0xDEADBEEF equivalent for 64-bit development?

问题

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?

回答1:

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.

回答2:

0xBADC0FFEE0DDF00D

回答3:

According to Wikipedia, BADC0FFEE0DDF00D is used on IBM RS/6000 64-bit systems to indicate uninitialized CPU registers.

回答4:

Most current 64-bit systems let you use only the lowest 2⁴⁸–2⁵² bits of the address space; higher bits of the address must be all-zero. Some chips (e.g. amd64) also let you use the highest 2⁴⁸–2⁵². Addresses outside these ranges cannot ever be mapped to accessible memory; the hardware simply won't allow it.

I therefore recommend you use a value close to 2⁶³, which is nowhere near either of the possibly-usable spaces. If the leading four hex digits are 7ff8, the value will be a double precision floating-point NaN, which is convenient. So my suggested cute hexadecimal phrase is 0x7FF8BADFBADFBADF.

By the way, you really don't want to use a value close to 0, because that makes it hard to tell an offset dereference of NULL — a structure member access, for instance — from a dereference of the poison pattern.

回答5:

I'm assuming you've already discounted NULL (i.e. 0 without the typecast). It's definitely the safest choice, as, in theory, a valid pointer could point to the memory address 0xDEADBEEF (Or any other non-NULL memory address).

回答6:

0xDEADBEEFBAADF00D might work.

回答7:

I don't have a good choice for you, but here's a list of hex words that you can use to make your phrase.

回答8:

Two 0xDEADBEEFs should be enough, I think..

回答9:

I see several answers claiming NULL is a good choice, but I disagree.

NULL is often used as a valid return value from functions. It indicates a failure return or an unknown value. This is a different meaning than "uninitialized pointer."

Using a debugger on the code and seeing NULL would then leave two possibilities: the pointer was never initialized or it had failed a memory allocation.

Setting the uninitialized pointer to 0xDEADBEEF or the 64-bit equivalent means that a NULL pointer indicates an intentional value.

回答10:

It depends on the OS and the environment, of course. I don't think 0xDEADBEEF is necessarily a bad pointer in an arbitrary 32-bit system, either.

Realistically, any modern OS should be access-protecting the first few pages of process memory, so NULL should be a good invalid pointer value. Conveniently enough, it's already pre-defined for you.

回答11:

0x42 could work on both 32bit and 64bit ? (It should still trigger a crash since it is close enough to the NULL pointer, and given that it's rather large, chances are you would not have it within a regular dereference of a structure field with the structure pointer being NULL).

回答12:

As the system I worked on basically runs on x86_64 platform, the value I use is:

0xDEADBEEFDEADBEEF

Reasons are:

• On x86_64 platform, only the low-order 48 bits are used for virtual memory address in current implementation, meaning any value > 2^48 should work: https://en.wikipedia.org/wiki/X86-64
• As 0xDEADBEEF is already very well known for this purpose in 32bit, 0xDEADBEEFDEADBEEF in 64bit is just more 'backward compatible'

来源：https://stackoverflow.com/questions/1258051/0xdeadbeef-equivalent-for-64-bit-development