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

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.