Segfault when calling show on an Integer of a certain size

You're not going to believe this.

$ ghc -e 'let k = show (10^184000) in k == k'
True
$ ghc -e 'let k = show (10^187000) in k == k'
True
$ ghc -e 'let k = show (10^186000) in k == k'
Bus error

That's right: there is a problem which affects Integer values which are 186000 digits long, but which does not affect values which are 187000 digits long.

So 10^184000 works fine, 10^187000 works fine, but 10^186000 doesn't. What about 10^185000? Well, it depends on your version of GHC. And on chance. GHC 7.10.0.20150123 is always happy with 10^185000, but GHC 7.8.3 crashes about two-thirds of the time:

$ ghc -e 'let k = show (10^185000) in k == k'
True
Segmentation fault

And it's a different kind of crash, too! A segmentation fault instead of a "bus error".

I have tried all the lengths in [1000,2000,..,100000], and some lengths are fine, some lengths have a bus error, and some lengths segfault. The most helpful lengths I've encountered give an error message about malloc:

$ ghc -e 'let k = show (10^264000) in k == k'
True
ghc(72417,0x107081000) malloc: *** error for object 0x107300000: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap

Sometimes it gives a slightly different error message:

$ ghc -e 'let k = show (10^264000) in k == k'
ghc(72453,0x107381000) malloc: *** error for object 0x107200128: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug
Abort trap

Anyway, a lot more people posted their results in the following reddit thread, without realizing that the problem had to do with the length: http://www.reddit.com/r/haskell/comments/31yajd/can_you_explain_this/

So far, only folks on OS X have managed to reproduce the problem. The problem occurs with ghci, runhaskell and ghc -e, but not with compiled binaries.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	hvr
Operating system
Architecture