I lost a lot of time in understanding the message "sendto Invalid argument" when I execute the following code.
This code is a simple UDP sender improved with some reliability feature.
My goal is to send a file. I've reported only the code which may be useful.
Can anyone help me?
Thank you in advance.

typedef struct{
  char* filename;
  unsigned long encoded_file_size;
}first_message;
typedef struct {
     char data[DATALENGTH];
     int nseq;
}packet; 
  memset((char *)&dest, 0, sizeof(dest));
  dest.sin_family = AF_INET;
  dest.sin_port = htons(PORT);
  dest.sin_addr.s_addr = inet_addr(argv[1]); /*destination's address*/
  if ((sd = socket(AF_INET,SOCK_DGRAM,0))<0) {
  perror("socket creation failed");
  exit(EXEC_FAILED);
  fin = fopen(argv[2],"rb");
  if (fin != NULL){
      fm.filename = find_fileName(argv[2]);
      fm.encoded_file_size = htonl(get_file_size(fin));
       printf("sizeof(fm) %ld",sizeof(fm));    
/*HERE THE ERROR*/
      while (send_first(sd,&fm,sizeof(fm),"unable to send first message",(struct sockaddr*)&dest)<0){
      send_first(sd,&fm,2*sizeof(fm),"unable to send first message",(struct sockaddr*)&dest);}
.....
}  /*end of main*/
ssize_t send_first(int sd, void* buf,size_t len,char* err, const struct sockaddr* to){
 unsigned long byte_sent;
 if(byte_sent = sendto(sd, buf, len, 0, to, sizeof(to))<0){
      perror(err); }
 sleep(TIMEOUT);
 if(fcntl(sd,F_SETFL,O_NONBLOCK)==-1){
    perror("Impossible to set NONBLOCK mode for the socket");
 return recvfrom(sd, buf,len,0,NULL,NULL);