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.CHECK.MUST checker flags situations in which a pointer that's been checked for a null value is subsequently dereferenced explicitly or through a function call without being checked 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  void npd_check_must() {
2    char *p = getSomeValue();
3    if (p != NULL) { }
4    p[0] = 0;
5  }

Although *p is checked for null at line 3, it's explicitly dereferenced at line 4 without being checked for null. (This example makes the defect obvious, but in more typical application code, the dereference would be harder to spot.) This type of vulnerability can produce unexpected and unintended results.

Fixed code example

1  void npd_check_must() {
2    char *p = getSomeValue();
3    if (p != NULL) {p[0] = 0;}
4  }

In the fixed version, the dereference is dependent on the check for null.

Related checkers

External guidance

Extension

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