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

[Ryan Johnson]

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.

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.

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

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.

Ryan

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