Not able to get PInvoke error message on Linux and MacOs

[DevMk]

Hello,

I see that Marshal.GetLastWin32Error() returns proper errno, however Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error()) always throws "The operation completed successfully" instead corresponding error.

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication {
 public class Program {
  [DllImport("libSystem.dylib", EntryPoint = "fopen", SetLastError = true)] // libc.so.6 for linux
  extern static private IntPtr FOpen(string path, string mode); // fopen was used for example. Same flow for other calls


  public static void Main(string[] args) {
   var f = FOpen("some-not-existing-file", "r");

   if (f == IntPtr.Zero) {
    Console.WriteLine("Error code is {0}", Marshal.GetLastWin32Error());

    Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error());
   }
  }
 }
}

Actual result

Admins-Mac:console admin$ dotnet --version
1.0.1

Admins-Mac:console admin$ dotnet run
Error code is 2  <--- File not found error code

Unhandled Exception: System.Runtime.InteropServices.COMException: The operation completed successfully.
 (Exception from HRESULT: 0x80070000)
   at System.Runtime.InteropServices.Marshal.ThrowExceptionForHRInternal(Int32 errorCode, IntPtr errorInfo)
   at ConsoleApplication.Program.Main(String[] args) in /Users/admin/console/Program.cs:line 24

Expected result:

Exception with "No such file or directory" message

[mellinoe]

I believe this is because you are calling GetLastWin32Error twice, essentially (once directly and once through the GetHRForLastWin32Error method). The first call is giving you the correct error code, but the second call returns "success" because the first call wiped the error code out. @stephentoub Is that accurate?

[DevMk]

Yes, definitely. Thank you.

Is there a way to get both GetLastWin32Error and GetHRForLastWin32Error? Or get HR based on error.

One for special error handling, second one for exception message.

[danmoseley]

@DevMk you could inline this.

        public static int GetHRForLastWin32Error()
        {
            int dwLastError = GetLastWin32Error();
            if ((dwLastError & 0x80000000) == 0x80000000)
                return dwLastError;
            else
                return (dwLastError & 0x0000FFFF) | unchecked((int)0x80070000);
        }

[danmoseley]

I think this is resolved -- please reopen if not.

[stephentoub]

I believe this is because you are calling GetLastWin32Error twice, essentially (once directly and once through the GetHRForLastWin32Error method). The first call is giving you the correct error code, but the second call returns "success" because the first call wiped the error code out. @stephentoub Is that accurate?

Hmm... this appears to be a difference between desktop and core. This program:

using System;
using System.Runtime.InteropServices;

class Program
{
    static void Main()
    {
        CopyFile("doesntexist", "blah", true);
        Console.WriteLine(Marshal.GetLastWin32Error());
        Console.WriteLine(Marshal.GetLastWin32Error());
    }

    [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
    static extern bool CopyFile(string lpExistingFileName, string lpNewFileName, bool bFailIfExists);
}

on desktop prints:

2
2

but on core prints:

2
0

@yizhang82, @jkotas, is this difference by design?

[stephentoub]

Ah, nevermind, I realized my example was flawed. This code:

using System;
using System.Runtime.InteropServices;

class Program
{
    static void Main()
    {
        CopyFile("doesntexist", "blah", true);
        int a = Marshal.GetLastWin32Error();
        int b = Marshal.GetLastWin32Error();
        Console.WriteLine(a);
        Console.WriteLine(b);
    }

    [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
    static extern bool CopyFile(string lpExistingFileName, string lpNewFileName, bool bFailIfExists);
}

prints out:

2
2

on core. The issue in my first example (and in the original example in this issue) is that Console.WriteLine is using a P/Invoke with SetLastError, and the known, by-design difference in core is that SetLastError zeros out the error code when making a P/Invoke (dotnet/coreclr#614).

[lostmsu]

Looks like the solution @danmosemsft proposed is not valid. I am using libc symlink call, and while GetLastWin32Error correctly returns EEXIST (17), simply adding 0x8007000 to it using the method suggested in GetHRForLastWin32Error and then passing result to ThrowExceptionForHR does not give a meaningful error message.

[janvorli]

@lostmsu the GetLastWin32Error is supposed to return windows error codes, not the Unix ones. So the error code 17 is ERROR_NOT_SAME_DEVICE (if the code is in decimal). See https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes--0-499-.

[lostmsu]

@janvorli GetLastWin32Error is currently returning and possibly also cleaning errno on Linux, which is sort of what I expect it to do.

This issue though is about getting errno and a meaningful error message after a failed PInvoke call on Linux and MacOS. The code in question was proposed as a workaround, and it does not solve the problem.

I think the issue should be reopened.