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May 2002

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announce: StableTable Printer for GHC
by Bernard James POPE 01 May '02

01 May '02

Hi all, Ever had that deep desire to see what was in that pesky Stable Pointer/Name table? Probably not, but I did, and I wrote some code to print it out in all its glory from the safe confines of a Haskell program. I thought I'd share it with you just in case one day you too want to see what is lurking in there. You can find the code here: http://www.cs.mu.oz.au/~bjpop/code.html Here's an example of what it can do: ----------------------------------------------------- > module Main where > > import Stable > foreign import ccall "printStableTable" printStableTable :: IO () > > main = do let list = [1,2,3,4] ++ list > print $ take 5 list > trueSName <- makeStableName True > listSPtr1 <- newStablePtr list > listSPtr2 <- newStablePtr list > printStableTable If you run this program, you get as output: [1,2,3,4,1] --- Begin Stable Table --- stable_ptr_free = 3 0.val = NULL 0.ref = 0 0.sn_obj = NULL 1.val = (addr = 0x80963e0) True 1.ref = 0 1.sn_obj = <STABLE_NAME> 2.val = (addr = 0x500c11c4) (: 1 (: 2 (: 3 (: 4 cycle)))) 2.ref = 2 2.sn_obj = NULL 3.val = free at pos 4 3.ref = 0 3.sn_obj = NULL 4.val = free at pos 5 4.ref = 0 4.sn_obj = NULL ... blah blah blah, until the end of the table ... ----------------------------------------------------- Some features: It shows each entry in the table, and implicitly the free list, starting at stable_ptr_free. Data structures are printed in all their glory, including the current address of the value and the representation of the value. Cycles are detected! (See 2.val), but other types of sharing are not yet shown, however it is possible with tweaks. Why might you want this? 1. Debugging the stable pointer table implementation. 2. Debugging your own code. Stable pointers introduce the potential for nasty space leaks in Haskell programs. Much like malloc/free in C. If you forget to free your pointers then the space is retained. If you suspect that your program is leaking space in this way, then just whip out the table printer and see what is looks like. 3. Checking how much sharing you have in your data (with tweaks, as I said ealier you can show sharing). What do you need to use it? Just a current version of GHC. It should compile out of the box without any modification to GHC. Oh, and you have to compile your program with -prof to see all the data constructors. How does it print the data structures with cycles? The library also comes with yet another library for prinitng arbitrary Haskell Data structures, done via the FFI interface. NOTE: this is different from the GHCinternals library. We can detect cycles more easily if we are just printing things and not building new values on the heap at the same time. Probably this extra library should live somewhere by itself, but I have just bundled them together at the moment. Cheers, Bernie.

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