How to ask server to create file with specific name from client end in MFC csocket programming?

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Hello! I'm transferring files from server application folder to client application folder once after the connection is made. I'm giving file name to create new text file in client program. Instead, I want the server to send any kind of message or command to ask the client to create new text file with particular name.

What I have tried:

This is the code in client end where I'm giving filename to create.
int br=SockConnection.Receive(buff,1000);
CFile Destfile;
Destfile.Open(L"Myreceivedfile", CFile::modeCreate | CFile::modeWrite);
Destfile.Write (buff,br);

Instead, I want to server to inform the client to create file with particular name. Pls help. Thank you.

Posted 21-Jan-19 21:40pm

rmds

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Richard MacCutchan 22-Jan-19 5:04am

Get the server to send the filename as the first message.

rmds 22-Jan-19 5:28am

Pls correct me if I'm wrong. You mean to say like this?
"SockConnection.Send(L"Myinputfiledata", 0);"

Richard MacCutchan 22-Jan-19 5:53am

Sort of. But you need to ensure that both client and server understand what each message is about. So a message with the filename should contain some sort of identifier so it is not mistaken for ordinary data. You should design a protocol between your terminals so they can validate the information that is being passed between them. Do not assume that the content of a message is what you think it should be.

rmds 22-Jan-19 7:03am

Explanation is crystal clear! Let's say I have multiple Receive() in client application and I'm in need to send message or data from server. How to send the message to specific Receive()? I indeed googled for examples but no luck. Please throw some light with example. Thank you!

Richard MacCutchan 22-Jan-19 7:19am

I already explained what you need to do. Each message should include a header that identifies what the content of the message is. So to start a new file, send a message with a header plus a file name. To send some file content send a header plus some data. The header should include a flag that identifies the type of record, and a length field that tells the receiver how long the message is.

Rick York 22-Jan-19 11:52am

To elaborate on what Richard wrote, the previous generation of my app had multiple servers we would ask to perform tasks and they were connected with sockets. Each message sent to them had a header that had some version bytes, a message length, and a command. The command told the server what to do with the data of the message. One command told the server the message contained a copy of the app and it needed to update itself. That was an interesting little process. Anyway, my point is I recommend adopting something similar : incorporate a header into the message to tell the other side what to do with the data.

rmds 23-Jan-19 0:33am

I understand but, I never actually done it. I'm trying since yesterday but unable to find how the send() with command header looks like. Can atleast show me glimpse of send() with command header with any kind of example? Thank you very much.

Rick York 23-Jan-19 3:16am

send takes a pointer to a character buffer and a length so you have to assemble the character buffer to send. Memcpy your header to the start of the buffer and follow that with the data. You do the reverse on the other end. Memcpy the start of the buffer to your header and then the data follows the header. You know how big the header is so you know where to find the data. Alternatively, you could use streams to set it up and unpack it on the other end.

rmds 23-Jan-19 6:52am

After going through lot, i did this...
Server:
typedef struct{
char* filename[20];
}Data_Buffer;
Data_Buffer data_buffer;
data_buffer.filename[21] = "myinputfile";

int nRet = SockConnection.Send((void*)&data_buffer, sizeof(data_buffer));

Client:
typedef struct{
char* filename[21];
}Data_Buffer;
Data_Buffer data_buffer;
int nRet = SockConnection.Receive((void*)&data_buffer, sizeof(data_buffer));
CFile Destfile;
CString filename = (LPCTSTR)&data_buffer;
Destfile.Open(filename, CFile::modeCreate | CFile::modeWrite);
Destfile.Write(Buf, br);

I get file name as garbage values.. What went wrong?

Rick York 23-Jan-19 10:39am

Your definitions are wrong. You can't send a pointer across a socket and assume it is valid and an array of pointers won't work either. My header has a four version bytes, a message length field, and a command field. That's it. Everything has to be static data with no pointers. The message length is important because if you have more than about 1500 bytes of data it will have to go in multiple packets and the length will tell you how much you need to receive.

rmds 24-Jan-19 2:38am

Can you please give me glimpse of header you're referring to with message, length and command. Just wanna see how it looks like to get an idea to implement. Thank you

Rick York 24-Jan-19 10:42am

typedef struct
{
   size_t length;
   INT    command;
   UCHAR  version[4];
} MsgHeader;

rmds 23-Jan-19 5:37am

I really appreciate your time for this. Could you please clarify is this what you mean by header? :
typedef struct COMMAND_HEADER {
char* SET_FILE_NAME;
int data_size;
}
please correct me if I'm wrong

rmds 24-Jan-19 23:56pm

Thank you rick and richard!

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