MSC19-C. For functions that return an array, prefer returning an empty array over a null value

There are situations in which a function may return an array based on its length. In the case that an array of length zero is being returned, null should not be used. An empty array must be used to ensure the caller function can handle the return value correctly.

While C does not keep track of the length of an array, two popular methods have emerged to emulate this behavior. The first is to wrap the array in a struct with an integer storing the length. The second is to place a sentinel value at the end of the data in the array.

Noncompliant Code Example (struct)

The erroneous behavior results form getStock() returning null while main() forgets to add in a check for such a value. In this noncompliant code example, the check for item != null is missing from the if condition in function main().

In the example below, there is an inventory system keeping track of the total number of different items (denoted length) as well as the stock of each item. Adding a new item would increase length in the struct and a value would be assigned for stock. If there are no items being stocked, however, then the below code

#include <stdio.h>

enum { INV_SIZE=20 };

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

int *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\n", item[0]);
  
  return 0;
}

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

Compliant Solution

This compliant solution eliminates the null return and simply returns the item array as is even if it is zero-length. The main function can effectively handle this situation without exhibiting erroneous behavior.

#include <stdio.h>

enum { INV_SIZE=20 };

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

int *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);

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

int *getStock(Inventory iv) {
  return iv.stockOfItem;
}

Risk Assessment

Returning null rather than a zero-length array may lead to vulnerabilities when the client code does not handle null properly.

Other Languages

This guideline appears in the Java Secure Coding Standard as MET03-J. For methods that return an array or collection prefer returning an empty array or collection over a null value.

Related Vulnerabilities

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

References

[[Bloch 08]] Item 43: return empty arrays or collections, not nulls

      49. Miscellaneous (MSC)