ghc-debug is a set of libraries which allow you to inspect the heap of a running Haskell program from an external debugger.

For example, you could use this library to

Getting Started

There are two parts to using ghc-debug. Firstly the application you want to inspect has to be instrumented using the withGhcDebug function from GHC.Debug.Stub. This just wraps the normal main function of your executable, when it is executed it will create a socket by which a debugger can connect and issue requests to. The location of the socket can be controlled by setting the GHC_DEBUG_SOCKET variable when the executable is run.

import GHC.Debug.Stub

main = withGhcDebug normalMain

Note: To enable source information you should also compile your application and dependencies with -finfo-table-map and optionally -fdistinct-constructor-tables.

A simple debugger

The most productive way to use ghc-debug is to write your own heap analysis scripts. Fortunately, this is also quite simple. Here is a simple, complete, debugger which connects to the /tmp/ghc-debug socket, requests the GC roots and then decodes the first one up to depth 10 before printing the result to the user.

import GHC.Debug.Client

main = withDebuggeeConnect "/tmp/ghc-debug" p1

p1 :: Debuggee -> IO ()
p1 e = do
  pause e
  g <- run e $ do
        (r:_) <- gcRoots
        buildHeapGraph (Just 10) r
  putStrLn (ppHeapGraph (const "") h)

The API for writing debuggers is described in the GHC.Debug.Client module.

There are many more examples in the test/Test.hs file.


A convenient way to use ghc-debug is to take a snapshot of the heap and then perform further analysis on the snapshot rather than connecting to a running process. Snapshotting utilities are in the GHC.Debug.Snapshot module. A snapshot can be created using the makeSnapshot program, it will pause the process and then save a snapshot to the /tmp/ghc-debug-snapshot file.

import GHC.Debug.Client
import GHC.Debug.Snapshot

main = withDebuggeeConnect "/tmp/ghc-debug" (\d -> makeSnapshot d "/tmp/ghc-debug-snapshot")

A snapshot can be then used for further analysis. For example, we can run p1 on the snapshot by using snapshotRun instead of withDebuggeeConnect. The same programs can be used with snapshots but requests such as pausing and resuming are just ignored.

import GHC.Debug.Client

main = snapshotRun "/tmp/ghc-debug-snapshot" p1

High-Level Analysis

There are also some more high-level analysis tools already packaged with the library. Mostly as an idea about what sort of thing you could program yourself.

These analysis modes are implemented in terms of the more low-level traversal functions.

Other Resources

How does it work?

We call the process we want to debug the debuggee and the process which does the debugging the debugger. Whilst the debuggee is running it calls the C function start which creates a unix domain socket (which is set from GHC_DEBUG_SOCKET). The debugger starts and connects to the socket.

Once the debugger is connected it can send requests to the debuggee to control and inspect the RTS.

How do I use it?

The project needs to built with a development version of GHC from this branch. Then you can use normal cabal commands to build this library. The easiest way to do this is with nix. The nix shell uses the development version of GHC so you don’t have to build it yourself.

$ nix-shell
$ cabal new-build all

Automated Testing

There are hspec tests, that can be run with cabal:

cabal new-test all

Unexpected Build Failures

If you encounter dependencies failing to build but there’s a patch for the library in head.hackage then you may need to delete ~/.cabal/packages/ so that the fresh patch is visible. This is probably a bug in cabal!