int stack_pop( struct stack_state *ss, void **user_data );
struct stack_state *ss
- A stack state as allocated by stack_new.
- The address of a pointer into which the user data will be placed. If the stack is empty, *user_data is not modified, as NULL is a valid user data value.
The return value is 1 upon successful pop, 0 upon unsuccessful pop. Popping fails only if the stack is empty. Note that on an unsuccessful pop, *user_data is untouched; it is not set to NULL, since NULL is a valid user data value. Only the return value indicates whether or not the pop was successful.