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Debugging CoreCLR

These instructions will lead you through debugging CoreCLR on Windows and Linux. They will be expanded to support macOS when we have good instructions for that.

Debugging CoreCLR on Windows

  1. Perform a build of the repo.
  2. Open solution <reporoot>\bin\obj\Windows_NT.<platform>.<configuration>\CoreCLR.sln in Visual Studio. <platform> and <configuration> are based on type of build you did. By default they are 'x64' and 'Debug'.
  3. Right click the INSTALL project and choose ‘Set as StartUp Project’
  4. Bring up the properties page for the INSTALL project
  5. Select Configuration Properties->Debugging from the left side tree control
  6. Set Command=$(SolutionDir)..\..\product\Windows_NT.$(Platform).$(Configuration)\corerun.exe
    1. This points to the folder where the built runtime binaries are present.
  7. Set Command Arguments=<managed app you wish to run> (e.g. HelloWorld.exe)
  8. Set Working Directory=$(SolutionDir)..\..\product\Windows_NT.$(Platform).$(Configuration)
    1. This points to the folder containing CoreCLR binaries.
  9. Press F11 to start debugging at wmain in corerun (or set a breakpoint in source and press F5 to run to it)
    1. As an example, set a breakpoint for the EEStartup function in ceemain.cpp to break into CoreCLR startup.

Steps 1-8 only need to be done once, and then (9) can be repeated whenever you want to start debugging. The above can be done with Visual Studio 2013.

Using SOS with windbg or cdb on Windows

If you know the path of the sos.dll for the version of your runtime, load it like .load c:\path\to\sos\sos.dll. Use can use the lm command to find the path of the "coreclr.dll" module. .loadby sos coreclr should also work.

For more information on SOS commands click here.

Debugging CoreCLR on OS X

To use lldb on OS X, you first need to build it and the SOS plugin on the machine you intend to use it. See the instructions in building lldb. The rest of instructions on how to use lldb for Linux on are the same.

Debugging CoreCLR on Linux

Only lldb is supported by the SOS plugin. gdb can be used to debug the coreclr code but with no SOS support. Visual Studio 2015 RTM remote debugging isn't currently supported.

  1. Perform a build of the coreclr repo.
  2. Install the corefx managed assemblies to the binaries directory.
  3. cd to build's binaries: cd ~/coreclr/bin/Product/Linux.x64.Debug
  4. Start lldb (the version the plugin was built with, currently 3.9): lldb-3.9 corerun HelloWorld.exe linux
  5. Now at the lldb command prompt, load SOS plugin: plugin load
  6. Launch program: process launch -s
  7. To stop annoying breaks on SIGUSR1/SIGUSR2 signals used by the runtime run: process handle -s false SIGUSR1 SIGUSR2
  8. Get to a point where coreclr is initialized by setting a breakpoint (i.e. breakpoint set -n LoadLibraryExW and then process continue) or stepping into the runtime.
  9. Run a SOS command like sos ClrStack or sos VerifyHeap. The command name is case sensitive.

You can combine steps 4-8 and pass everything on the lldb command line:

lldb-3.9 -o "plugin load" -o "process launch -s" -o "process handle -s false SIGUSR1 SIGUSR2" -o "breakpoint set -n LoadLibraryExW" corerun HelloWorld.exe linux

For .NET Core version 1.x and 2.0.x, is built for and will only work with version 3.6 of lldb. For .NET Core 2.1, the plugin is built for 3.9 lldb and will work with 3.8 and 3.9 lldb.

SOS commands

This is the full list of commands currently supported by SOS. lldb is case-sensitive unlike windbg.

Type "soshelp <functionname>" for detailed info on that function.

Object Inspection                  Examining code and stacks
-----------------------------      -----------------------------
DumpObj (dumpobj)                  Threads (clrthreads)
DumpArray                          ThreadState
DumpStackObjects (dso)             IP2MD (ip2md)
DumpHeap (dumpheap)                u (clru)
DumpVC                             DumpStack (dumpstack)
GCRoot (gcroot)                    EEStack (eestack)
PrintException (pe)                ClrStack (clrstack)
                                   bpmd (bpmd)

Examining CLR data structures      Diagnostic Utilities
-----------------------------      -----------------------------
DumpDomain                         VerifyHeap
EEHeap (eeheap)                    FindAppDomain
Name2EE (name2ee)                  DumpLog (dumplog)
DumpMT (dumpmt)                    CreateDump (createdump)
DumpClass (dumpclass)
DumpMD (dumpmd)
DumpModule (dumpmodule)
DumpIL (dumpil)

Examining the GC history           Other
-----------------------------      -----------------------------
HistInit (histinit)                FAQ
HistRoot (histroot)                Help (soshelp)
HistObj  (histobj)
HistObjFind (histobjfind)
HistClear (histclear)


By default you can reach all the SOS commands by using: sos [command_name] However the common commands have been aliased so that you don't need the SOS prefix:

bpmd            -> sos bpmd
clrstack        -> sos ClrStack
clrthreads      -> sos Threads
clru            -> sos U
createdump      -> sos CreateDump
dso             -> sos DumpStackObjects
dumpclass       -> sos DumpClass
dumpheap        -> sos DumpHeap
dumpil          -> sos DumpIL
dumplog         -> sos DumpLog
dumpmd          -> sos DumpMD
dumpmodule      -> sos DumpModule
dumpmt          -> sos DumpMT
dumpobj         -> sos DumpObj
dumpstack       -> sos DumpStack     
eeheap          -> sos EEHeap
eestack         -> sos EEStack
gcroot          -> sos GCRoot
histinit        -> sos HistInit
histroot        -> sos HistRoot
histobj         -> sos HistObj
histobjfind     -> sos HistObjFind
histclear       -> sos HistClear
ip2md           -> sos IP2MD
name2ee         -> sos Name2EE
pe              -> sos PrintException
soshelp         -> sos Help

Debugging core dumps with lldb

It is also possible to debug .NET Core crash dumps using lldb and SOS. In order to do this, you need all of the following:

  • The crash dump file. We have a service called "Dumpling" which collects, uploads, and archives crash dump files during all of our CI jobs and official builds.
  • On Linux, there is an utility called createdump (see doc) that can be setup to generate core dumps when a managed app throws an unhandled exception or faults.
  • To get matching runtime and symbol binaries for the core dump use the symbol downloader CLI extension:
    • Install the .NET Core 2.1 SDK.
    • Install the symbol downloader extension: dotnet tool install -g dotnet-symbol. Make sure you are not in any project directory with a NuGet.Config that doesn't include as a source.
    • Run dotnet symbol coredump to download the runtime binaries and symbols.
    • Check out the coreclr and corefx repositories at the appropriate commit for the appropriate source.
    • For further information see: dotnet-symbol.
  • lldb version 3.9. The SOS plugin (i.e. provided is now built for lldb 3.9. In order to install lldb 3.9 just run the following commands:
~$ echo "deb llvm-toolchain-trusty-3.9 main" | sudo tee /etc/apt/sources.list.d/llvm.list
~$ wget -O - | sudo apt-key add -
~$ sudo apt-get update
~$ sudo apt-get install lldb-3.9

Once you have everything listed above, you are ready to start debugging. You need to specify an extra parameter to lldb in order for it to correctly resolve the symbols for Use a command like this:

lldb-3.9 -O "settings set target.exec-search-paths <runtime-path>" -o "plugin load <>" --core <core-file-path> <host-path>
  • <runtime-path>: The path containing, as well as the rest of the runtime and framework assemblies.
  • <core-file-path>: The path to the core dump you are attempting to debug.
  • <host-path>: The path to the dotnet or corerun executable, potentially in the <runtime-path> folder.
  • <>: The path to, should be in the <runtime-path> folder.

lldb should start debugging successfully at this point. You should see stacktraces with resolved symbols for At this point, you can run plugin load <>, and begin using SOS commands, as above.

lldb-3.9 -O "settings set target.exec-search-paths /home/parallels/Downloads/System.Drawing.Common.Tests/home/helixbot/dotnetbuild/work/2a74cf82-3018-4e08-9e9a-744bb492869e/Payload/shared/Microsoft.NETCore.App/9.9.9/" -o "plugin load /home/parallels/Downloads/System.Drawing.Common.Tests/home/helixbot/dotnetbuild/work/2a74cf82-3018-4e08-9e9a-744bb492869e/Payload/shared/Microsoft.NETCore.App/9.9.9/" --core /home/parallels/Downloads/System.Drawing.Common.Tests/home/helixbot/dotnetbuild/work/2a74cf82-3018-4e08-9e9a-744bb492869e/Work/f6414a62-9b41-4144-baed-756321e3e075/Unzip/core /home/parallels/Downloads/System.Drawing.Common.Tests/home/helixbot/dotnetbuild/work/2a74cf82-3018-4e08-9e9a-744bb492869e/Payload/shared/Microsoft.NETCore.App/9.9.9/dotnet

Disabling Managed Attach/Debugging

The "COMPlus_EnableDiagnostics" environment variable can be used to disable managed debugging. This prevents the various OS artifacts used for debugging like the named pipes and semaphores on Linux/MacOS and shared memory on Windows from being created.

export COMPlus_EnableDiagnostics=0

Using Visual Studio Code

  • Install Visual Studio Code
  • Install the C# Extension
  • Open the folder containing the source you want to debug in VS Code
  • Open the debug window: ctrl-shift-D or click on the button on the left
  • Click the gear button at the top to create a launch configuration, select .NET Core from the selection dropdown
  • In the .NET Core Launch (console) configuration do the following
    • delete the preLaunchTask property
    • set program to the full path to corerun in the test directory
    • set cwd to the test directory
    • set args to the command line arguments to pass to the test
      • something like: [ "xunit.console.netcore.exe", "<test>.dll", "-notrait", .... ]
  • Set a breakpoint and launch the debugger, inspecting variables and call stacks will now work