EXPR_NULL
Expression uses NULL pointer
Problem
Diagnosis (at runtime)
Repair

EXPR_NULL

Expression uses NULL pointer

This error is generated whenever an expression operates on the NULL pointer.

Problem

The following code fragment declares a pointer, a, which is initialized to zero by virtue of being a global variable. It then manipulates this pointer, generating the EXPR_NULL error.

	1:	/*
	2:	 * File: expnull.c
	3:	 */
	4:	char *a;
	5:
	6:	main()
	7:	{
	8:		char *b;
	9:
	10:		b = &a[1]; 
	11:		return (0);
	12:	}

Diagnosis (at runtime)

1.	[expnull.c:10] **EXPR_NULL**
	>> 		b = &a[1];
	
2.		Expression uses null pointer: a[1]

3.		Stack trace where the error occurred:
			main() expnull.c, 10

1. Source line at which the problem was detected.
2. Description of the problem and the expression that is in error.
3. Stack trace showing the function call sequence leading to the error.

Repair

One potential cause of this error is shown in this example. The pointer a is a global variable that will be initialized to zero by the compiler. Since this variable is never modified to point to anything else, it is still NULL when first used.

One way the given code can be corrected is by adding an assignment as follows

	/*
	 * File: expnull.c (modified)
	 */
	char *a;

	main()
	{
 	     char *b, c[10];
	     a = c;
             b = &a[1];
	     return (0);
        }

It could also be corrected by allocating a block of memory.

A second possibility is that the pointer was set to zero by the program at some point before its subsequent use and not re-initialized. This is common in programs which make heavy use of dynamically allocated memory and which mark freed blocks by resetting their pointers to NULL.

A final common problem is caused when one of the dynamic memory allocation routines, malloc, calloc, or realloc, fails and returns a NULL pointer. This can happen either because your program passes bad arguments or simply because it asks for too much memory. A simple way of finding this problem with Insure++ is to enable the RETURN_FAILURE error code via your .psrc file and run the program again. It will then issue diagnostic messages every time a system call fails, including the memory allocation routines.

EXPR_DANGLING

EXPR_UNINIT_PTR