Bug Report LibGpiodDriver OpenPin Null Check And Exception Handling

August 27, 2025 by StackCamp Team 68 views

Introduction

Hey guys, we've got a serious issue to discuss today regarding the LibGpiodDriver in the .NET IoT library. Specifically, there's a bug in how the OpenPin method handles null checks and exceptions. This can lead to some misleading errors and unexpected behavior, which we definitely want to get sorted out. So, let's dive into the details and see what's going on and how we can fix it.

Describe the bug

The bug lies within the OpenPin method of the LibGpiodDriver class, specifically in this line of code:

https://github.com/dotnet/iot/blob/main/src/System.Device.Gpio/System/Device/Gpio/Drivers/LibGpiodDriver.cs#L232

The issue is that the code checks if the construction of the LineHandle is null. However, constructors in C# (and most languages) cannot return null. Only the legendary Jon Skeet might be able to pull that off! 😉

This seemingly innocuous check has a significant consequence. It masks the real problem that occurs when opening a pin fails due to an underlying issue with the libgpiod library. Instead of throwing a relevant exception immediately, the code proceeds, and the error manifests later as an incorrect ObjectDisposedException. This makes debugging a real pain because the actual cause of the problem is hidden behind a misleading exception. We need to address this to ensure developers get clear and accurate error messages when things go wrong. This will help them to identify and resolve issues more efficiently.

Steps to reproduce

To reproduce this bug, follow these steps:

Choose a pin number that will cause the underlying LibGpiodV1.gpiod_chip_get_line call to fail. This could be a pin that doesn't exist or is already in use.
Create a new instance of LibGpiodDriver for gpiochip0:
```
var drv = new LibGpiodDriver(0); // gpiochip0
```

Create a GpioController using the driver:

using var gpioController = new GpioController(drv);

Attempt to open the problematic pin:
```
gpioController.OpenPin(pin)
```

By following these steps, you'll likely encounter the bug and observe the incorrect exception being thrown. This is a critical issue because it misdirects developers, making it harder to diagnose and fix problems in their code. It's like getting a flat tire and the car telling you the engine is broken – totally misleading!

Expected behavior

The expected behavior is that the code should throw a more appropriate exception, such as ArgumentException or another relevant exception, immediately after the LibGpiodV1.gpiod_chip_get_line call fails. This exception should also include information about the actual error, which can be obtained by decoding the LastError property. Providing specific error details is crucial for developers to understand what went wrong and how to resolve it.

In general, any calls to interop methods (methods that interact with native libraries) should always check for failure at the point of the call. This is a best practice in software development that ensures errors are caught and handled promptly. By doing so, we prevent the propagation of errors and provide clearer, more actionable feedback to the user.

Actual behavior

Instead of throwing a relevant exception, the code throws a mysterious ObjectDisposedException later in the process, specifically at:

System.Device.Gpio.Libgpiod.V1.LineHandle.get_Handle()

This is incredibly misleading because the ObjectDisposedException suggests that an object was accessed after it had been disposed, which isn't the root cause of the problem. The real issue is the failure to open the pin in the first place. Throwing ObjectDisposedException because the underlying native handle is null is also incorrect and adds to the confusion. It's like saying you're out of gas when the real problem is a broken fuel pump – the information is technically related, but it doesn't help you fix the immediate issue.

Versions used

Here are the versions of the relevant components used in the reported environment:

.NET SDK

> dotnet --info
.NET SDK:
 Version:           9.0.302
 Commit:            bb2550b9af
 Workload version:  9.0.300-manifests.183aaee6
 MSBuild version:   17.14.13+65391c53b

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19045
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.302\

.NET workloads installed:
 [maui-windows]
   Installation Source: VS 17.14.36310.24
   Manifest Version:    9.0.51/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maui\9.0.51\WorkloadManifest.json
   Install Type:              Msi

 [android]
   Installation Source: VS 17.14.36310.24
   Manifest Version:    35.0.78/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.android\35.0.78\WorkloadManifest.json
   Install Type:              Msi

 [maccatalyst]
   Installation Source: VS 17.14.36310.24
   Manifest Version:    18.5.9207/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maccatalyst\18.5.9207\WorkloadManifest.json
   Install Type:              Msi

 [ios]
   Installation Source: VS 17.14.36310.24
   Manifest Version:    18.5.9207/9.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.ios\18.5.9207\WorkloadManifest.json
   Install Type:              Msi

Configured to use loose manifests when installing new manifests.

Host:
  Version:      9.0.7
  Architecture: x64
  Commit:       3c298d9f00

.NET SDKs installed:
  2.1.802 [C:\Program Files\dotnet\sdk]
  5.0.416 [C:\Program Files\dotnet\sdk]
  9.0.302 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

.NET SDK (on the machine where the app is run)

$ dotnet --info
.NET SDK:
 Version:           8.0.412
 Commit:            819e1a9566
 Workload version:  8.0.400-manifests.9cf71931
 MSBuild version:   17.11.31+933b72e36

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  18.04
 OS Platform: Linux
 RID:         linux-arm64
 Base Path:   /usr/local/bin/dotnet/sdk/8.0.412/

.NET workloads installed:
Configured to use loose manifests when installing new manifests.
There are no installed workloads to display.

Host:
  Version:      8.0.18
  Architecture: arm64
  Commit:       ef853a7105

.NET SDKs installed:
  8.0.412 [/usr/local/bin/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 8.0.18 [/usr/local/bin/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 8.0.18 [/usr/local/bin/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
  None

Environment variables:
  DOTNET_ROOT       [/usr/local/bin/dotnet]

global.json file:
  Not found

System.Device.Gpio

Iot.Device.Bindings

Conclusion and Next Steps

Alright, folks, we've pinpointed a pretty critical bug in the LibGpiodDriver that can lead to misleading exceptions and make debugging a nightmare. The incorrect null check and the subsequent ObjectDisposedException are definitely areas we need to address.

The next step is to get this fixed! The best approach would be to:

Remove the unnecessary null check on the LineHandle constructor.
Check for failures immediately after calling interop methods like gpiod_chip_get_line.
Throw a more specific exception, such as ArgumentException, with a clear error message that includes details from LastError.

By tackling these points, we can significantly improve the reliability and usability of the LibGpiodDriver. This will not only make life easier for developers using the library but also ensure that our IoT solutions are more robust and easier to maintain. Let's get this fixed and keep making awesome IoT projects! 🚀

Thanks for reading, and stay tuned for updates on this issue. Keep coding, everyone! 💻