Question

I’m developing a small haskell program that uses an external static library I’ve developed in C++. It accesses the lib through ghc’s FFI (foreign function interface). Inside this library I would like to do some output to the console. However, it looks to me like the c++ side of things does not have a correct handle to stdout because output does not appear on the console. So then, my questions are:

• Does ghc hijack these three streams (stdout, stdin, stderr) or is libstdc++ simply not initializing them because I’m linking with ghc?
• Do my FFI imports need to be “safe” if they write to stdout?
• How can I pass stdout to a C function? Should I simply pass it directly or do I need a C type?

Additional notes: I’m linking libstdc++ directly to the executable (i.e. ghc -lstdc++ …) which I naively assumed would be the correct way of doing this. Seems to work well

Disclaimer: Still pretty new to Haskell, so baby steps for now ;P

Answer 1

Your problem does appear to be that libstdc++ is not being initialized. I’m not entirely sure why — -lstdc++ is sufficient on my system — but see if it works the other way around.

Main.hs:

{-# LANGUAGE ForeignFunctionInterface #-}
module Main where
foreign export ccall "Main_main" main :: IO ()
foreign import ccall driver_callback :: IO ()
main = putStrLn "Now in Haskell" >> driver_callback

driver.cc:

#include <iostream>

extern "C" {
# include "HsFFI.h"

#  ifdef __GLASGOW_HASKELL__
#   include "Main_stub.h"
    extern void __stginit_Main(void);
#  endif

    void driver_callback(void) {
        std::cout << "Back in C++" << std::endl;
    }
}

int main(int argc, char **argv) {
    hs_init(&argc, &argv);
# ifdef __GLASGOW_HASKELL__
    hs_add_root(__stginit_Main);
# endif

    std::cout << "Starting in C++" << std::endl;

    Main_main();

    hs_exit();
    return 0;
}

Compiling:

$ ghc -c --make Main
[1 of 1] Compiling Main             ( Main.hs, Main.o )
$ ghc --make -no-hs-main -lstdc++ Main driver.cc
Linking Main ...
$ ./Main
Starting in C++
Now in Haskell
Back in C++