The database server allocates an area in the virtual portion
of shared memory to store nonshared data for the functions that a
thread executes. This area is called the stack.
For information about how to set the size of the stack, see Stacks.
The stack enables a virtual processor to protect the nonshared
data of a thread from being overwritten by other threads that concurrently
execute the same code. For example, if several client applications
concurrently perform SELECT statements, the session threads for each
client execute many of the same functions in the code. If a thread
did not have a private stack, one thread might overwrite local data
that belongs to another thread within a function.
When a virtual processor switches to a new thread, it loads a stack
pointer for that thread from a field in the thread-control block.
The stack pointer stores the beginning address of the stack. The virtual
processor can then specify offsets to the beginning address to access
data within the stack. The figure illustrates how a virtual processor
uses the stack to segregate nonshared data for session threads.Figure 1. Virtual processors segregate
nonshared data for each user
