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Many functions have the option of returning a pointer to an object or returning NULL if a valid pointer cannot be produced. Some functions return arrays, which appear like a pointer to an object. However, if a function has the option of returning an array or indicating that a valid array is not possible, it should not return NULL. Instead, the function should return an empty array. Often, code that calls a function that returns an array intends merely to iterate over the array elements. In this case, the calling code need not change—iterating over the elements works correctly even if the returned array is empty, so the calling code need not check the return value for NULL.

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The problem arises in this setup when no items are being stocked. getStock would recognize that length = 0 and would return NULL. In this noncompliant code example, erroneous behavior results from getStock returning NULL while main neglects to check for such a value. It results in an abnormal program termination after returning to the main function.

#include <stdio.h>
enum { INV_SIZE=20 };

typedef struct {
  size_t stockOfItem[INV_SIZE];
  size_t length;
} Inventory;

size_t *getStock(Inventory iv);

int main(void) {
  Inventory iv;
  size_t *item;

  iv.length = 0;

  /*
   * Other code that might modify the inventory but still
   * leave no items in it upon completion.
   */

  item = getStock(iv);

  printf("Stock of first item in inventory: %d%zd\n", item[0]);
  
  return 0;
}

size_t *getStock(Inventory iv) {
  if (iv.length == 0) {
    return NULL;
  }
  else {
    return iv.stockOfItem;
  }
}

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This compliant solution eliminates the NULL return and simply returns the item array, even if it is zero-length. The main function can effectively handle this situation without exhibiting erroneous behavior. Since the array lives on the stack, it must prevent returning a value in the stack frame (as mandated by DCL30-C. Declare objects with appropriate storage durations). So the getStack() function also takes a pointer to Inventory, so that it can return a pointer to its contents safely.

#include <stdio.h>

enum { INV_SIZE=20 };

typedef struct {
  size_t stockOfItem[INV_SIZE];
  size_t length;
} Inventory;

size_t *getStock(Inventory* iv);

int main(void) {
  Inventory iv;
  size_t i;
  size_t *item;

  iv.length = 0;

  /*
   /* Other code that might modify the inventory but still
   * leave no items in it upon completion.
   */
  
  item = getStock(&iv);

  if (iv.length != 0) {
    printf("Stock of first item in inventory: %d%zd\n", item[0]);
  }
  
  return 0;
}

size_t *getStock(Inventory* iv) {
  return iv.stockOfItem->stockOfItem;
}

Noncompliant Code Example (Sentinel Value)

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The following code attempts to return an array of the items in stock, sorted by the amount of each item in stock. The arraySort function incorrectly returns NULL instead of a pointer to an empty array when no items are in stock. The null return is improperly handled by the main function, which is attempting to print out the returned array, and an abnormal program termination results.

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

enum { FINAL_ITEM=SIZE_MAX, INV_SIZE=20 };

size_t *arraySort(size_t *array);

int main(void) {
  size_t i;
  size_t stockOfItem[INV_SIZE];
  size_t *sortedArray;

  /* Other code that might use stockarray but leaves it empty */

  sortedArray = arraySort(stockOfItem);
  
  for (i = 0; sortedArray[i] != FINAL_ITEM; i++) {
	printf("Item stock: %d%zd", sortedArray[i]);
  }
  
  return 0;
}

/* Create new sorted array */
size_t *arraySort(size_t *array) {
  size_t i;
  size_t *sortedArray;

  for(i = 0; array[i] != FINAL_ITEM; i++);
  
  if (i == 0) {
    return NULL;
  }

  sortedArray = (size_t*) malloc(sizeof(size_t)*i);
  if (sortedArray == NULL) {
    /* Handle memory error */
  }

  /* Add sorted data to array */

  return sortedArray;
}

Compliant Solution (Sentinel Value)

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#include <stdio.h>
#include <stdint.h>
#include <malloc.h>

enum { FINAL_ITEM=SIZE_MAX, INV_SIZE=20 };

size_t *arraySort(size_t *array);

int main(void) {
  size_t i;
  size_t stockOfItem[INV_SIZE];
  size_t *sortedArray;


  /* Other code that might use stockarray but leaves it empty */

  sortedArray = arraySort(stockOfItem);
  
  for (i = 0; sortedArray[i] != FINAL_ITEM; i++) {
    printf("Item stock: %d%zd", sortedArray[i]);
  }
  
  return 0;
}

/* Create new sorted array */
size_t *arraySort(size_t *array) {
  size_t i;
  size_t *sortedArray;

  for(i = 0; array[i] != FINAL_ITEM; i++);

  if (i == 0) {
    size_t *emptyArray = (size_t*) malloc(sizeof(size_t));
    if(emptyArray == NULL) {
      /* Handle memory error */
    }
    emptyArray[0] = FINAL_ITEM;
    return emptyArray;
  }
  
  sortedArray = (size_t*) malloc(sizeof(size_t)*i);
  if (sortedArray == NULL) {
    /* Handle memory error */
  }

  /* Add sorted data to array */

  return sortedArray;
 }

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Returning NULL rather than a zero-length array can lead to vulnerabilities when the client code does not handle NULL properly. Abnormal program termination can result when the calling function performs operations on NULL.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

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