c - Why do I need to use & to pass a pointer to a struct in this stack implementation -

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• what reason using double pointer when adding node in linked list? 9 answers

in following program need pass argument function using &-operator although expect pointer , function expecting pointer. why need this?

the program implements simple stack using linked lists , incomplete types in c. here 3 necessary files:

stack.h

#ifndef stack_h #define stack_h  #include <stdbool.h>  struct stack {         int number;         struct stack *next; };  /*  * declare pointer stack structure thereby making use of incomplete  * types. clients pull in stack.h able declare variables of type  * pstack pointers stack structures. */ typedef struct stack *pstack;  bool is_empty(pstack *s); void make_empty(pstack *s); void push(pstack *s, int new_num); int pop(pstack *s);  #endif /* stack_h */ 

stack.c

#include <stdio.h> #include <stdlib.h> #include "stack.h"  bool is_empty(pstack *s) {         return !s; }  void make_empty(pstack *s) {         if (!is_empty(s))                 pop(s); }  int pop(pstack *s) {         struct stack *tmp;         int i;          if (is_empty(s)) {                 exit(exit_failure);         }         tmp = *s;         = (*s)->number;         *s = (*s)->next;         free(tmp);         return i; }  void push(pstack *s, int new_num) {         struct stack *new_node = malloc(sizeof(struct stack));         if (!new_node) {                 exit(exit_failure);         }         new_node->number = new_num;         new_node->next = *s;         *s = new_node; } 

stackclient.c

#include <stdio.h> #include <stdlib.h> #include "stack.h"  int main(void) {   pstack s1;   int n;    push(&s1, 1);   push(&s1, 2);    n = pop(&s1);   printf("popped %d s1\n", n);   n = pop(&s1);   printf("popped %d s1\n", n);    exit(exit_success); } 

again, thought using

typedef struct stack *pstack; 

and later on in main()

pstack s1; 

i'm declaring pointer linked list stack , hence fine pass s1 push() using

push(s1, 1); 

but need use

push (&s1, 1); 

why?

your functions declared take pstack* argument, pointer pointer stack struct. use pstack. you'll need replace instances of (*s) s in implementations of functions.

edit: pointed out in comments, write (*s) in function implementations , rely on behavior correctness, need keep argument pstack*. conceptually, because stack variable (s1) literally top of stack itself, , must modified push , pop.