Boost asio - async reading established number of chars from stdin

问题

I wanna write boost::asio app which is reading from stdin with boost::asio::streambuf. Anyway the only function which works on streambuf made from STDIN_FILENO is boost::asio::async_read_until. The other ones throws errors. Is there any possibility to read 100 first character from stdin with boost asio function?

回答1:

In principle this just works

#include <boost/asio.hpp>
#include <boost/asio/posix/stream_descriptor.hpp>

using namespace boost::asio;
using boost::system::error_code;

#include <iostream>

int main()
{
    io_service svc;
    posix::stream_descriptor in(svc, STDIN_FILENO);

    char buf[100];
    async_read(in, buffer(buf,sizeof(buf)), [&](error_code ec, size_t br) { 
            std::cout << std::string(buf, br) << std::flush; 
            if (ec)
                std::cerr << ec.message(); 
        });

    svc.run();
}

When used as

cat input.txt | ./test | wc -c

will just output 100 as expected (and echo the input). We can even use live terminal input:

./test | wc -c

When the input is shorter than 100 bytes, you get the ec.message() "End of file" printed too.

What does not work on Linux is:

 ./test < input.txt

you receive:

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::system::system_error> >'
  what():  assign: Operation not permitted

This is because regular files are not supported for async operations: Strange exception throw - assign: Operation not permitted

来源：https://stackoverflow.com/questions/23720222/boost-asio-async-reading-established-number-of-chars-from-stdin