Memory mapped region is not present in core dump

[bhargav M.P]

We have application software running on freebsd 10 and it has  memory
mapped region(mapped using mmap, used for IPC) and those addresses are
not in the core file. Wierd thing is we don't see this when the core dump
is collected using gcore and all the memory mapped region are present in
the core file.
 But if the program crashes (seg fault or abort) the core dump doesn't
contain memory mapped addresses. application software is NOT setting
MAP_NOCORE
when memory mapping. Why this behavior.?whats different when the core file
collected when the program crashes.?