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

Paolo Carlesi pcarlesi at gmail.com
Mon Dec 14 14:46:16 EST 2015


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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> 



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