CWE Rule 476

NULL Pointer Dereference

Since R2023a

Description

Rule Description

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.

Polyspace Implementation

The rule checker checks for Dereference of a null pointer.

Examples

Dereference of a null pointer

Issue

This issue occurs when you use a pointer with a value of NULL as if it points to a valid memory location. If you dereference the zero address, such as 0x00, Polyspace^® considers the null address as equivalent to NULL and raises this defect.

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 (or source code tooltips in Polyspace as You Code) 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 this event history, you can search for previous references of variables relevant to the defect using right-click options in the source code and find related events. See also Interpret Bug Finder Results in Polyspace Desktop User Interface or Interpret Bug Finder Results in Polyspace Access Web Interface (Polyspace Access).

See examples of fixes below.

Extend Checker

A default Bug Finder analysis might not raise this defect when the input values are unknown and only a subset of inputs cause an issue. To check for defects caused by specific system input values, run a stricter Bug Finder analysis. See Extend Bug Finder Checkers to Find Defects from Specific System Input Values.

Example — Null pointer error

#include <stdlib.h>

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

 *p=arr[0];  //Noncompliant
 /* 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;
}

Check Information

Category: Pointer Issues

Version History

Introduced in R2023a