Debugging with GDB Manual (5900-1473; WDB 6.2; January 2011)

When this setting is turned on, any time a memory block is allocated or deallocated, HP
WDB scrambles the space and overwrites it with a specific pattern.
This change to the memory contents increases the chance that erroneous behaviors will
cause the program to fail. Examples of such behavior include attempting to access space
that is freed or depending on initial values of malloc() blocks.
You can now look at the stack trace to understand where and how the problem occurred.
NOTE: Turning on scrambling slows down the program slightly, because at every
malloc() and free() call, the space involved must be overwritten.
14.10.9.5 Detect dangling pointers and dangling blocks
A pointer is a Dangling pointer if the block of memory it points to, has been freed by
the application. The block is called Dangling Block.
The same freed block could be subsequently allocated to the application in response to
another memory allocation request. In this scenario, if the application incorrectly tries to
write into the freed memory block using the dangling pointer, it could result in incorrect
or an undefined program behavior, as the new owner or function owning the same
allocated block would find different values in the heap block.
NOTE: Software literature names this concept as premature free or Reading/writing
freed memory using a pointer.
HP WDB tracks the dangling pointers and dangling blocks using a modified version of
Garbage collection. The enabler for doing this is by retaining all the freed blocks internally
within RTC without actually freeing it as long as possible. It displays all the potential
pointers to the freed dangling blocks, in the application data space.
The pointers are potential because the pointers need not be actual pointers and could
be a datum value and hence there are chances of false positives in the dangling report.
NOTE: HP WDB tries to help as much as possible to detect if these pointers are of type
datum or real pointers. In a -g compiled binary, HP WDB performs a look-up on a
symbol table to find the symbol name and type to find the symbol name of the potential
pointer and if its of pointer type, then the corresponding dangling block is really dangling
(not a false positive).
HP WDB turns on these checks, only when you specify set heap-check
retain-freed- blocks on.
14.10.9.6 Detect in-block corruption of freed blocks
HP WDB detects all the attempts of a program to write to the freed dangling blocks using
dangling pointers. We detect such in-block corruptions and are reported as part of the
existing info corruption command output.
170 HP-UX Configuration-Specific Information