Intercepting calls to pthread_create (or any pthreads function)

["Philip J. Mucci" <mucci at cs dot utk dot edu>]

Hi all,

I'm trying to intercept calls to pthread_create so as I can call a 
monitor function in the context of the newly created thread. I thought I 
might be able to do this with LD_PRELOAD and some dlopen() tricks, but 
unfortunately, I keep getting segfaults. I'm guessing that by dlopen'ing 
the new pthreads library, I'm initializing an entirely new copy of the 
library, which is not what I want.

So my question is, how can I intercept a call and then call the real 
version? Is it possible? The real reason I need to do this is to enable 
a profiling timer in the child thread.

Thanks,

#define _REENTRANT
#include <pthread.h>
#include <stdio.h>
#include <dlfcn.h>

typedef struct {
     void *arg;
     void *(*fn)(void *);
} real_args;

void *create_head(void *arg)
{
   real_args *a = arg;

   fprintf(stderr,"PID %d TID %d\n",
	  getpid(),(int)pthread_self());
   return(a->fn(a->arg));
}

int pthread_create(pthread_t  *thread,
		   __const pthread_attr_t *attr,
		   void * (*start_routine)(void *), void * arg)
{
   real_args args;
   static void *handle = NULL;
   static int (*real_pthread_create)(pthread_t  *,
			     __const pthread_attr_t *,
			  void * (*)(void *), void *) = NULL;

   args.arg = arg;
   args.fn = start_routine;

   if (handle == NULL)
     {
       handle = dlopen("/usr/lib/libpthread.so",RTLD_LAZY);
       if (handle == NULL)
	{
	  fprintf(stderr,"dlopen(%s) failed\n","/usr/lib/libpthread.so");
	  exit(1);
	}
       real_pthread_create = dlsym(handle,"pthread_create");
       if (handle == NULL)
	{
	  fprintf(stderr,"dlsym(%s) failed\n","pthread_create");
	  exit(1);
	}
     }
   return(real_pthread_create(thread,attr,create_head,&args));
}