For C++ development for 32-bit systems (be it Linux, Mac OS or
Windows, PowerPC or x86) I have initialised pointers that
would otherwise be undefined (e.g. they can not immediately
get a proper value) like so:
int *pInt = reinterpret_cast<int *>(0xDEADBEEF);
(To save typing and being DRY the right-hand side would normally be in a constant, e.g. BAD_PTR.)
If pInt is dereferenced before it gets a proper value then it will crash immediately on most systems (instead of crashing much later when some memory is overwritten or going into a very long loop).
Of course the behavior is dependent on the underlying hardware (getting a 4 byte integer from the odd address 0xDEADBEEF from a user process may be perfectly valid), but the crashing has been 100% reliable for all the systems I have developed for so far (Mac OS 68xxx, Mac OS PowerPC, Linux Redhat Pentium, Windows GUI Pentium, Windows console Pentium). For instance on PowerPC it is illegal (bus fault) to fetch a 4 byte integer from an odd address.
What is a good value for this on 64-bit systems?
Generally it doesn't matter exactly what pattern you write, it matters that you can identify the pattern in order to determine where problems are occurring. It just so happens that in the Linux kernel these are often chosen so that they can be trapped if the addresses are dereferenced.
Have a look in the Linux kernel at include/linux/poison.h. This file contains different poison values for many different kernel subsystems. There is no one poison value that is appropriate.
Also, you might check per-architecture include files in the Linux kernel source tree for info on what is used on specific architectures.