如何使用 c++ 在 Winsock 中将大字符串从客户端发送到服务器

Question

I am writing a server-client application using Winsock in c++ for sending a file line by line and I have a problem in sending huge string. The line size is very huge.

For getting the message from the client by the server I use the code below.

int result;
char message[200];

while (true)
{
    recv(newSd, (char*)&message, sizeof(message), 0);

    cout << "The Message from client: " << message << ";";
}

The above code working fine if I send small length of the message. But, what I wanted is to send an unknown size of lines in a file.

How to send a big unknown string instead of char message[200];

Answer 1

TCP is a byte stream, it knows nothing about messages or lines or anything like that. When you send data over TCP, all it knows about is raw bytes, not what the bytes represent. It is your responsibility to implement a messaging protocol on top of TCP to delimit the data in some meaningful way so the receiver can know when the data is complete. There are two ways to do that:

• send the data length before sending the actual data. The receiver reads the length first, then reads however many bytes the length says.

• send a unique terminator after sending the data. Make sure the terminator never appears in the data. The receiver can then read until the terminator is received.

You are not handling either of those in your recv() code, so I suspect you are not handling either of them in your send() code, too (which you did not show).

Since you are sending a text file, you can either:

• send the file size, such as in a uint32_t or uint64_t (depending on how large the file is), then send the raw file bytes.

• send each text line individually as-is, terminated by a CRLF or bare-LF line break after each line, and then send a final terminator after the last line.

You are also ignoring the return value of recv() , which tells you how many bytes were actually received. It can, and usually does, return fewer bytes than requested, so you must be prepared to call recv() multiple times, usually in a loop, to receive data completely. Same with send() .