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

On 10/07/2012 12:46 PM, Claude SIMON wrote:
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
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++
operator, the component compiled with Cygwin g++ doesn't crash
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
rather than the C++ 'new' operator, I wonder which functions are used
the C++ 'new' operator to allocate memory (and naturally which
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
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
"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)'.
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
the opportunity to test the returned value, nor to use it. It's perhaps
Cygwin bug, or perhaps a JVM/JRE/JDK bug ; I don't know and I don't
(but if someone will make some investigation about that, I'm ready to
him or her if I can).

When you replace the 'malloc()' by the 'new' operator, then the
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
doesn't want to use the 'new' operator, I wish to know which functions
'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++
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.
#include <stdlib.h>
int main() { return (int) malloc(10); }

Does not crash. There must be some other code which is buggy.

As asked in another reply to this thread, I've made a test case, which can
be found at :
There is a README file which contains some further explanations.
If it needs to live in a CVS repo, it's not a simple test case. Those usually fit inline in emails (see above). Long test cases are acceptable if the problem can't be narrowed down further, but you'll need to make a substantial effort to exclude bugs in your own code before others will be interested to jump in. Like running a debug allocator.

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.
Great. Please do.

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.
Why not? Try it, you might be surprised.

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

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

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.
It's burned into gcc. That's why I highly doubt cygwin's code is directly causing the problem here.

To be blunt, you appear to be a help vampire [1]. You haven't actually done any visible legwork, even the things people have taken time to suggest you try. I am done with this thread unless/until you do something to dispel that perception.



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