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question about interposing routines in libSystem.B.dylib

Hi,

I'm attempting to develop an interposition library to catch a few system calls on the application side (i.e., interpose the system call's system library analogue).  It seems, however, whenever I attempt to build such a library that interposes, for example, malloc, I have a dependency on libSystem.B.dylib.  This is the library in which the malloc system library routine is stored.  otool also tells me dlsym (needed to do interposition) is contained in libSystem.B.dylib.  According to Apple documentation, it is not possible to interpose dependent libraries and it further seems I can't do interposition without this particular library. 

I'm using the following simple code for a dry run:

// *** BEGIN CODE ***

#include <stdio.h>
#include <dlfcn.h>

void *malloc(size_t size)
{
     static void * (*func)();

     printf ("malloc(%d) called\n", size);
    
     if(!func)
          func = (void *(*)()) dlsym(RTLD_NEXT, "malloc");

     return(func(size));
}

// *** END CODE ***

And building the dylib as follows:

$ gcc -dynamiclib -o interpose.dylib -fPIC interpose.c

Of course, this turns up the following messages about duplicate malloc symbols:

ld: warning multiple definitions of symbol _malloc
/var/tmp//cczzOOvT.o definition of _malloc in section (__TEXT,__text)
/usr/lib/gcc/powerpc-apple
-darwin8/4.0.0/../../../libSystem.dylib(malloc.So) definition of _malloc

And attemptnig to have it loaded via DYLD_INSERT_LIBRARIES results in a seg fault (not a suprise).

My sense is that it's just plain silly that it's not possible to do something like this.  Any ideas out there?  It's definitely possible to interpose malloc, open, etc. on other systems.  Does anyone know how I can achieve this under darwin?  Googling around hasn't turned up much.  And, if it matters, the calls I'm most interested in interposing are open, close, read, write, connect, accept, pipe, and fork.


Your help is greatly appreicated,

Will
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