Is the "top" of a stack different from both the start and the end?

Question

I'm using the C libYAML library for handling YAML configuration files, and I a little confused about this struct element in its code:

/** The stack of mapping pairs (key, value). */
struct {
    /** The beginning of the stack. */
    yaml_node_pair_t *start;
    /** The end of the stack. */
    yaml_node_pair_t *end;
    /** The top of the stack. */
    yaml_node_pair_t *top;
} pairs;

It uses three pointers, start, end, and top. Both start and end seem obvious, they're the beginning and end of the region of data, but what is top?

(For reference, this code appears at line 741 here.)

Answer 1

Start and end refer to the maximum boundaries of the stack. The top is a dynamic pointer referring to the current position, and changes as you call nested levels of functions.