Keep boost.process alive outside of the function is was called from - c++

Question

I'm using Visual Studio 2019 on Windows 10, using the boost.process library. I'm trying to make chess, and I'm using the stockfish engine as a separate executable. I need the engine to run throughout the entirety of the game, as that's how it's designed to be used.
Currently I have in ChessGame.h
class ChessGame
{
public:
void startStockFish();
void beginGame();
void parseCommand(std::string cmd);
private:
boost::process::child c;
boost::process::ipstream input;
boost::process::opstream output;
}
And in ChessGame.cpp
#include ChessGame.h
void ChessGame::startStockFish()
{
std::string exec = "stockfish_10_x32.exe";
std::vector<std::string> args = { };
boost::process::child c(exec, args, boost::process::std_out > input,
boost::process::std_in < output);
//c.wait()
}
void ChessGame::beginGame()
{
parseCommand("uci");
parseCommand("ucinewgame");
parseCommand("position startpos");
parseCommand("go");
}
void ChessGame::parseCommand(std::string cmd)
{
output << cmd << std::endl;
std::string line;
while (std::getline(input, line) && !line.empty())
{
std::cout << line << std::endl;
}
}
And in main.cpp
ChessGame chessGame = ChessGame(isWhite); //isWhite is a boolean that control who the player is, irrelevent to the question
//std::thread t(&ChessGame::startStockFish, chessGame);
chessGame.startStockFish();
chessGame.beginGame();
The problem is that I believe as soon as the function startStockFish finishes it terminates c, as nothing is outputted to the terminal as described above, but if I use beginGame() within startStockFish() it outputs as expected. Also, if I uncomment the line c.wait() and the funtion waits for stockfish to exit, it gets stuck as stockfish never gets the exit command. If I instead try running startStockFish on a separate thread in main (as seen above) I
get the following two errors:
the argument to a feature-test macro must be a simple identifier.
In file 'boost\system\detail\config.hpp' line 51
and
'std::tuple::tuple': no overloaded function takes 2 arguments.
In file 'memory' line 2042
Also, I don't want to use threads as I can imagine that will have its own issues with the input and output streams.
So is there a way for me to keep the process alive out of this function, or do I need to reorganise my code some other way? I believe having the process being called in main would work, but I really don't want to do that as I want to keep all the chess-related code in ChessGame.cpp.

Answer 1

Ok I believe that adding c.detach(); after initialising the boost.process child in startStockFish() has done what I want, as the program no longer terminates c when the function ends. Input appears to work fine with a detached process, simply writing output << cmd << std::endl; where cmd is the desired command as a std::string has no issues. However, output does have some issues, the usual method of
std::string line;
while (std::getline(input, line) && !line.empty())
{
// Do something with line
}
somewhat works, but std::getline(input, line) will get stuck in an infinite loop when there are no more lines to output. I couldn't find a direct solution to this, but I did find a work around.
Firstly I changed the initialisation of the boost.process child to
boost::process::child c(exec, args, boost::process::std_out > "console.txt", boost::process::std_in < output);
And then changed input to a std::ifstream, a file reader stream. Then to get the output I used
input.open("console.txt");
std::string line;
while (std::getline(input, line))
{
// Do something with line
}
input.close();
I also added remove("console.txt"); to the beginning of startStockFish() to attain a fresh text file.
I'm not confident that this is the best solution, as I am worried about what would happen if stockfish tried to write to console.txt as input was reading from it, but that hasn't seemed to occur or doesn't seemed to be an issue if it has occurred, so right now it is an adequate solution.