Null pointer

NULL pointer dereferenced

Description

This defect occurs when you use a pointer with a value of NULL as if it points to a valid memory location.

Risk

Dereferencing a null pointer is undefined behavior. In most implementations, the dereference can cause your program to crash.

Fix

Check a pointer for NULL before dereference.

If the issue occurs despite an earlier check for NULL, look for intermediate events between the check and the subsequent dereference. Often the result details show a sequence of events that led to the defect. You can implement the fix on any event in the sequence. If the result details do not show the event history, you can trace back using right-click options in the source code and see previous related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface.

See examples of fixes below.

Examples

Null pointer error

#include <stdlib.h>

int FindMax(int *arr, int Size) 
{
 int* p=NULL;

 *p=arr[0];
 /* Defect: Null pointer dereference */

 for(int i=0;i<Size;i++)
  {
   if(arr[i] > (*p))
     *p=arr[i];    
  }

 return *p;
}

The pointer p is initialized with value of NULL. However, when the value arr[0] is written to *p, p is assumed to point to a valid memory location.

Correction — Assign Address to Null Pointer Before Dereference

One possible correction is to initialize p with a valid memory address before dereference.

#include <stdlib.h>

int FindMax(int *arr, int Size) 
{
 /* Fix: Assign address to null pointer */
 int* p=&arr[0];       

 for(int i=0;i<Size;i++)
  {
   if(arr[i] > (*p))
     *p=arr[i];    
  }

 return *p;
}

Result Information

Group: Static memory

Language: C | C++

Default: On

Command-Line Syntax: NULL_PTR

Impact: High

CWE ID: 476, 690