Checkers:NPD.FUNC.MUST

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Reference > C/C++ checkers > NPD.FUNC.MUST

Possible null pointer 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.FUNC.MUST checker flags situations in which a pointer value from a function call that might return null is subsequently dereferenced explicitly or passed to a function that dereferences it without checking it for null.

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  int global;
2  
3  int *xmalloc() {
4    if (global) return &global;
5    return 0; // xmalloc() may return NULL
6  }
7  
8  void npd_func_must(int flag, char *arg) {
9    int *p = xmalloc(); // xmalloc() may return NULL
10   *p = 1;  // pointer is dereferenced without validation
11 }

Depending on the conditional statement at line 4, function xmalloc may pass a null pointer to npd_func_must, in which it's dereferenced. This type of vulnerability can produce unexpected and unintended results.

Fixed code example

1  int global;
2  
3  int *xmalloc() {
4    if (global) return &global;
5    return 0; // xmalloc() may return NULL
6  }
7  
8  void npd_func_must(int flag, char *arg) {
9    int *p = xmalloc(); // xmalloc() may return NULL
10   if (p!= 0) // check for null
11   *p = 1;  // pointer is dereferenced 
12 }

In the fixed code, *p is checked for null at line 10 before the dereference.

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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