The lua stack layout in Lua 5.4

Stack Layout

In the lua_State structure, there are three pointers(L->stack, L->stack_last and L->top) that relate to stack management. Additionally, there is a structure representing information about the current called function, which is CallInfo* ci. In the CallInfo structure, there are two pointers(ci->func and ci->top) that also relate to stack management. Here, we will dive into the layout of entire stack and show the internal meaning of these pointers.

L->stack and L->stack_last used to represents the start of and the end of the total Lua stack area, respectively. The sizo of total stack may shrink or grow as needed, depending on the stack's usage.

ci->func points to the stack position where the function frame starts, and ci->top points to the maximum stack position that current function can use. These two pointers define the function's stack frame.

L->top represents the current position of the stack's top, which is where a new values are pushed. This pointer only used to track the active top of current stack, so its value can be greater then and equal to ci->func but less then ci->top(it cannot exceed ci->top).

The relationship of these pointer is L->stack <= ci->func < L->top <= ci->top < L->stack_last.

so, we have:

• L->top - (ci->func + 1) represents the number of elements that currently exists in stack within the function's frame.
• L->top - (ci->func + 1) represents the index of top of stack. If the stack is empty, we have L->top == ci->func + 1.
• ci->top - L->top represents the unused space of current function's frame.