Casting a struct for sending to socket

**chrisjmoss** · 04-08-2008

Hi Guys

I am trying to send a struct to a UDP socket, but I'm not entirely sure how to cast the structure.

Code:

struct CONNECT_REQUEST {
	char headersize ;//	= 0x06;		//static EIB header size
	char protocolversion;//	= 0x10;		//protocol version (1.0)
	char servicetype[1];//	= 0x0205;	//service type in this case a connect_request
	char totallength[1];//	= 0x0018;	//total length (24 byte)
	char controllength;//	= 0x08;		//HPAI header for control endpoint
	char protocoltype;//	= 0x01;		//0x01 UDP, 0x02 TCP
	char IPcontrol[3];//;			//four chars for conrtol endpoint IP
	char portcontrol[1];//			//two chars for control endpoint port must be gleaned during discovery
	char datalength	;//	= 0x08;		//HPAI header length for data endpoint
	char IPdata[3];	//			//fours chars for data endpoint IP use same IP as control endpoint
	char portdata[1];//	= 0x0e57;	//set data port to 3671 default for EIB
	char connectheader;//	= 0x02;		//connection type structue length
	char connecttype;//	= 0x04;		//connection type in this case tunnelling may need 2 extra bytes.
	};

struct CONNECT_REQUEST connect;					//creat struct called connect of type CONNECT_REQUEST

Code:

  /* construct CONNECT_REQUEST structure */
  connect.headersize		= 0x06;		//static EIB header size
  connect.protocolversion	= 0x10;		//protocol version (1.0)
  connect.servicetype[1]	= 0x02,0x05;	//service type in this case a connect_request
  connect.totallength[1]	= 0x00,0x18;	//total length (24 byte)
  connect.controllength		= 0x08;		//HPAI header for control endpoint
  connect.protocoltype		= 0x01;		//0x01 UDP, 0x02 TCP
  connect.IPcontrol[0] = oca, connect.IPcontrol[1] = ocb, connect.IPcontrol[2] = occ, connect.IPcontrol[2] = ocd;//set IP endpoint
  connect.portcontrol[0] = recv_str[12]&255, connect.portcontrol[13]&255;		//set port number for control endpoint
  connect.datalength		= 0x08;		//HPAI header length for data endpoint
  connect.IPdata[0] = oca, connect.IPdata[1] = ocb, connect.IPdata[2] = occ, connect.IPdata[3] = ocd;//set data end point IP
  connect.portdata[1]		= 0x0e,0x57;	//set data port to 3671 default for EIB
  connect.connectheader		= 0x02;		//connection type structue length
  connect.connecttype		= 0x04;		//connection type in this case tunneling 04

  if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)					//Creat UDP socket
	DieWithError("UDP socket creation failed\n");

  /* construct UDP address structure */
  memset(&udp_addr, 0, sizeof(udp_addr));					//set udp_addr struct to 000's using memset
  udp_addr.sin_family      = AF_INET;						//set family type to AF_INET
  udp_addr.sin_addr.s_addr = htonl((oca,'.',ocb,'.',occ,'.',ocd));		//set address as any
  udp_addr.sin_port        = htons(mc_port);					//set the port as the same multi cast port

  if ((sendto(sock, &connect, sizeof(connect), 0, (struct sockaddr *) &udp_addr, sizeof(mc_addr))) != sizeof(connect)) 	//send data to socket
	DieWithError("sendto() sent incorrect number of bytes");

  close(sock);

  exit(0);
}

This code compiles, but I get the error Invalid argument when sending to the socket. I have also tried casting to a pointer, but this gives an error when comilping.

Any help would be greatly appreciated

**Dino** · 04-08-2008

Code:

char servicetype[1];//	= 0x0205;	//service type in this case a connect_request
char totallength[1];//	= 0x0018;	//total length (24 byte)
...
char IPcontrol[3];//;			//four chars for conrtol endpoint IP
...
char portcontrol[1];//			//two chars for control endpoint port must be gleaned during discovery
...
char IPdata[3];	//			//fours chars for data endpoint IP use same IP as control endpoint
char portdata[1];//	= 0x0e57;	//set data port to 3671 default for EIB

Methinks you are off in your count a few places.

**chrisjmoss** · 04-08-2008

Code:

char IPcontrol[3];		//four chars for conrtol endpoint IP
char IPdata[3];			//fours chars for data endpoint IP use same IP as control endpoint
char portdata[1];             //set data port to 3671 default for EIB

because they are not strings i.e not terminated with \0 can I not just use the array elements [0].[1],[2] and [3]. giving me the 4 required characters.

**Dino** · 04-08-2008

In this case,

Code:

char IPcontrol[3];

you are saying the array is 3 bytes long, plain and simple. If you want it to be 4 bytes long, code

Code:

char IPcontrol[4];

I think you are confusing array length with array indexing. The highest element when indexing a 4 byte array is 3, as in:

Code:

 
IPcontrol[0] (the 1st element) 
IPcontrol[1] (the 2nd element) 
IPcontrol[2] (the 3rd element) 
IPcontrol[3] (the 4th element)

**chrisjmoss** · 04-08-2008

Todd thanks for the help thus far, I was being a bit of a div and got confused with array size and indexing, but I have sorted that out now.

Code:

char servicetype[2];//	= 0x0205;	//service type in this case a connect_request
	char totallength[2];//	= 0x0018;	//total length (24 byte)
	char controllength;//	= 0x08;		//HPAI header for control endpoint
	char protocoltype;//	= 0x01;		//0x01 UDP, 0x02 TCP
	char IPcontrol[4];//;			//four chars for conrtol endpoint IP
	char portcontrol[2];//			//two chars for control endpoint port must be gleaned during discovery
	char datalength	;//	= 0x08;		//HPAI header length for data endpoint
	char IPdata[4];	//			//fours chars for data endpoint IP use same IP as control endpoint
	char portdata[2];//

I'm still having having issues sending the structure with the sendto function do I need to cast from the structure or create a pointer to it.

Code:

 struct CONNECT_REQUEST connect;					//creat struct called connect of type CONNECT_REQUEST

if ((sendto(sock, &connect, sizeof(connect), 0, (struct sockaddr *) &udp_addr, sizeof(udp_addr))) != sizeof(connect)) 	//send data to socket
	DieWithError("sendto() sent incorrect number of bytes");

**Dino** · 04-08-2008

"issues" is a bit vague. We have no error messages to work with here.

**Salem** · 04-08-2008

> connect.portdata[1] = 0x0e,0x57;
Also, the comma operator does not incrementally assign each value to each successive array element.

Also, check that your struct is actually the size you want.
Using a struct like this to communicate with an external API is generally non-portable.

Code:

unsigned char message[SOME_SIZE];
message[MSG_HEADER_SIZE] = 0x06;
message[MSG_PROTOCOL]    = 0x10;
message[MSG_SERVICE_LSB] = 0x05;
message[MSG_SERVICE_MSB] = 0x02;
// and so on

Tedious to be sure, but it's the only way to be sure of isolating yourself from padding, alignment and endian issues. Though TBH, a struct composed entirely of char should be OK.

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