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

On 12/07/2012 8:12 AM, Claude SIMON wrote:
Ryan Johnson wrote:
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
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.

#include <stdlib.h>
int main() { return (int) malloc(10); }

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

The test case I provided is EXACTLY the code your wrote above, only
applied to a Java native component (and without the 'return', which is not
needed for the test case), and it DOES crash. So ?
Sorry, I should have actually looked at the repo before assuming the test case was a monstrosity. By way of penance, I've now looked, downloaded, tweaked, and tested it.

Tweaks:
- wrap the .cpp in `extern "C"' and remove the stdcall_alias linker flag (could have led to ABI issues)
- made jcmc.h a makefile target that calls my local javah (I'm paranoid)

My setup:
cygwin 1.7.14
jdk 1.6.0_24 (64-bit native Windows)
jre ? (32-bit native Windows)
g++ 4.7.1 (cygwin), 4.5.2 (64-bit mingw)

Attempt 1:
$ cat jcmc.java
class jcmc {
static private native void bug();
static public void main ( String[] args ) {
System.out.print( "Loading library 'jcmc'..." );
System.out.flush();
System.loadLibrary("jcmc");
System.out.println( " 'jcmc' library loaded !" );
System.out.print( "Calling 'malloc' function from a native component..." );
System.out.flush();
bug();
System.out.println( " 'malloc' calling succeed !" );
}
}

$ cat jcmc.cpp
#include "jcmc.h"
#include <stdlib.h>
extern "C" {
JNIEXPORT void JNICALL Java_jcmc_bug( JNIEnv *, jclass ) { new( char[10] ); }
}

$ make
rm -f jcmc.h jcmc.class jcmc.dll jcmc.o
jdk/bin/javac jcmc.java
jdk/bin/javah jcmc
g++ -c -g -Ijdk/include -Ijdk/include/win32 "-D__int64=long long" jcmc.cpp
g++ -shared -o jcmc.dll jcmc.o
java -cp . jcmc
Loading library 'jcmc'... 'jcmc' library loaded !
Calling 'malloc' function from a native component...Exception in thread "main" java.lang.UnsatisfiedLinkError: jcmc.bug()V
at jcmc.bug(Native Method)
at jcmc.main(jcmc.java:16)
This more or less was what I think you tried. I don't seg fault, but the malloc() function is not available at runtime. Since JNI is by definition not portable, and the java in my path is from some 32-bit JRE, I decided to try again with the JDK's 64-bit java.

Attempt 2:
$ make
rm -f jcmc.h jcmc.class jcmc.dll jcmc.o
jdk/bin/javac jcmc.java
jdk/bin/javah jcmc
g++ -c -g -Ijdk/include -Ijdk/include/win32 "-D__int64=long long" jcmc.cpp
g++ -shared -o jcmc.dll jcmc.o
./jdk/bin/java -cp . jcmc
Loading library 'jcmc'...Exception in thread "main" java.lang.UnsatisfiedLinkError: C:\cygwin\home\Ryan\experiments\epeios\bugs\jcmc\jcmc.dll: Can't load IA 32-bit .dll on a AMD 64-bit platform
at java.lang.ClassLoader$NativeLibrary.load(Native Method)
at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1803)
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1728)
at java.lang.Runtime.loadLibrary0(Runtime.java:823)
at java.lang.System.loadLibrary(System.java:1028)
at jcmc.main(jcmc.java:9)

That's clear enough... time to switch to 64-bit mingw.

Attempt 3:
$ make
rm -f jcmc.h jcmc.class jcmc.dll jcmc.o
jdk/bin/javac jcmc.java
jdk/bin/javah jcmc
x86_64-w64-mingw32-g++ -static-libgcc -static-libstdc++ -c -g -Ijdk/include -Ijdk/include/win32 "-D__int64=long long" jcmc.cpp
x86_64-w64-mingw32-g++ -static-libgcc -static-libstdc++ -shared -o jcmc.dll jcmc.o
./jdk/bin/java -cp . jcmc
Loading library 'jcmc'... 'jcmc' library loaded !
Calling 'malloc' function from a native component... 'malloc' calling succeed !
Et voilà . Don't mix 32/64 bit (if applicable), don't try to call JNI stuff with a different JVM than was used to create it, and life is good. This has nothing to do with either cygwin or new vs. malloc AFAICT. Note that I used new char[10] in all the examples above, so I don't know why you were able to run with operator new and not malloc. I don't know why you got a seg fault either; I assume you haven't seen such clear error messages as I got, or you could have solved this yourself in about five minutes. Maybe you have a different (older?) JVM that's less careful about checking runtime linker dependencies? A broken dll trampoline could easily cause a seg fault.

Note that cygwin has no 64-bit capabilities for the time being. Mingw is a windows-targeted cross compiler that runs under cygwin but produces native windows binaries, so it doesn't have any posix functions available; the -static flags tell mingw to make a truly stand-alone executable that has only standard windows runtime dependencies.

Ryan


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