Accessing Buffer Contents

[efweber999]

Version of Beast: 330 Compiler: gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0

Hi,

I'm having difficulty accessing the buffer contents in the websockets-server-async example. Following the documentation for Accessing Buffer Contents, I added this line to libs/beast/example/websocket/server/async/websockets_server_async.cpp:

        // Echo the message
        ws_.text(ws_.got_text());
        // Added this line only.
        unsigned char* p1 = static_cast<unsigned char*>(buffer_.data());
        ws_.async_write(
            buffer_.data(),
            beast::bind_front_handler(
                &session::on_write,
                shared_from_this()));

Compiling produced the following error:

websockets_server_async.cpp: In member function ‘void session::on_read(boost::beast::error_code, std::size_t)’:
websockets_server_async.cpp:134:71: error: invalid static_cast from type ‘boost::beast::basic_flat_buffer<std::allocator<char> >::mutable_buffers_type’ {aka ‘boost::asio::mutable_buffer’} to type ‘unsigned char*’
  134 |         unsigned char* p1 = static_cast<unsigned char*>(buffer_.data());

I've tried various ideas to get past this, but I'm stuck. Help would be appreciated. Thanks.

Here is some additional information. I used b2 to compile the examples.

./b2 -j8 -d+1 -d+2 --debug-building libs/beast/example cxxstd=11 \
--build-dir=/home/beast \
variant=release \
2>&1 | tee /home/beast/beast_build.log

I then used the g++ compile settings directly from beast_build.log to re-compile websockets-server-async as I tried things.

# Compile
g++ \
-std=c++11 \
-fvisibility-inlines-hidden \
-fPIC \
-m64 \
-pthread \
-O3 \
-finline-functions \
-Wno-inline \
-Wall \
-fvisibility=hidden \
-DBOOST_ALL_NO_LIB=1 \
-DBOOST_ASIO_DISABLE_BOOST_ARRAY=1 \
-DBOOST_ASIO_DISABLE_BOOST_BIND=1 \
-DBOOST_ASIO_DISABLE_BOOST_DATE_TIME=1 \
-DBOOST_ASIO_DISABLE_BOOST_REGEX=1 \
-DBOOST_ASIO_NO_DEPRECATED=1 \
-DBOOST_ASIO_SEPARATE_COMPILATION \
-DBOOST_BEAST_ALLOW_DEPRECATED \
-DBOOST_BEAST_SEPARATE_COMPILATION \
-DBOOST_COROUTINES_NO_DEPRECATION_WARNING=1 \
-DNDEBUG \
-D_GNU_SOURCE=1 \
-D_XOPEN_SOURCE=600 \
-I. \
-I/usr/local/boost_1_79_0 \
-I/usr/local/boost_1_79_0/libs/beast \
-c \
-o websocket_server_async.o \
websocket_server_async.cpp

# Link
g++ \
-o websocket-server-async \
-Wl,--start-group \
websocket_server_async.o \
lib-asio.a \
lib-beast.a \
-Wl,-Bstatic \
-Wl,-Bdynamic \
-lrt \
-Wl,--end-group \
-fPIC \
-m64 \
-pthread \
-std=c++11 \
-fvisibility=hidden \
-fvisibility-inlines-hidden

[madmongo1]

You are not using the buffer sequence returned by the dynamic buffer's data() method correctly.

Here is an example of how it works:

#include <boost/beast/core.hpp>
#include <boost/asio.hpp>
#include <iostream>


int
main()
{
    boost::beast::flat_buffer buffer_;

    std::string src = "Hello, World!";

    auto inbuf = boost::asio::buffer(src);
    auto target = buffer_.prepare(inbuf.size());
    boost::asio::buffer_copy(target, inbuf);
    buffer_.commit(inbuf.size());

    // this function returns the buffer sequence representing the output area of the 
    // dynamic buffer bufseq
    auto bufseq = buffer_.data();

    // for a flat_buffer, the buffer sequence us actually a single buffer
    char* p1 = static_cast<char*>(bufseq.data());
    std::size_t len = bufseq.size();

    std::cout.write(p1, len);
    std::cout.put('\n');

}

expected output:

Hello, World!

https://godbolt.org/z/j4MYPaPn9

[efweber999]

Got it. Thank you.