Re: Differences between C++ 'new' operator and 'malloc()' (NOT a C/C++ question)

["Claude SIMON" <sc dot cygwin dot com at zeusw dot org>]

Ryan Johnson wrote:
> On 05/07/2012 9:36 AM, Claude SIMON wrote:
>> Ryan Johnson wrote:
>>> On 04/07/2012 5:45 AM, Claude SIMON wrote:
>>>> When I compile the component with Visual C++, it works. When I compile
>>>> the
>>>> component with g++... it crashes.
>>>>
>>>> With 'gdb', I found that the problem happens when calling the 'malloc'
>>>> function (as soon as the function is called, NOT when the returned
>>>> allocated memory is used). When I replace the 'malloc' by a the C++
>>>> 'new'
>>>> operator, the component compiled with Cygwin g++ doesn't crash
>>>> anymore.
>>>> I thought that the C++ 'new' operator calls the 'malloc' function, but
>>>> this seems not to be the case. As I want to use 'malloc'-like
>>>> functions
>>>> rather than the C++ 'new' operator, I wonder which functions are used
>>>> in
>>>> the C++ 'new' operator to allocate memory (and naturally which
>>>> functions
>>>> are used in the C++ 'delete' operator to free the memory).
>>> Operator new() and malloc() are explicitly *not* interchangeable (for
>>> many reasons, not least of which that the Standard says so). If you
>>> were
>>> to free new'ed memory, or delete malloc'ed memory, the resulting heap
>>> corruption could easily manifest as a crash the next time you tried to
>>> allocate something... or it might just silently clobber data and lead
>>> to
>>> "spooky action at a distance."
>>>
>> Thank you for the answer, but I am aware of this and my problem has
>> nothing to do with it, nor, as stated in the subject, with having some
>> lacuna in C/C++ programming.
>>
>> Let's try to be a little more explicit despite my poor English.
>>
>> Let's consider a Java native component which only calls a 'malloc(1)'.
>> It
>> doesn't even test the returned value (it is usually not a good idea, but
>> it doesn't matter here).
>>
>> This component :
>> - compiled with g++ under Linux : works,
>> - compiled with g++ under Mac OS : works,
>> - compiled with Visual C++ under Windows : works,
>> - compiled with g++ under Cygwin : CRASHES !
>>
>> It crashes as soon the 'malloc(1)' function is called. You don't even
>> have
>> the opportunity to test the returned value, nor to use it. It's perhaps
>> a
>> Cygwin bug, or perhaps a JVM/JRE/JDK bug ; I don't know and I don't
>> bother
>> (but if someone will make some investigation about that, I'm ready to
>> help
>> him or her if I can).
>>
>> When you replace the 'malloc()' by the 'new' operator, then the
>> component
>> compiled with g++ under Cygwin works too.
>> The 'new' operator, among other things, allocates memory, as 'malloc()'
>> does, but obviously it doesn't use 'malloc()' as it doesn't crash. So,
>> because I can't use 'malloc()' in my Java native components, and because
>> I
>> doesn't want to use the 'new' operator, I wish to know which functions
>> the
>> 'new' operator uses to allocate memory, so I can use them in my Java
>> native component so they would no more crash when compiled with g++
>> under
>> Cygwin.

> A crash inside malloc is 99.99% likely due to a bug in user code (wild
> pointer, double-free, smashed stack, etc). The fact that your code
> doesn't crash under other circumstances does precisely *nothing* to rule
> out a bug in your code if bad has been observed anywhere (it just proves
> the platforms really are different). The buggy code may have nothing to
> do with malloc, other than having the bad luck of clobbering a data
> structure the latter needs. Even a single mix-up of new/malloc usage
> (perhaps due to losing track of a pointer's provenance) is also enough.

Indeed. The problem is... the crash happens even when there is no other
code which could be buggy.
As asked in another reply to this thread, I've made a test case, which can
be found at :
	http://cvs.savannah.gnu.org/viewvc/epeios/bugs/jcmc/?root=epeios
There is a README file which contains some further explanations.

> This is all standard memory management debugging stuff that's off topic
> for this list. If at some point you have some evidence besides "it
> crashes when I run it under cygwin" *that* would be a topic for this list.

With the test case above, I think that it is easy to establish if the
problem is off or on topic.

> My suggestion: run under the debugging malloc library of your choice
> and/or Valgrind and see what that turns up.

Should be interesting to see if a alternative 'malloc' would also crash,
but would not solve my problem given what I wrote above.

> As to your question, new() usually calls malloc under the hood (with
> extra bookkeeping), but five minutes with gdb will give you a definitive
> answer.
>

I don't manage to make 'gdb' step into a 'new' call...

Beside the crash thing, all I'm interested into, is if someone here can
show me the implementation of the 'new' operator as used in Cygwin, or
give me an address where I can found the source code of this 'new'
implementation, or where I may ask this questions to obtain a response to
one of this question.

-- 
Claude SIMON (sc.cygwin.com@zeusw.org)



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