SetStdHandle could not redirect stdout and stderr in native DLLs to files in C#

Question

I need to call some native DLLs in C# and redirect the stdout and stderr in the native DLLs to files. Have tried a very simple example but couldn't get it working.

My experiment started with building a native DLL (StdStreamRedirect.dll) with below code.

#include <stdio.h>

extern __declspec(dllexport) void Test();

void Test()
{
    fprintf(stderr, "%s", "STDERR\n");

    fprintf(stdout, "%s", "STDOUT\n");
}

Then I created a C# console application as shown below.

namespace StdStreamRedirectionTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var stdErrFilePath = "stderrredirect.txt";
            var stdOutFilePath = "stdoutredirect.txt";
            var errFileStream = File.Open(stdErrFilePath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite);
            SetStdHandle(-12, errFileStream.SafeFileHandle.DangerousGetHandle());
            var outFileStream = File.Open(stdOutFilePath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite);
            SetStdHandle(-11, outFileStream.SafeFileHandle.DangerousGetHandle());

            var dllAddress = LoadLibrary(@"StdStreamRedirect.dll");
            var procAddress = GetProcAddress(dllAddress, "Test");
            var action = Marshal.GetDelegateForFunctionPointer(procAddress, typeof(Action)) as Action;
            action?.Invoke();

            errFileStream.Flush();
            outFileStream.Flush();
        }

        [DllImport("Kernel32.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi, SetLastError = true)]
        public static extern IntPtr LoadLibrary([MarshalAs(UnmanagedType.LPStr)]string fileName);

        [DllImport("Kernel32.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        public static extern IntPtr GetProcAddress(IntPtr modulePtr, [MarshalAs(UnmanagedType.LPStr)] string procName);

        [DllImport("Kernel32.dll", SetLastError = true)]
        private static extern int SetStdHandle(int device, IntPtr handle);
    }
}

I was expecting the stdout and stderr from the native DLL would be redirected into stdoutredirect.txt and stderrredirect.txt but nothing was found in the two files after ran the C# console app.

To eliminate the possibility that the native dll wasn't called correctly in the first place, I also tried to comment out the std stream redirect code (the first six lines of Main method), and ran the c# console app, the stdout and stderr were printed in the console correctly.

Would you please help to see what went wrong in my code? Thanks a lot.

Update

I figured it out. It turned out that there was nothing wrong with the C# code which redirects the stdout and stderr. The problem was from the C code. According to this answer, at least the stdout is buffered. That's why I didn't see them in the redirected files because they weren't flushed yet so that there was nothing to redirect. After I forced the stdout and stderr to flush as shown below, it worked!

void Test()
{
    fprintf(stderr, "%s", "STDERR\n");
    fflush(stderr);
    fprintf(stdout, "%s", "STDOUT\n");
    fflush(stdout);
}

I'll still keep this question here. Because it is still not clear to me that why after the the C# app terminated, the buffered stdout and stderr were still not flushed and got redirected?

That's not probably the issue, but you are not checking return values from the called PInvoke methods. Also, unless you really need to use PInvoke, I'd recommend you to try https://learn.microsoft.com/en-us/dotnet/api/system.console.setout?view=netframework-4.8 instead. — Eugene Podskal, Aug 06 '19 at 06:24
@Eugene Podskal I removed the return values check of PInvoke methods on purpose in order to simplify the code pasted here. Thanks for the comment :) — Shinbo, Aug 08 '19 at 00:21
Very interesting, an identical approach does not work for me. I'm trying to capture from within a C# Console program the stdout & stderr output of a C++ DLL function invocation. @Shinbo after you make the 2 SetStdHandle calls in your program, do you still see the C++ program's output? I do see it in my Console program, which makes me believe that the SetStdHandle redirections do not work. — eugen_nw, Jun 21 '23 at 16:30

SetStdHandle could not redirect stdout and stderr in native DLLs to files in C#

0 Answers0