In this section:

Overview

This problem occurs when an attempt is made to dereference a pointer that points to a block of memory that has been freed.

Code	Description	Enabled	Reported	Platform
WRITE_DANGLING	Writing to a dangling pointer		Runtime	Windows/Unix

Problem

The following code attempts to use a piece of dynamically allocated memory after it has already been freed.

/*
 * File: writdngl.c
 */
#include <stdlib.h>

main()
{
	char *a = (char *)malloc(10);

	free(a);
	*a = 'x';
	return (0);
}

Diagnosis at Runtime

[writdngl.c:11] **WRITE_DANGLING**
>>		 *a = 'x';
Writing to a dangling pointer: a

---- Associated Common Weakness Enumerations ----
CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
CWE-416: Use after free
CWE-825: Expired pointer dereference

Pointer:0x000173e8
In block:0x000173e8 thru 0x000173f1 (10 bytes)
		block allocated at:
			malloc() (interface)
			main() writdngl.c, 8
Stack trace where memory was freed:
			main() writdngl.c, 10
Stack trace where the error occurred:
			main() writdngl.c, 11
	**Memory corrupted. Program may crash!!**

• Line 2: Source line at which the problem was detected.
• Line 3: Description of the problem and the expression that is in error.
• Line 5-8: CWEs associated with this problem.
• Line 10: Value of the dangling pointer variable.
• Line 11: Description of the block to which this pointer used to point, including its size, name, and the line at which it was allocated.
• Line 15: Indication of the line at which this block was freed.
• Line 17: Stack trace showing the function call sequence leading to the error.

Repair

Check that the de-allocation that occurs at the indicated location should indeed have taken place. Also check that the pointer you are using should really be pointing to a block allocated at the indicated place.

References

The table below shows Common Weakness Enumerations associated with this error.