problem when mapping malloc to GC_malloc.

Hans Boehm
Thu Jun 18 16:30:00 GMT 2009

On Thu, 18 Jun 2009, abhishek desai wrote:

> Hi,
> My JNI code includes redefinitions to the malloc, free and realloc
> functions (shown below). These functions call GC_malloc, GC_free and
> GC_realloc respectively. This is done so that any calls to the malloc
> get allocated through the garbage collector. However this is failing
> with segfault. Any clues why this does not work ?
> I am using this code along with the libgcj library linked dynamically
> with my application.
> void *malloc(size_t size)
> {
>        return GC_malloc(size);
> }
> void *realloc(void *ptr, size_t size)
> {
>        return GC_realloc(ptr, size);
> }
> void free(void *ptr)
> {
>        GC_free(ptr);
> }
> regards
> abhishek
The collector itself supports a REDIRECT_MALLOC option that might get you 
closer.  In general, this is very hard.

There are other functions (calloc, memalign, etc.) that you would also 
have to replace, so that their clients don't end up using the original 
malloc with GC_free.  This is the easy part.

The hard part is that if you replace malloc, low level parts of the system 
will also end up using GC_malloc, and sometimes squirrel away pointers to 
the results in places the GC doesn't really know about.  Recent versions 
of the GC (7.1+) contains some hacks to try to handle this on Linux.  But 
the multithreaded versions still are sometimes not 100% robust.  Gcj's 
version is unlikely to work in this mode, except possibly in 
single-threaded mode.

A real fix here would probably require some new hooks in glibc and the 
startup and libpthread code.


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