Debugging the RTS

Simon Peyton Jones simonpj at
Fri May 15 11:17:50 UTC 2015

Good idea.  I think it’d great if Jost (or someone else) felt able to update those wiki pages with the info in his email.



From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Lennart Kolmodin
Sent: 15 May 2015 11:31
To: Jost Berthold
Cc: ghc-devs at
Subject: Re: Debugging the RTS

Searching for online resources brought me to and

This mail seems to be a more detailed description than available on the wiki. That page refers to GHC 6.12.
Would it make sense to add some info to the RuntimeSystem<> page?

2015-05-14 16:21 GMT+02:00 Jost Berthold <berthold at<mailto:berthold at>>:
Hi Eric,

definitely not bit-rotted - the #if DEBUG code is what you get when you compile with -debug.

Short roadmap:

In general, GHC is built in many "ways", the -debug way is one.
Some ways (like -threaded, -eventlog, -debug) are RTS-only, others (like -prof) lead to different library code.

Ways are defined in compiler/main/DynFlags.hs as a Haskell data structure.
In _dynamic_flags_, the actual flag strings for the ghc invocation are defined (like -prof, -threaded), they will activate the respective _Way_.
_wayTag_, in turn, defines short names used as a suffix for *.o and *.a files, for instance *.p_o for profiling, *.l_o for eventlog.
A number of other functions in there customise behaviour depending on the ways. Note _wayOptc_ which sets some options for the C compiler, like -DTRACING for the -eventlog way. However, it does not define DEBUG for the debug way!

In mk/<> you will find all the short names, and some more options are defined (WAY_*_HC_OPTS). These definitions are for the driver script, and pass on the right (long-name) options to the Haskell compiler to activate what is inside DynFlags (like -prof for WAY_p_HC_OPTS).
Here we find
WAY_debug_HC_OPTS= -static -optc-DDEBUG -ticky -DTICKY_TICKY
which is what you were looking for

To build a GHC with debug-way RTS, you need to add GhcRtsWays += debug to your<> (also see mk/<>{.in}, which gathers the default ways to build depending on platform and build configuration).
I guess this might produce quite a number of new warnings and errors if you do it on a new platform...

Once you have built this, you can compile with -debug and pass RTS options to get debug traces and activate sanity checks, like so:
ghc -debug -myprogram.hs -o myprogramDebug
./myprogramDebug +RTS -Ds -DS
(-DS = sanity checks on, -Ds = scheduler tracing)
The usage message (see /rts/RtsFlags.c) tells you more options you can use.

Happy debugging!
/ Jost

On 05/14/2015 10:00 PM, ghc-devs-request at<mailto:ghc-devs-request at> wrote:
Date: Thu, 14 May 2015 18:36:04 +1000
From: Erik de Castro Lopo <mle+hs at<mailto:mle%2Bhs at>>
To: ghc-devs at<mailto:ghc-devs at>
Subject: Debugging the RTS
Message-ID: <20150514183604.2476baaaacd6a85ef1430b37 at<mailto:20150514183604.2476baaaacd6a85ef1430b37 at>>
Content-Type: text/plain; charset=US-ASCII

Hi all,

I'm trying to debug a AArch64/Linux specific RTS issue and digging
around in the rts C code, I see a large amount of code wrapped in
"#if DEBUG" type statements, but no way to enable in either
mk/<> or it seems in any of the other build related settings
files in the mk/ directory.

The only way I have found to enable this debug code that actually
works is to enable this DEBUG code is to edit mk/<> as follows:


However, once I do that I get a bunch of C synatx errors.

I am doing this wrong or has the DEBUG code bit rotted?


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