Debugging the RTS

Lennart Kolmodin kolmodin at
Fri May 15 10:30:59 UTC 2015

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

> 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 wrote:
>> Date: Thu, 14 May 2015 18:36:04 +1000
>> From: Erik de Castro Lopo <mle+hs at>
>> To: ghc-devs at
>> Subject: Debugging the RTS
>> Message-ID: <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?
>> Cheers,
>> Erik
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