TCP Write Raw data

[^NyAw^]

Hi,

I'm trying to write raw data thorugh TCPSocket. I have a struct like (Modbus protocol):

//Modbus header
	typedef struct MB_Request
	{
		uchar function_code;
		quint16 start_adr;
		quint16 quantity_regs;
	};
 
	//Modbus/TCP header, containing the Modbus header
	typedef struct MBAP_Header
	{
		quint16 transaction_id;
		quint16 protocol_id;
		quint16 len;
		uchar unit_id;
	};
 
	typedef struct MB_Message
	{
		MBAP_Header mb_hdr;
		MB_Request mb_req;
	};

//Modbus header
	typedef struct MB_Request
	{
		uchar function_code;
		quint16 start_adr;
		quint16 quantity_regs;
	};

	//Modbus/TCP header, containing the Modbus header
	typedef struct MBAP_Header
	{
		quint16 transaction_id;
		quint16 protocol_id;
		quint16 len;
		uchar unit_id;
	};

	typedef struct MB_Message
	{
		MBAP_Header mb_hdr;
		MB_Request mb_req;
	};

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What I don't know is how to send a variable (MB_Message type) as raw data. The documentation show that the data have to be stored as "char *". How have I to do this?

Thanks,

[jacek]

You can cast a pointer to the structure to char *, but beware that these structures are aligned and for example size of MB_Request is more than 5 bytes. If you use GCC you can add __attribute__((packed)) to pack them. Also make sure that integers have proper endianness.

[^NyAw^]

Hi,

You can cast a pointer to the structure to char *

const char* data = (char*)&message;
int ilen = strlen(data);
tcpSocket->write(data,ilen);

const char* data = (char*)&message;
int ilen = strlen(data);
tcpSocket->write(data,ilen);

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Using this I only recive 1 byte.
Maybe have I to create a "char*" containing the same values of the struct?

Thanks,

[jacek]

strlen() is for zero-terminated strings, not for data --- use sizeof message instead.

[^NyAw^]

Hi,

Ok, it seems to work now.
When the reciver gets the raw data, I have to cast the data to a struct pointer. Right?

The other question is:

but beware that these structures are aligned and for example size of MB_Request is more than 5 bytes.
Also make sure that integers have proper endianness

What do you mean with that the structures are aligned? And how can I maintain the proper endianess, I think that I'm sending the data as big endian.

Thanks,

[jacek]

What do you mean with that the structures are aligned?

Check what sizeof( MB_Request ) returns. It contains only 5 bytes of data, yet it's bigger because of performance reasons (i.e. it contains holes between fields). Most likely the protocol expects packed data, so you have to tell your compiler to treat those structures differently.

http://en.wikipedia.org/wiki/Data_structure_alignment

And how can I maintain the proper endianess, I think that I'm sending the data as big endian.

QSysInfo::ByteOrder will tell you whether you are using big-endian or little-endian system and you have to convert values to endianness required by the protocol.

[^NyAw^]

Hi,

Check what sizeof( MB_Request ) returns. It contains only 5 bytes of data

It returns me "14". The structs are 7 bytes(Modbus/TCP header) + 5 bytes (Modbus header) that sum 12. So I think that the struct "MB_Message" contains 1 byte for the first struct and another byte for the second struct.

Thanks,

[^NyAw^]

Hi,

Ok, thanks for the link of "Data structure alignment".

So, the "Modbus/TCP header" is 7 bytes. The "Modbus header" is 5 bytes. A total of 12 bytes that are 12*8 = 96 bits that are 32 bit aligned.
So, why "sizeof(MB_Message)" is returning me 14 bytes?

Thanks,

[jacek]

So, why "sizeof(MB_Message)" is returning me 14 bytes?

Because there are one-byte holes after unit_id and function_code fields.

I've attached a small application. It prints the addresses of all fields, so you can see how they're placed in the memory. If you use GCC, you can uncomment the #define in line 7 and comment out the one in line 6 to see how unaligned (a.k.a. packed) structures look like.

[^NyAw^]

Hi,

Ok, so every struct have to be 32-bit aligned.

Thanks,

[jacek]

Ok, so every struct have to be 32-bit aligned.

Yes, but only in memory on 32-bit (and pseudo-64bit) machines. Other machines might have different alignment. Also usually you don't align data in files* or in network communication.

* Unless the file format was designed for fast access.