[gambit-list] Segfault on OSX using Gambit's C Interface

Marc Feeley feeley at iro.umontreal.ca
Wed Dec 16 18:00:18 EST 2015


I have improved the FFI by introducing the ___return(val) macro which should be used from now on instead of ___result=val and ___result_voidstar=val.  So your *->My function can be coded like this:

(define *->My (c-lambda (My*/nonnull) My "___return(*___arg1);"))

This will hide the implementation of the FFI and the use of ___result and ___result_voidstar, which was confusing.

Check the updated Gambit manual for details.

Marc

> On Dec 14, 2015, at 2:46 PM, Paolo Carlesi <pcarlesi at gmail.com> wrote:
> 
> Thank you for the workaround!
> 
> What is the best way to generate a foreign c-type object ?
> 
> This seems to me the best approach (sorry if it's obvious to everybody but me):
> 
> (define make-My (c-lambda () My "___result_voidstar = ___EXT(___alloc_rc)(sizeof(struct My));"))
> (define My->* (c-lambda (My) My*/nonnull "___result_voidstar = &___arg1;"))
> 
> has the advantage of not wasting time / space creating a new foreign object and a copy, like *->My below does.
> It also doesn't trigger the c-lambda bug, not that I have verified carefully yet.
> Members can easily be set up with a syntax like (My-x-set! (My->* xoxo) #xdeadbeef) .
> Is this approach correct?
> 
> Thanks Again
> 
> Paolo
> 
> Il giorno 14/dic/2015, alle ore 18:02, Marc Feeley <feeley at iro.umontreal.ca> ha scritto:
> 
>> There appears to be a bug in the FFI when using a c-lambda with inline code that returns a foreign object.  So for now you will have to define a C function that does the work and use the name of that function in the c-lambda.  In other words:
>> 
>> (c-declare #<<c-declare-end
>> 
>> My indirect_My_star(My *ptr) { return *ptr; }
>> 
>> c-declare-end
>> )
>> 
>> (define *->My (c-lambda (My*/nonnull) My "indirect_My_star"))
>> 
>> Marc
>> 
>> 
>>> On Dec 14, 2015, at 4:55 AM, Paolo Carlesi <pcarlesi at gmail.com> wrote:
>>> 
>>> Hello,
>>> 
>>> I'm experiencing regular segfaults on OSX 10.9.5, with the sample code shown below. 
>>> Tested with Gambit 4.8.1 + GCC 4.9  and with packaged Gambit 4.7.3 + LLVM.
>>> No issue at all on Linux, GCC 4.8, with both Gambit 4.2.8 and Gambit 4.8.2.
>>> Of course I may be doing something silly.
>>> 
>>> A simple struct, "My" is generated inside a c procedure, then passed multiple times to another c-lambda procedure (*->My).
>>> It's a simplified model of the code produced by a FFI SDL2 Library I'm using.
>>> The doc says that the foreign object (xoxo in the example) should not be released while the scheme code is still referencing it.
>>> I am seeing the refcount going wildly in the negative, one for each call to *->My
>>> The only way -- clumsy -- I have found to avoid this is to increment the reference count with ___addref_rc ( not shown here ).
>>> 
>>> What's wrong here?
>>> Thanks!
>>> 
>>> 
>>> LLDB output:
>>> 
>>> Process 48149 launched: '/Users/paolo/scheme/spazz/fromlin' (x86_64)
>>> Process 48149 stopped
>>> * thread #1: tid = 0x40e3c9, 0x000000010000770c fromlin`___release_rc + 28, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
>>>  frame #0: 0x000000010000770c fromlin`___release_rc + 28
>>> fromlin`___release_rc + 28:
>>> -> 0x10000770c:  movq   %rax, (%rdx)
>>> 0x10000770f:  movq   %rdx, 0x8(%rax)
>>> 0x100007713:  jmp    0x100010240               ; ___free_mem
>>> 0x100007718:  nopl   (%rax,%rax)
>>> 
>>> stack trace:
>>> 
>>> (lldb) bt
>>> * thread #1: tid = 0x40e3c9, 0x000000010000770c fromlin`___release_rc + 28, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
>>> * frame #0: 0x000000010000770c fromlin`___release_rc + 28
>>>  frame #1: 0x0000000100002579 fromlin`___release_fn0 + 9
>>>  frame #2: 0x0000000100009009 fromlin`___garbage_collect + 3497
>>>  frame #3: 0x00000001000091bf fromlin`___alloc_scmobj + 255
>>>  frame #4: 0x000000010000d4eb fromlin`___POINTER_to_SCMOBJ + 43
>>>  frame #5: 0x000000010000d5e4 fromlin`___STRUCT_to_SCMOBJ + 20
>>>  frame #6: 0x0000000100002112 fromlin`___H__20_fromlin + 3378
>>>  frame #7: 0x0000000100004aa4 fromlin`___call + 372
>>>  frame #8: 0x0000000100004bc6 fromlin`___setup_pstate + 262
>>>  frame #9: 0x00000001000063be fromlin`___setup + 3950
>>> (lldb) 
>>> 
>>> 
>>> ;; SAMPLE CODE
>>> 
>>> (c-declare #<<c-declare-end
>>> 
>>> typedef struct My {
>>> int r;
>>> } My;
>>> 
>>> c-declare-end
>>> )
>>> 
>>> (begin
>>> (c-define-type My (struct "My"))
>>> (c-define-type My* (pointer My (My*)))
>>> (c-define-type My*/nonnull (nonnull-pointer My (My*)))
>>> (c-define-type My*/release-rc (nonnull-pointer My (My*) "___release_My")))
>>> 
>>> 
>>> (begin
>>>  (c-declare #<<c-declare-end
>>> static ___SCMOBJ ___release_My( void* ptr ) {
>>>  ___EXT(___release_rc)( ptr );
>>>  return ___FIX(___NO_ERR);
>>> }
>>> c-declare-end
>>>  )
>>> 
>>>  (define alloc-My (c-lambda () My*/release-rc "___result_voidstar = ___EXT(___alloc_rc)( sizeof( struct My ));"))
>>>  (define *->My (c-lambda (My*/nonnull) My "___result_voidstar = (My*)___arg1;"))
>>>  (define My-r (c-lambda (My*/nonnull) unsigned-int8 "___result = ___arg1->r;"))
>>>  (define My-r-set! (c-lambda (My*/nonnull unsigned-int8) void "___arg1->r = ___arg2;")))
>>> 
>>> (define (evil)
>>> (let ((xoxo (alloc-My)))
>>>    (My-r-set! xoxo #xff)
>>>    (let loop ((c 1))
>>>        (begin
>>>            (if (= 0 (remainder c 10000))
>>>                (begin (display c) (newline)))
>>>            (*->My xoxo)
>>>            (loop (+ 1 c))))))
>>> 
>>> (evil)
>>> 
>>> ;;END CODE
>>> 
>>> 
>>> 
>>> 
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>>> Gambit-list mailing list
>>> Gambit-list at iro.umontreal.ca
>>> https://webmail.iro.umontreal.ca/mailman/listinfo/gambit-list
>> 
> 



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