how to set pointer to a memory to NULL using memset?

Question

I have a structure

typedef struct my_s {

   int x;
   ...
} my_T;

my_t * p_my_t;

I want to set the address of p_my_t to

Answer 1

Don't use memset to initialize a null pointer as this will set the memory to all bits zero which is not guaranteed to be the representation of a null pointer, just do this:

p_my_t = NULL;

or the equivalent:

p_my_t = 0;

Answer 2

Per your reply (elsewhere in this post) stating:

Thanks, here is what I an trying to do

• two processes, A and B
• malloc p_my_t in A, B has N threads and can access it
• start deleting in A but I can not simply free it since threads in B may still using.
• so I call a function, pass address of p_my_t to B to set its address to NULL in B so no others threads in B can use anymore
• After call back from B, I then free memory in A

What you need is some form of synchronization between all your threads and processes. I'm not sure of how you are sharing this object between processes, but I suspect you are using shared memory.

Normally I would recommend using a shared pointer class (such as Boost's shared_ptr class), but I'm not sure of how well that would work in this scenario. You may want to consider tweaking your class so that it tracks its own references and can be used with the Boost intrusive_ptr class.

That way, process A can simply forget about the object, and when process B is finished, the instance of my_T will know that there are no more references left and clean itself up.

The synchronization would come into play here when my_T is adding or removing references internally (so you don't run into nasty race conditions where it thinks it should clean itself up but is really still in use).

One other approach that has a bit more of a "kluge" feel to it is to give each instance of my_T an "is-valid" flag so that all processes/threads using it will know whether or not to continue doing so.

For more details on Boost's various pointer classes, check out their documentation.

Answer 3

From reading your multi-thread comments I should say there is no safe sequence of code to accomplish your task. You will have to step back and reexamine your algorithm.