Im trying to create a array as a pointer using a function, and assigning random integers to that array.

But inside the for loop i am getting the following warning message, on equals sign:

Warning Here ↓↓↓ - Incompatible integer to pointer conversion.
 array[index] = rand() % 100;

incompatible integer to pointer conversion assigning to 'int *' from 'int'

And when i execute, i get the following error:

signal: segmentation fault (core dumped)

Below is my code, with lots of comments to specify what is happening.

#include <stdio.h>
#include <stdlib.h> // Lib for rand() to work

// Function: Create array and its lenght, save to a pointer.
int createArray(
    int **array,
    size_t *size) { // Initiate pointers of array and array size
  printf("What will be the size of the array?: ");
  scanf("%ld", &*size); // For formatting using "size_t" we use %ld

  for (int index = 0; index != *size; index  ) {

    /*
     * Warning Message on Equals Sign below: incompatible integer to pointer
     * conversion assigning to 'int *' from 'int' */

    array[index] = rand() % 100; // Equal sign is giving me a warning. (??)
  }
  printf("\n --- Array created. Pointers: *size, *array --- \n");
  return 0;
}

// Function: Print elements of pointer array specifying its size from pointer.
void *arrayElements(int array[], size_t size) {
  printf("\n --- Printing Elements of array --- \n");
  // Loop through the array by incrementing value of i
  for (int i = 0; i < size; i  ) {
    printf("%d ", array[i]);
  }
}

int main(void) {
  int *array; // Instance pointer (Persistent variable -- For remembering what it does)
  size_t size;
  createArray(&array, &size);
  arrayElements(array, size);

  return 0;
}

If i change array[index] = rand() % 100; to *array[index] = rand() % 100;, the warning goes away but i get illegal instruction error when executing.

What i want to achieve: Pass a array and its size as pointers using a function, then print its elements from another function using those pointers.

Please, can someone help me? Thank you!

CodePudding user response：

Couple of things wrong,

1. scanf("%ld", &*size), the two operators &* cancel each other, so &*size is just redundant. scanf("%ld", size) is enough. (If you are allowed to use, the standard format specifier for size_t is %zu).
2. You also need to check the return value of scanf since you are using the variable to allocate space for your array, it's crucial that the conversion succeeded.
3. You never allocate memory for array.
4. You are trying to store an int to an int * variable.

Since array is of type int ** a single dereference array[index] yields a pointer to int

array[index] --> *(array index)

*array[index] won't work either because of operator precedence

([] operator will be evaluated first and the result will be dereferenced)

What you want is (*array)[index] = rand() % 100;

or array[0][index] = rand() % 100;

or even *(*array index) = rand() % 100;

int createArray(
    int **array,
    size_t *size)
{ 
// first make sure you didn't get NULL pointers
if (array == NULL || size == NULL) {
    fprintf(stderr, "createArray: null pointer\n");
    return 1;
}
// Initiate pointers of array and array size
    printf("What will be the size of the array?: ");
    if (scanf("%zu", size) != 1) { // conversion failed
        fprintf(stderr, "scanf: error reading size\n");
        return 1; 
    }

    *array = malloc(*size * sizeof(int)); // dereference array pointer and allocate memory
    // alternatively, you can use array[0] = ... which is the same

    if (*array == NULL) { // allocation failed
        fprintf(stderr, "malloc: error allocating memory for array\n");
        return 1;
    }

    for (size_t index = 0; index < *size; index  ) // use size_t since size is size_t
    {
        // dereference first and assign value to array[index]
        (*array)[index] = rand() % 100;
    }
    printf("\n --- Array created. Pointers: *size, *array --- \n");
    return 0;
}

I also noticed that void *arrayElements(int array[], size_t size) returns a void pointer. Since you don't actually return anything,

void arrayElements(int array[], size_t size) should be enough.