Sending raw data with write() in boost::asio

Question

I've recently decided to use boost::asio for my sockets, but now I'm running into a problem: documentation seems to be lacking.

What I want to do is write a function that will send a message consisting of the following structure:

2 bytes of an unsigned integer (uint16_t) for an opcode all bytes
all bytes after that (a flexible amount) being any type of data (cast to void*). This data will be operated upon based on the opcode

For example, if the opcode is 1, perhaps defined as OPCODE_LOGIN, then the bytes following the opcode could contain a string containing login information, etc.

bool sendMessage(tcp::socket* sock, uint16_t opcode, void* data)
{
    void* fullData = malloc(sizeof(uint16_t) + sizeof(data));
    memcpy(fullData, (void*)opcode, sizeof(opcode));
    memcpy(&fullData + sizeof(uint16_t), data, sizeof(data));
    boost::asio::write(sock, boost::asio::buffer(fullData, sizeof(fullData)));
    // by the way, at this point, is it safe to delete fullData to prevent memory leaks?
    return true;

}

This does not compile, however. I get a cryptic compilation error regarding the call to write:

1>------ Build started: Project: client, Configuration: Debug Win32 ------
1>  main.cpp
1>c:\boost\boost_1_47\boost\asio\impl\write.hpp(46): error C2228: left of '.write_some' must have class/struct/union
1>          type is 'boost::asio::basic_stream_socket<Protocol> '
1>          with
1>          [
1>              Protocol=boost::asio::ip::tcp
1>          ]
1>          did you intend to use '->' instead?
1>          c:\boost\boost_1_47\boost\asio\impl\write.hpp(59) : see reference to function template instantiation 'size_t boost::asio::write<SyncWriteStream,ConstBufferSequence,boost::asio::detail::transfer_all_t>(SyncWriteStream &,const ConstBufferSequence &,CompletionCondition,boost::system::error_code &)' being compiled
1>          with
1>          [
1>              SyncWriteStream=boost::asio::ip::tcp::socket *,
1>              ConstBufferSequence=boost::asio::mutable_buffers_1,
1>              CompletionCondition=boost::asio::detail::transfer_all_t
1>          ]
1>          c:\users\josh\documents\visual studio 2010\projects\client\client\main.cpp(53) : see reference to function template instantiation 'size_t boost::asio::write<boost::asio::ip::tcp::socket*,boost::asio::mutable_buffers_    1>(SyncWriteStream &,const ConstBufferSequence &)' being compiled
1>          with
1>          [
1>              SyncWriteStream=boost::asio::ip::tcp::socket *,
1>              ConstBufferSequence=boost::asio::mutable_buffers_1
1>          ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

As you can see, the error message points directly into Boost's write.hpp file, and not to any of my own code.. I believe I'm calling write() incorrectly in some way, but after a solid hour of googling and researching references and examples (all of which either use another overloaded write() or are using data with specifically defined sizes/structures), I haven't been able to determine what exactly I'm doing wrong here.

Can someone help me debug this compilation error?

This is probably irrelevant at this point, but I'd like to point out that `sizeof(data)` is equivalent to `sizeof(void *)`, which is most definitely not what you want here. You should pass the data's actual length as another parameter. `sizeof(fullData)` suffers from the same problem. — abel1502, Aug 10 '21 at 08:54

score 16 · Accepted Answer · edited May 23 '17 at 11:53

documentation seems to be lacking

The highest voted question in boost-asio is about documentation, start there :-)

Can someone help me debug this compilation error?

The write() free function expects a reference type as the first parameter. Not a pointer as you have in your example

bool sendMessage(tcp::socket* sock, uint16_t opcode, void* data)
{
    void* fullData = malloc(sizeof(uint16_t) + sizeof(data));
    memcpy(fullData, (void*)opcode, sizeof(opcode));
    memcpy(&fullData + sizeof(uint16_t), data, sizeof(data));
    boost::asio::write(*sock, boost::asio::buffer(fullData, sizeof(fullData)));
    //                 ^^^^ correct type now
    // by the way, at this point, is it safe to delete fullData to prevent memory leaks?
    return true;
}

at this point, is it safe to delete fullData to prevent memory leaks?

Yes, write() is a blocking call. It is done with your buffer when the call returns. I strongly suggest making this code exception safe however, look into using new and a boost::scoped_array if you wish to create your buffer with dynamic storage duration.

That worked great, thanks! :) And I did see the "highest voted question" many times in my googling, and I followed its advice, but not to much avail... the examples used different overloaded versions of write(), the class reference built upon many boost streams that I'm not familiar with, etc.. — Josh1billion, Oct 16 '11 at 21:40

Sending raw data with write() in boost::asio

1 Answers1