The struct pointer in C

Question

The struct problem.

I wrote a code implementing a stack. If I pass sqStack* sq to this function init_stack(), the code ended up with an error. As seen in the comment of the following code.
But then I found out if I pass sqStack &sq to init_stack() function, the code works.
Can anyone explain to me? Thanks!

#include <stdio.h>
#include <stdlib.h>

#define init_size 10
#define increment 1

typedef struct sqStack
{
    int* top;
    int* base;
    int stack_size;
}sqStack;

int init_stack(sqStack* sq)
{
    if(sq->base==NULL)
    {
       sq->base = (int*)malloc(init_size*sizeof(int));
    }
    if(sq->base==NULL) exit(-1);
    sq->stack_size=init_size;
    sq->top=NULL;
    sq->top=sq->base;
    return 1;
}
int push(sqStack* sq, int e)
{
    if(sq==NULL) exit(-1);
    if(sq->top-sq->base==sq->stack_size)
    {
        int* q = (int*)realloc(sq->base,  
        (init_size+increment)*sizeof(int));
        if(q==NULL) exit(-1);
        sq->base=q;
        sq->stack_size += increment;
        sq->top=sq->base+sq->stack_size;

    }
    *sq->top++=e;//Thread 1: EXC_BAD_ACCESS  If I pass sqStack* sq to this function, error occurs. But if I pass sqStack &sq, the code works.


    return 1;
}

int pop(sqStack* sq,int*e)
{
    if(sq==NULL) exit(-1);  
    if(sq->base==sq->top)  exit(-1);   
    sq->top-=1;   
    *e=*sq->top;   
    return 1;    
}

int empty(sqStack* sq)
{
    if(sq->base==sq->top) return 1;
    else return 0;
}


int main()
{
    sqStack* sq;
    init_stack(sq);
    push(sq,1);
    int e=
    pop(sq,e);
    printf("%d\n",*e);
/* sqStack sq;
    init_stack(&sq);
    push(&sq,1);
    int e;
    pop(&sq,&e);
    printf("%d\n",e);*/
    return 0;
}

In either case, the output is 1.

In the main function you have the pointer sq, but where does it point? I recommend you change it to be not a pointer, and use the address-of operator & when a pointer to it is needed. — Some programmer dude
– Some programmer dude, Commented Jan 28, 2019 at 14:40
This is a classic problem. Either return the allocated node or use a pointer to pointer. In C, you should pass a pointer to a X to a function where you want your X to be modified. In this case, since you want a pointer to be modified, you ought to pass a pointer to a pointer. — Nick S
– Nick S, Commented Jan 28, 2019 at 14:45

Ctx · Accepted Answer · 2019-01-28 14:52:34Z

6

Here you dereference an uninitialized (dangling) pointer:

sqStack* sq;
init_stack(sq);

(in init_stack()):

if(sq->base==NULL)
   ...

which immediately leads to undefined behaviour.

Better do it this way:

sqStack sq;
init_stack(&sq);

Now you properly allocate space for sqStack on your process stack and pass a pointer to that space to init_stack(). Every time you want to pass a pointer to that structure (e.g. in pop and push) you have to use &sq now.

Alternatively, you can dynamically allocate memory for sqStack like this:

sqStack *sq = malloc(sizeof(sqStack));
init_stack(sq);

which also reserves memory (this time on the heap).

A third variant is to leave the allocation of the structure to the init_stack() function. In this case, you have to pass a double-pointer to init_stack, so the address can be written to it (error-checking to be added by yourself):

int init_stack(sqStack** _sq) {
    sqStack* sq;
    sq = *_sq = malloc(sizeof(sqStack));
    sq->base = malloc(init_size*sizeof(int));
    ...

and in your main:

sqStack *sq;
init_stack(&sq);
...

edited Jan 28, 2019 at 14:52

answered Jan 28, 2019 at 14:44

Ctx

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6 Comments

Liu Over a year ago

Thanks! And is sq->top=NULL; in init_stack() necessary in the code??

Ctx Over a year ago

@Liu No, since you overwrite this in the next line sq->top = sq->base; you can omit that.

Liu Over a year ago

Thanks! I know my problem with my code now, but why I used the same way to create a sequence, and it worked out well, why??

Liu Over a year ago

If I dynamically allocate memory for sqStack sqStack *sq = malloc(sizeof(sqStack)); init_stack(sq); sq wasn't initialized and after passing it to push, the code crushed, how should I fix it??

Ctx Over a year ago

@Liu You have to show the full code for that (i.e. edit your question and add it there)

|

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

Problem solution

The problem lies in the fact that you reserve memory for sq as a pointer. Why is this problematic?

Well, we only have space for an address to a sqStack. If we pass sq to init_stack(), we dereference sq. This leads to nothing, as we initialised it as a pointer, and thus, if we try to assign values to its fields, we don't have space for it!

How to solve the issue? Simply initialise sq as a sqStack instead of a pointer to it:

sqStack sq;
init_stack(&sq);  /* pass by reference here */
/* code continued */

Suggestion

Also, I think a better struct to use would be

typedef struct sqStack {
  int *elements;
  int top;
}

In this manner, you can use the top as a means to keep track of your stack's size, and simply store the elements in a specially allocated array. Pushing would then look like

void push(sqStack *sq, int element) {
  sq->top++;
  if (sq->top - 1 == 0) {
    /* we need to allocate space */
    sq->elements = malloc(sizeof(int));
  } else {
    /* we need to reallocate space */
    sq->elements = realloc(sq->elements, sq->top * sizeof(int));
  }
  sq->elements[sq->top - 1] = element;
}

...and revise the other functions in a similar way. But this is only a suggestion.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 28, 2019 at 14:57

some-coder

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4 Comments

Liu Over a year ago

Thanks! And is sq->top=NULL; in init_stack() necessary in the code??

some-coder Over a year ago

@Liu I've added a suggestion that indirectly addresses your question.

Liu Over a year ago

a suggestion ??

Liu Over a year ago

Thanks! I know my problem with my code now, but why I used the similar way to create a sequence, and it worked out well, why??

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