Thread: Adding To A char array

  1. #1
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    Adding To A char array

    Hello,

    I have a client /server example that I am trying to modify. The client will send a message to the server and then echo it back to the client.

    I want to append a string to the message when it gets to the server (just before it goes to the client)

    Here is the original source
    doc/html/boost_asio/example/cpp03/echo/async_tcp_echo_server.cpp - 1.55.0


    Here is my non working attempt but for some reason I just cant get it to work.

    Code:
            string sToSend(data_);
            sToSend += "<--- this string to append";
            //const char cs = sToSend.c_str();
          socket_.async_read_some(boost::asio::buffer(sToSend.c_str(), max_length),
              boost::bind(&session::handle_read, this,
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));
    Error:
    Code:
    /usr/include/boost/asio/detail/buffer_sequence_adapter.hpp|212|error: no matching function for call to ‘boost::asio::mutable_buffer::mutable_buffer(const boost::asio::const_buffers_1&)’|
    Original Code
    Code:
          tcp::socket socket_;
      enum { max_length = 1024 };
      char data_[max_length];boost::asio::async_write(socket_,          boost::asio::buffer(data_, bytes_transferred),          boost::bind(&session::handle_write, this,            boost::asio::placeholders::error));
    Code:
    //
    // async_tcp_echo_server.cpp
    // ~~~~~~~~~~~~~~~~~~~~~~~~~
    //
    // Copyright (c) 2003-2013 Christopher M. Kohlhoff (chris at kohlhoff dot com)
    //
    // Distributed under the Boost Software License, Version 1.0. (See accompanying
    // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
    //
    
    #include <cstdlib>
    #include <iostream>
    #include <boost/bind.hpp>
    #include <boost/asio.hpp>
    
    using boost::asio::ip::tcp;
    
    class session
    {
    public:
      session(boost::asio::io_service& io_service)
        : socket_(io_service)
      {
      }
    
      tcp::socket& socket()
      {
        return socket_;
      }
    
      void start()
      {
        socket_.async_read_some(boost::asio::buffer(data_, max_length),
            boost::bind(&session::handle_read, this,
              boost::asio::placeholders::error,
              boost::asio::placeholders::bytes_transferred));
      }
    
    private:
      void handle_read(const boost::system::error_code& error,
          size_t bytes_transferred)
      {
        if (!error)
        {
          boost::asio::async_write(socket_,
              boost::asio::buffer(data_, bytes_transferred),
              boost::bind(&session::handle_write, this,
                boost::asio::placeholders::error));
        }
        else
        {
          delete this;
        }
      }
    
      void handle_write(const boost::system::error_code& error)
      {
        if (!error)
        {
          socket_.async_read_some(boost::asio::buffer(data_, max_length),
              boost::bind(&session::handle_read, this,
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));
        }
        else
        {
          delete this;
        }
      }
    
      tcp::socket socket_;
      enum { max_length = 1024 };
      char data_[max_length];
    };

  2. #2
    and the hat of int overfl Salem's Avatar
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    Well the error message mentions boost::asio::mutable_buffer::mutable_buffer, which suggests that whatever gets passed in should be susceptible to modification.

    A string.c_str() fails to meet this requirement.

    You need to allocate a char* block of memory, copy your string to it and then free it when you're done.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Thank you for the reply. I have been trying to get this to work but so far I cannot.


    No matter what I do, the same amount of bytes is returned back to the client as was originally read. What am i doing wrong here?

    Code:
      void handle_read(const boost::system::error_code& error,
          size_t bytes_transferred)
      {
    
            // data_ here == "thisisatest"
         
        if (!error)
        {
            data_[0] = 'X';                                  // this works
            char * cstr;
            string str (data_);
            str = str + "app";
            cstr = new char [str.size()+1];
            strcpy (cstr, str.c_str());
    
            // cstr  at this point == "Xhisisatestapp"
            // sizeof(cstr)  == 8   this is not correct
    
    
          boost::asio::async_write(socket_,
              boost::asio::buffer(cstr, sizeof(cstr)),          // sizeof(cstr) is not right here  I even try to hardcode this to 12 but it did not work either (only 10 bytes returned to client)
              boost::bind(&session::handle_write, this,
                boost::asio::placeholders::error));
    
                delete cstr;
    
    // original code
    //      boost::asio::async_write(socket_,
    //          boost::asio::buffer(data_, bytes_transferred),
    //          boost::bind(&session::handle_write, this,
    //            boost::asio::placeholders::error));
        }
        else
        {
          delete this;
        }
      }

  4. #4
    and the hat of int overfl Salem's Avatar
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    Well it's either str.size() or strlen(cstr)

    It's also
    delete [] cstr;
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
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    Thank you! strlen worked for fixing the size.

    I cant thank you enough for your help!
    Last edited by EverydayDiesel; 01-21-2017 at 05:23 PM.

  6. #6
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    is there a better alternative to getting the char* to a string?

    They used this (which is great for writing out to the console but I need it in a string variable)
    Code:
    cout.write(reply, reply_length);
    // this works (only because the last line with the substr) but seems more like a hack
    Code:
        #define SSTR( x ) static_cast< std::ostringstream & >(( std::ostringstream() << std::dec << x ) ).str()
    
    
        string sReply = SSTR(reply);
        sReply = sReply.substr(0, request_length);

    This has garbage at the end of it
    Code:
        stringstream ss;
        string sReply;
        ss << reply;
        ss >> sReply;

  7. #7
    C++まいる!Cをこわせ!
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    Use a vector:

    std::vector<char> Buf(1024);
    // Write to buffer
    // Null terminate buffer if boost doesn't
    // Now let's add the buffer to the string
    MyStr += &Buf[0];

    It would be easier if you used boost's streams to do network I/O. I'm sure there must be an example somewhere.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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