C - Segmentation Fault 11 When use the array in function

Question

first of all I looked other titles about segmentation faut 11 error.

This one is very close to me. But cannot solve my problem. How to fix segmentation fault 11 error

This code take 2 array from user with readArray function Here is the code

int *readArray(int p[]) {
  int i = 0;
  printf("Enter the array :\n");
  for (i = 0; i <= 10; i++) {
    printf("a%d : ", i);
    scanf("%d", &p[i]);
  }
  return p;
}

int main() {

  int *firstArray;
  int *secondArray;
  int *resultArray;

  firstArray = readArray(firstArray);
  // printArray(firstArray);

  secondArray = readArray(secondArray);
  // printArray(secondArray);

  // addArray(firstArray,secondArray,resultArray);
  // printArray(resultArray);
  printf("\n");
  return 0;
}

I can take first one from user but cannot take second. It gets error segmentation fault 11.

How can I solve it?

Thank you all! FIXED CODE

 void readArray(int p[]) {
  int i = 0;
  printf("Enter the array :\n");
  for (i = 0; i < 11; i++) {
    printf("a%d : ", i);
    scanf("%d", &p[i]);
  }

}

int main() {

  int firstArray[11];
  int secondArray[11];
  int resultArray[11];

  readArray(firstArray);
  // printArray(firstArray);

  readArray(secondArray);
  // printArray(secondArray);

  // addArray(firstArray,secondArray,resultArray);
  // printArray(resultArray);
  printf("\n");
  return 0;
}

actually 11 element is ok for me. But I can't send it as array to fuction so I use pointer. — mehmet salih bindak, Nov 23 '17 at 21:17
The tail is trying to wag the dog. `readArray` returns the same pointer it was passed, and assigning that to the pointer that was passed is useless. — Weather Vane, Nov 23 '17 at 21:20
dear @EugeneSh. are you say that, I have to `myArray = (int *) malloc(11);` for 11 integer ? — mehmet salih bindak, Nov 23 '17 at 21:21
`readArray(firstArray)` has undefined behavior; `firstArray` is uninitialized. — melpomene, Nov 23 '17 at 21:21
`malloc(11)` allocates 11 bytes, not 11 integers. And don't cast `malloc()`. — melpomene, Nov 23 '17 at 21:21
...but that would be the direction, yes. But I would go with static allocation such as `int firstArray[11]`. — Eugene Sh., Nov 23 '17 at 21:22
Re the "problem solved" edit: you still have `for (i = 0; i <= 10; i++)` where 10 is not the 11 that you used for the array lengths. — Weather Vane, Nov 23 '17 at 21:42
Well done, but you edited the *original* code posted, not the *problem solved* code. — Weather Vane, Nov 23 '17 at 21:48
Please do not edit your question in that way, read the [tour] for more information. [help] — Stargateur, Nov 23 '17 at 21:55
@mehmetsalihbindak A subtle issue: `void *readArray(...)` should be `void readArray(...)`; the first is a function that returns a pointer-to-`void`, while the second is a function that doesn't return anything, which is what you want here. — frslm, Nov 23 '17 at 22:58

score 1 · Accepted Answer · answered Nov 23 '17 at 21:28

Your firstArray, secondArray, and resultArray pointers are all uninitialized, so attempting to access p[i] in readArray() is undefined.

Instead of:

int * firstArray;
int * secondArray;
int * resultArray;

You can use:

int firstArray[11];
int secondArray[11];
int resultArray[11];

If you really want to dynamically allocate the memory, then you'd have to use malloc() instead:

int * firstArray = malloc(sizeof(int) * 11);
int * secondArray = malloc(sizeof(int) * 11);
int * resultArray = malloc(sizeof(int) * 11);

And delete them after you're done using them:

free(firstArray);
free(secondArray);
free(resultArray);

By the way, since you're passing in an array (or pointer) to readArray() and modifying its contents, you don't need to return anything. It's sufficient to define void readArray(int p[]) {...} (or void readArray(int * p) {...} if you're using pointers).

@mehmetsalihbindak aside: please try to avoid using magic numbers. The array length is 11 but you use 10 in the loop `for (i = 0; i <= 10; i++)`. This makes the code harder to follow and maintain. Use a `const int` or a `#define` for the array length and drive everything from that. So (assuming 11 will be defined) the loop should be `for (i = 0; i < 11; i++)`. — Weather Vane, Nov 23 '17 at 21:36

C - Segmentation Fault 11 When use the array in function

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