[ocaml-ctypes] Spinning process on osx

[Daniel Bünzli]

I managed to get a little bit more info by compiling libffi with `--enable-debug` and everything compiled with -g under ocaml.

Using lldb it seems something bad happens during the setup of ffi_call call in ctypes's ctypes_call function. See the `->' in the second disassemble below which points to this line:

https://github.com/ocamllabs/ocaml-ctypes/blob/ocaml-ctypes-0.2.2/src/ctypes-foreign-base/ffi_call_stubs.c#L302

Daniel

> lldb bug.native 
Current executable set to 'bug.native' (x86_64).
(lldb) run
Process 43924 launched: '/Users/dbuenzli/bug/bug.native' (x86_64)
Process 43924 stopped
* thread #1: tid = 0x26ff7d, 0x00000001007a37b0, queue = 'com.apple.main-thread, stop reason = EXC_BAD_ACCESS (code=2, address=0x1007a37b0)
    frame #0: 0x00000001007a37b0
-> 0x1007a37b0:  xorb   %ch, 65539(%rbx)
   0x1007a37b6:  addb   %al, (%rax)
   0x1007a37b8:  addl   (%rax), %eax
   0x1007a37ba:  addb   %al, (%rax)
(lldb) bt
* thread #1: tid = 0x26ff7d, 0x00000001007a37b0, queue = 'com.apple.main-thread, stop reason = EXC_BAD_ACCESS (code=2, address=0x1007a37b0)
    frame #0: 0x00000001007a37b0
    frame #1: 0x0000000100076d50 bug.native`ctypes_call(function=4303202416, callspec_=4303295728, argwriter=4302976944, rvreader=4303196432) + 304 at ffi_call_stubs.c:298
    frame #2: 0x000000010003ae3c bug.native`.L141 + 24
(lldb) up
frame #1: 0x0000000100076d50 bug.native`ctypes_call(function=4303202416, callspec_=4303295728, argwriter=4302976944, rvreader=4303196432) + 304 at ffi_call_stubs.c:298
(lldb) di --mixed -c 13 -s $pc-20
bug.native`ctypes_call + 284 at ffi_call_stubs.c:293
   0x100076d3c:  clc    
   0x100076d3d:  addq   %rdi, %r15
bug.native`ctypes_call + 288 at ffi_call_stubs.c:298
   0x100076d40:  callq  0x1000870c0               ; caml_copy_int64
   0x100076d45:  movq   %r14, %rdi
   0x100076d48:  movq   %rax, %rsi
   0x100076d4b:  callq  0x100094bd0               ; caml_callback
bug.native`ctypes_call + 304 at ffi_call_stubs.c:302
-> 0x100076d50:  movq   -64(%rbp), %rax
   0x100076d54:  movq   64(%rax), %rdi
bug.native`ctypes_call + 312 at ffi_call_stubs.c:300
   0x100076d58:  movq   -56(%rbp), %rax
   0x100076d5c:  movq   8(%rax), %rsi
bug.native`ctypes_call + 320 at ffi_call_stubs.c:302
   0x100076d60:  movq   %r15, %rdx
   0x100076d63:  movq   %r13, %rcx
   0x100076d66:  callq  0x1000979f0               ; symbol stub for: caml_system__code_end
(lldb) frame variable
(value) function = 4303202416
(value) callspec_ = 4303295728
(value) argwriter = 4302976944
(value) rvreader = 4303196432
(caml__roots_block) caml__roots_function = {
  next = 0x0000000000000000
  ntables = 4
  nitems = 1
  tables = {
    [0] = 0x00007fff5fbff6d8
    [1] = 0x00007fff5fbff6d0
    [2] = 0x00007fff5fbff6c8
    [3] = 0x00007fff5fbff6c0
    [4] = 0x000000010003ad20
  }
}
(caml__roots_block) caml__roots_function = {
  next = 0x0000000000000000
  ntables = 4
  nitems = 1
  tables = {
    [0] = 0x00007fff5fbff6d8
    [1] = 0x00007fff5fbff6d0
    [2] = 0x00007fff5fbff6c8
    [3] = 0x00007fff5fbff6c0
    [4] = 0x000000010003ad20
  }
}
(caml__roots_block *) caml__frame = <register r12 is not available>
(int) caml__dummy_function = 0
(size_t) arg_array_offset = <variable not available>
(char *) return_slot = <register r15 is not available>
(void (*)()) cfunction = <variable not available>
(callspec *) callspec = <no location, value may have been optimized out>
(int) roffset = <no location, value may have been optimized out>
(size_t) bytes = <no location, value may have been optimized out>
(char *) callbuffer = <no location, value may have been optimized out>