Pointer to pointer to char in struct, segmentation fault

Question

So I have created a struct, in which one of the variable is a pointer to a dynamic array of chars. So I implemented it as a pointer to pointer. Then I used a separate function to initialize the struct:

#include<stdio.h>
#include<stdlib.h>
//create a struct
typedef struct{
    //use a double pointer
    char **dynamicArray; 
    int size;
    int topValue; 
}Stack; 

/*
    Inintializes the stack
    Dyanmic Array will have a size of 2
*/
void intializeStack(Stack *stack){
    stack->size = 2; 
    stack->topValue = 0;

    //create a dyanmic array of char and set the value in the struct to the address for the newly created array
    char *dyanmic; 
    dyanmic = malloc(2 * sizeof(char)); 
    stack->dynamicArray = &dyanmic; 
}
int main(){

    Stack stack; 
    intializeStack(&stack);

    printf("stack.size: %d\n", stack.size);
    printf("stack.topValue: %d\n", stack.topValue); 
    int i; 
    for (i = 0; i < stack.size; i++){
        *(stack.dynamicArray)[i] = 'r'; 
        printf("%d value of the dynamic array: %c\n", i, *(stack.dynamicArray)[i]);
    }

    printf("Check if the stack is empty: %s\n",isEmpty(&stack)?"true":"false");

    return 0; 
}

The array is initially set to a size of 0. The problem is when I try to access the second element in the array, i get a segmentation fault error.

Segmentation fault (core dumped)

Am I doing something wrong in the implementation?

Undefined behavior for using the address of an object after the end of the object's lifetime. — user3185968
– user3185968, Commented Sep 19, 2017 at 19:45
Why the pointer-to-pointer? From everything I can see, you want single-indirection; not double. — WhozCraig
– WhozCraig, Commented Sep 19, 2017 at 19:48
@WhozCraig Because I have to dynamically change it's size, so I could create a temp array that's bigger than then set the pointer to that. — Vineet Patel
– Vineet Patel, Commented Sep 19, 2017 at 19:49
The memory of what? You're retaining the address of a local automatic variable that ceases to exist as soon as your function returns. There is a big difference between &dyanmic and dyanmic. See it live. — WhozCraig
– WhozCraig, Commented Sep 19, 2017 at 19:58

Blunt Jackson · Accepted Answer · 2017-09-19 20:08:14Z

1

The following construct is confusing and ultimately incorrect:

for (i = 0; i < stack.size; i++){
      *(stack.dynamicArray)[i] = 'r'; 
      printf("%d value of the dynamic array: %c\n", i, *(stack.dynamicArray)[i]);
}

You are actually referencing the FIRST level of ** by this construction. Try this instead:

for (i = 0; i < stack.size; i++){
    stack.dynamicArray[0][i] = 'r';
    printf("%d value of the dynamic array: %c\n", i, stack.dynamicArray[0][i]);
}

answered Sep 19, 2017 at 20:08

Blunt Jackson

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0___________ · Accepted Answer · 2017-09-19 20:35:57Z

0

Abstracting ffrom the sense of this :)

char **dyanmic; 
dyanmic = malloc(sizeof(char *));
*dyanmic = malloc(2 * sizeof(char)); 
stack->dynamicArray = dyanmic;

answered Sep 19, 2017 at 20:35

0___________

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