5.1-R: zero byte core file.

While using 5.1-RELEASE, I find that if my application program seg 
faults, it produces "programname.core"; but it is 0 bytes.
I ran the exact same program on another machine that was running 
4.4-RELEASE, and I do get a core file that I can use with gdb.
I'd really appreciate if someone could help me resolve this.

Additional details: 
- It is not specific to the application program. I tried a 2 line program:
    char p[8];
    memcpy(p, "1234567890123456789012345678901234567890", 40);
 with same results on 5.1-R(0 byte core file) and 4.4-R(usable core file)

- "ulimit -a" on the 5.1-R machine gives
   core file size        (blocks, -c) unlimited

- Just to be sure I used getrlimit() to find what the limit  for 
RLIMIT_CORE is in my processes, and it is RLIM_INFINITY.

- I did the basic checks like write permission on current directory, it 
looks fine.

Can someone help me resolve this? 

Thanks,
Yogeshwar.