making use of the returned array in C

Question

I am a beginner to C. I have a function which returns an array and I am trying to print it but it is giving me segmentation fault, don't exactly know what I am doing wrong. Help would be much appreciated. Below is my code.

I want to know how I can make use of an array returned by a function (like printing the array and sorting them) and no, I cannot do them all in the main function, I have to make the function return the array and then print/sort the array.

#include <stdio.h>  
int *getarray()  
{  
    int arr[5] = {0,1,3,4,5};

    return arr;  
}  
int main()  
{  
  int *n;  
  
 
  n=getarray();  
  printf("\nElements of array are :");  
  for(int i=0;i<5;i++)  
    {  
        printf("%d", n[i]);  
    }  
    return 0;  
}

programmer · Accepted Answer · 2022-07-25 12:48:29Z

2

The array you defined

int arr[5] = {0,1,3,4,5};

Has "automatic storage duration", which just means that the array will only exist during the execution of the function getarray(). Once the function ends the array ceases to exist and the memory occupied by it is freed.

So when you try to access it you are causing undefined behaviour since your accessing some random part of memory you are not supposed to.

To fix this you can dynamically allocate memory for the array using malloc():

 int *getarray() { 
   
         int *arr = malloc(sizeof(int) * 5);

         if (arr == NULL) {

              printf("malloc error");
              return NULL;
         }
  
         arr[0] = 0;
         arr[1] = 1;
         arr[2] = 3;
         arr[3] = 4;
         arr[4] = 5;
        
         return arr; 
}

Or you could define the array as static which means it will have "static storage duration" aka the array will remain for as long as the program does.

int *getarray() { 
   
         static int arr[5] = {0,1,3,4,5}; 
         return arr; 
}

edited Jul 25, 2022 at 12:48

answered Jul 24, 2022 at 9:51

programmer

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1 Comment

programmer Over a year ago

You could also use a struct to return the array but I won't discuss that here to avoid any confusion

Josh Kuber · Accepted Answer · 2022-07-24 09:43:49Z

2

This is one of the basics in truly understanding and using C properly.

you are creating an array within a function, thus it's scope is limited to the lifetime of the function itself; thus, you are effectivey in main() referencing a pointer to memory which no longer exists.

answered Jul 24, 2022 at 9:43

Josh Kuber

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Comments

m.ruchann · Accepted Answer · 2022-07-24 10:26:25Z

I will answer this with a beginner-friendly approach. In C, every function has its own working area. When a function is called, this can also be main (), and working space is created, called an activation record. This activation record includes some local variables, return values, and some other information. It's like a temporary working environment. If the function did its job, for example, did some computation, it may return something and quits. After the function terminates, this returned thing should not depend on the memory place it is created. In your case, the array is a contiguous memory area, and it is created in the stack in your code. After the function quits, there is nothing in that address since it is cleaned. In other words, the activation record has been cleaned.

There are a couple of ways to achieve this goal:

Global array: This array would sit in the data segment of the memory and will stay there until the program quits. Every function will have access to this array (without needing it as a parameter).

Static keyword: Using static directly implies that you are putting this array into the data segment of your memory, and it will stay there until the program quits. However, static variables may be local (for example, if you use it in your getArray() function). There is a trick in the static keyword; if you use a function including a static keyword multiple times, the static variable is not initialized every time. You can skip that in other calls.

Heap: You need to understand pointers and dynamic memory allocation—research malloc(), NULL pointer, pointers, pointers-array relation, etc.

user1322654 · Accepted Answer · 2022-07-24 11:49:57Z

1

C has different ways of allocating memory, stack, heap and static. In the code you're trying to return a pointer to a stack allocated variable which gets erased when the procedure returns. https://craftofcoding.wordpress.com/2015/12/07/memory-in-c-the-stack-the-heap-and-static/

answered Jul 24, 2022 at 11:49

user1322654

Comments

Gerhard · Accepted Answer · 2022-07-26 07:51:23Z

Your declaration int arr[5] has function scope and is located in memory on the stack. When the function returns the memory will get reused for other functions stack needs. Allocate the array where you will need it and pass the pointer around.

Add a declaration for memcpy:

#include <stdio.h>
#include <string.h>

getarray now gets a pointer where it can store data. The function uses memcpy to copy the local data to the memory of the pointer (int * out_arr).

void getarray(int * out_arr) {
  int arr[5] = {0, 1, 3, 4, 5 };
  memcpy(out_arr, arr, 5 * sizeof(int));
}

Now in main we allocate memory on the heap and get a reference to this memory from malloc. We have full control of this memory and we can pass this around without problems.

int main() {
  int * n = malloc(5 * sizeof(int));

  getarray(n);
  printf("\nElements of array are :");
  for (int i = 0; i < 5; i++) {
    printf("%d", n[i]);
  }

  free(n);
  return 0;
}

The memory needs to be released withfree() when done with n unless you exit the program.

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