<string.h> //memset
#include<stdlib.h> //exit(0)
#include<netinet/in.h>
#include<sys/socket.h>
#define BUFLEN 512 //Max length of buffer
#define PORT 10003 //The port on which to listen for incoming data
}

[code]....