[Haskell-cafe] RE: Examining the Haskell stack (read at your own risk ; -))

2 Dec 2005

      On 02 December 2005 14:17, Joel Reymont wrote:
...
On Dec 2, 2005, at 2:08 PM, Simon Marlow wrote:
...
It looks like your crash happened in the SSL library, and you have a
useful stack trace there.
This is contrived in that I already know where the error is and it
clearly points to SSL_free. I'm trying to figure out how I would have
gotten to that call to getHostByName.
...
r22 is a pointer to the stack, not a pointer to code, so you can't
disassemble it, you need to display memory (as I described in
separate mail).
Quoting you:
---
   gdb> p16 $r22
which prints 16 words of memory backwards (the way I like it) starting
at the addresss in $r22.  when displaying memory this way, gdb very
handily prints the symbol name for words that point into the program.
You can then pick things off the stack that look like return addresses
and disassemble them, if you want.
---
And p16 is defined in .gdbinit as:
define p16
pmem $arg0 16
end
Printing the 16 words gives me the printout below but where do I find
my Haskell function? The code tha causes the crash looks like this:
maybeFreeSSL :: MaybeSSL -> IO ()
maybeFreeSSL tmv =
     do putStrLn $ "maybeFreeSSL invoked"
        mssl <- atomically $ swapTMVar tmv Nothing
        case mssl of
          Nothing -> return ()
          Just (ssl, _, _) -> do sslFree ssl
                                 sslFree ssl
Is there a way to have maybeFreeSSL in the trace? I compiled the app
with -debug but the libraries and the above maybeFreeSSL code was
compiled without it.
Thanks, Joel
P.S.
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x00000019
0x00568fe0 in sk_pop_free ()
(gdb) p16 $r22
0x137a40c:      0x1ce63c 
0x137a408:      0x0
0x137a404:      0x0
0x137a400:      0x18fd0 
0x137a3fc:      0xd6dbc 
0x137a3f8:      0xd6f38 
0x137a3f4:      0x13ef148
0x137a3f0:      0x0
0x137a3ec:      0x2605c 
0x137a3e8:      0x25c80 
0x137a3e4:      0x13ef150
0x137a3e0:      0x27030 
0x137a3dc:      0x1d22cc 
0x137a3d8:      0x26f6c 
0x137a3d4:      0x13f38f4
0x137a3d0:      0x243f4 
0x137a3cc:      0xc3270 
Ok, you want a crash course in reading the Haskell stack.

Each xxx_info symbol is a return address.  The other values are the
contents of stack frames: values saved for use at the return address.

A return address also has an associated info table (use pinfo in the
.gdbinit I sent you for disaplying the info table, eg. pinfo s7FI_info).
Understanding info tables is beyond the scope of this message; please
see InfoTables.h in the GHC sources.

You probably want to know what each return address corresponds to:

  - the ones beginning "stg_" are in the runtime, you can see the code
for
    these in the GHC sources (grep in ghc/rts).

  - the ones that look like "s7FI_info" are local symbols.  You might be
    able to find which module it comes from by grepping the .o files in
    your program and the .a library files.  If you find it in a library,
    you can use 'nm' on the library to get a better idea of what
function
    the symbol is associated with.

    One problem is that because these are local symbols, you might find
    the same symbol in multiple places, and you have to make an educated
    guess as to which is the appropriate one (or use the disassembly to
    distinguish).

    To map this back to Haskell code, you need to recompile the original
    module and dump the intermediate code with -ddump-stg.
Understanding
    the output of -ddump-stg is beyond the scope of this message :-)

Cheers,
	Simon