数组指针 + 整数 (C++)

Question

I am not so familar with C++, so I want to ask, what the following code will do (I have it in an existing C++ project):

1: char* buf;
2: *buf = 0;
3: int readBytes = tcpSocket->read(buf, 10);
4: buf += readBytes;

Explanation is very simple, from a TCP Socket should be read 10 bytes and the read bytes are stored in the char* Buffer "buf". The return value is the number of read bytes.

But why I need line 4? Or better what is line 4 doing? In my understanding it is destroying my "buf" result, isn't it?

I hope someone can help me, and maybe explaining me, why I need this line 4.

BR ThW

Answer 1

TCP is a streaming protocol, that means there are no message boundaries in the data you receive. Therefore one might not get all the data one asks for in a single receive call, but have to loop and and read multiple times.

By doing eg buf += readBytes you advance the pointer buf by readBytes elements, so next time you receive the data will be written where the last receive call left of.

There are some things that you need to fix through, besides you using and dereferencing an uninitialized pointer, and that is that you can't read a fixed number of bytes every iteration in the loop, you need to decrease the amount of data to receive by readBytes as well. You also need to exit the loop once you read all the data.

Lets put all this together into a nice function which will always read the requested amount of data, as an example:

// tcpSocket is the socket to receive data from (I don't know the actual type)
// buffer is the destination buffer, where the received data should be written
// len is the number of bytes to receive
// Pre-condition: buffer must point to memory of at least len bytes
bool read_data(TCPSocketType* tcpSocket, char* buffer, size_t len)
{
    // Loop while there are still bytes to read
    while (len > 0)
    {
        ssize_t readBytes = tcpSocket->read(buffer, len);
        if (readBytes <= 0)
        {
            // There was an error (readBytes < 0)
            // Or the connection was closed (readBytes == 0)
            return false;
        }

        buffer += readBytes;  // Next position to write data into
        len -= readBytes;  // We don't need to read as much now
    }

    return true;  // Now we have read all of the data
}

Answer 2

Line 4 advances the pointer by the bytes you read, so that eg on the next call you don't overwrite your previous data.

As is stated in the comments line 2 is undefined behavior, because you didn't set your buf pointer to allocated memory.