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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