Checkers:NPD.CONST.DEREF

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Reference > C/C++ checkers > NPD.CONST.DEREF

Null-pointer constant value is dereferenced

Null is a special value in C/C++ that is used to indicate that a pointer doesn't point to any object. An attempt to access data using a null pointer causes a runtime error. When a program dereferences a pointer that is expected to be valid but turns out to be null, a null pointer dereference occurs. Null-pointer dereference defects often occur due to ineffective error handling or race conditions, and typically cause abnormal program termination. Before a pointer is dereferenced in C/C++ code, it must be checked to confirm that it is not equal to null.

The NPD checkers look for instances in which a null or possibly null pointer is dereferenced. The NPD.CONST.DEREF checker flags situations in which a null-pointer constant value is dereferenced either explicitly or through a function call.

Vulnerability and risk

Null-pointer dereferences usually result in the failure of the process. These issues typically occur due to ineffective exception handling.

Mitigation and prevention

To avoid this vulnerability:

  • Check for a null value in the results of all functions that return values
  • Make sure all external inputs are validated
  • Explicitly initialize variables
  • Make sure that unusual exceptions are handled correctly

Code examples

Vulnerable code example 

1  void xstrcpy(char *dst, char *src) {
2    if (!src) return;
3    dst[0] = src[0];
4  }
5  
6  void npd_const_deref(int flag, char *arg) {
7    xstrcpy(NULL, "Hello");
8  }

In this example, function xstrcpy may pass null pointer *dst to function npd_const_deref. This type of vulnerability can produce unexpected and unintended results.

Fixed code example

1  void xstrcpy(char *dst, char *src) {
2    if (!src) return;
3    if (!dst) return;
4    dst[0] = src[0];
5  }
6  
7  void npd_const_deref(int flag, char *arg) {
8    xstrcpy(NULL, "Hello");
9  }

In the fixed code, *dst is checked for null at line 3.

Related checkers

External guidance

Extension

This checker can be extended through the Klocwork knowledge base. See Tuning C/C++ analysis for more information.

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