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.MIGHT checker flags situations in which a pointer value from a function call that might return null might subsequently be 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;
7  
8  char *xmalloc() {
9    if (global) return &global;
10   return 0;
11 }
12 
13 void npd_func_might(int flag, char *arg) {
14   char *p = &arg;
15   if (flag) p = xmalloc(); // xmalloc() may return NULL
16   if (arg) { p = arg; } // p may get a new value here
17   xstrcpy(p, "Hello"); // p will be dereferenced in xstrcpy()
18 }

Depending on the conditional statement at line 9, function xmalloc may pass a null pointer to function npd_func_might and then xstrcpy, in which it's dereferenced, depending on the condition at line 16. 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  char global;
8  
9  char *xmalloc() {
10    if (global) return &global;
11   return 0;
12 }
13 
14 void npd_func_might(int flag, char *arg) {
15   char *p = &arg;
16   if (flag) p = xmalloc(); // xmalloc() may return NULL
17   if (arg) { p = arg; } // p may get a new value here
18   xstrcpy(p, "Hello"); // p will be dereferenced in xstrcpy()
19 }

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