[Serusers] what's the difference between the shm_malloc and pkg_malloc

When I browse the ser code, I found there are three kind of memory allocation.The most frequently used is shm_malloc() and pkg_malloc(). the first have lock functions and the latter is not. I found that the transaction struct cell and its field is allocated with shm_alloc, but others are allocated with pkg_alloc(). I added a pointer member into the cell struct. If I allocate a block of memory for my pointer member with pkg_malloc(), the ser will crash when some one want to establish a call through ser. If using shm_malloc(),all will go well. What I want to know is when and where I should use shm_malloc() and pkg_malloc(). Thanks for your instructions Best Regards Sun Zongjun