Socket: C++ Server reads text wrongly from Java client

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I have a client in Java and a server in C++ (using wsock32.lib). The client is very simple, just sends a text to server. I have the server loop to receive a message from client, like this:

 
    while(1){
                int nret = 0;    
            nret = recv(m_socket,buffer,100,0);
            if(nret == SOCKET_ERROR) {
               nret = WSAGetLastError();        
               //throw error
            }
            buffer[nret-1] = '\0';
        printf("Recv:%s\n",buffer);
    }
 

When the client sends a message, ended by "\n", e.g. "hello\n", I debug, and the process is hereafter:

- server calls recv for the first time. buffer gets "h????????????", nret = 1. so the buffer is got "" after truncating.
- it continues to call recv, buffer gets "ello????????", nret = 5. the buffer is "ello" then.

I don't know why the recv called 2 times. Please help me! Thank you in advance.

Best regards,
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