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
READ_DANGLING	Reading from 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: readdngl.c
 */
# include <stdlib.h>
main()
{
	char b;
	char *a = (char *)malloc(10);


	free(a);
	b = *a;
	return (0);
}

Diagnosis at Runtime

[readdngl.c:12] **READ_DANGLING**
>>			 b = *a;
Reading from 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() readdngl.c, 9
	stack trace where memory was freed:
					main() readdngl.c, 11
Stack trace where the error occurred:
				main() readdngl.c, 12

• 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 the 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: Stack trace showing where 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.