freeGroup: block size is zero

I'm trying to extract pixels from OpenGL and read them into a ForeignPointer, but getting a segfault while doing a peek operation.

The fault is on line 240 in peekArray. It crashes in different ways depending on the arguments to readPixelArray.

with readPixelArray 0 0 100 100:

(lldb) run
Process 26467 launched: '/Users/scott/Code/grids/dist/build/grids/grids' (x86_64)
0x00000001015b3010
Process 26467 stopped
* thread #1: tid = 0x23d3cc, 0x0000000100621ead grids`evacuate + 157, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xfffffffffffffff8)
    frame #0: 0x0000000100621ead grids`evacuate + 157
grids`evacuate + 157:
-> 0x100621ead:  movl   -0x8(%r12), %esi
   0x100621eb2:  leal   -0x1(%rsi), %ecx
   0x100621eb5:  cmpl   $0x3b, %ecx
   0x100621eb8:  ja     0x1006222a9               ; evacuate + 1177

with readPixelArray 0 0 200 200:

(lldb) run Process 26647 launched: '/Users/scott/Code/grids/dist/build/grids/grids' (x86_64) 0x00000001095d8010 grids: internal error: freeGroup: block size is zero

(GHC version 7.8.3 for x86_64_apple_darwin)

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

Process 26647 stopped

• thread #1 (closed): tid = 0x23f4d4, 0x00007fff8b2cc866 libsystem_kernel.dylib__pthread_kill + 10, queue = 'com.apple.main-thread', stop reason = signal SIGABRT frame #0: 0x00007fff8b2cc866 libsystem_kernel.dylib__pthread_kill + 10

libsystem_kernel.dylib`__pthread_kill + 10: -> 0x7fff8b2cc866: jae 0x7fff8b2cc870 ; __pthread_kill + 20

0x7fff8b2cc868: movq %rax, %rdi 0x7fff8b2cc86b: jmpq 0x7fff8b2c9175 ; cerror_nocancel 0x7fff8b2cc870: ret

The "freeGroup: block size is zero" message appears in the second output.

I'm stumped trying to debug this one. I didn't get as far as trying to go backwards in time debugging it because of lack of familiarity with lldb.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture

changed weight to 5

added RTS Tbug Trac import labels

Attached file Grids.hs ($1017).

According to my reading of https://www.opengl.org/sdk/docs/man2/xhtml/glReadPixels.xml, glReadPixels with format=GL_RGBA and type=GL_UNSIGNED_INT will write one unsigned int per channel per pixel, so it will overwrite the array you allocated by a factor of 4. Looks like it is overwriting some GC information as evacuate and freeGroup are GC-related functions (not surprising that the GC would run during peekArray, as it has to allocate a large list).

Yep, spot on! Thanks for the help, can close.

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added runtime crash label

added Pnormal label