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.GEN.MUST checker flags situations in which a pointer that's been assigned a null constant value 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  void xstrcpy(char *dst, char *src) {
2    if (!src) return;
3    dst[0] = src[0];
4  }
5  
6  char global_buf[256];
7  
8  void npd_gen_must(int flag) {
9    char *p = global_buf;
10   if (flag) p = 0; // NULL is assigned to p
11   xstrcpy(p, "Hello");
12 }

Klocwork flags a defect in this example, because constant NULL is assigned to 'p' if condition 'flag' is true and then dereferenced through a call to function xstrcpy. This 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  char global_buf[256];
8  
9  void npd_gen_must(int flag) {
10    char *p = global_buf;
11   if (flag) p = 0; 
12   xstrcpy(p, "Hello");
13 }

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.