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Arithmetic operation with NULL pointer

Arithmetic operation performed on NULL pointer

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Description

This defect occurs when an arithmetic operation involves a pointer whose value is NULL.

Risk

Performing pointer arithmetic on a null pointer and dereferencing the resulting pointer is undefined behavior. In most implementations, the dereference can cause your program to crash.

Fix

Check a pointer for NULL before arithmetic operations on the pointer.

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.

Examples

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#include<stdlib.h>

int Check_Next_Value(int *loc, int val) 
 {
  int *ptr = loc, found = 0; 
  
  if (ptr==NULL)
   { 
      ptr++; 
      /* Defect: NULL pointer shifted */

      if (*ptr==val) found=1;
   } 
   
  return(found);    
 }

When ptr is a NULL pointer, the code enters the if statement body. Therefore, a NULL pointer is shifted in the statement ptr++.

Correction — Avoid NULL Pointer Arithmetic

One possible correction is to perform the arithmetic operation when ptr is not NULL.

#include<stdlib.h>

int Check_Next_Value(int *loc, int val) 
 {
  int *ptr = loc, found = 0; 
  
  /* Fix: Perform operation when ptr is not NULL */
  if (ptr!=NULL)
   { 
      ptr++;

      if (*ptr==val) found=1;
   }
   
  return(found);    
 }

Result Information

Group: Static memory
Language: C | C++
Default: Off
Command-Line Syntax: NULL_PTR_ARITH
Impact: Low

Version History

Introduced in R2013b

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See Also

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