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

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

Answer 2

I think maybe you want

extern void set_t_pointer_to_null(my_T *pp);

and call

set_t_pointer_to_null(&p_my_t);

where

void set_t_pointer_to_null(my_T *pp) { *pp = NULL; }

I'm not sure it's worth defining a function to do this, but I think this answers the question you're trying to ask.

Answer 3

If I get it right, memset won't solve your problem. If A and B are separate processes, then p_my_t in process A will be different form p_my_t in process B. You just can't pass a pointer between different processes. I sugest you use some kind of IPC mechanism in order to sinchronyze your two processes (message queues, for example), and just using p_my_t = NULL instead of memset.

Answer 4

What exactly are you trying to do? p_my_t is already a pointer, but you haven't allocated memory for it. If you want to set the pointer to NULL, simply do

p_my_t = NULL;

Trying to dereference this pointer will result in a segmentation fault (or access violation on Windows).

Once the pointer actually pointers to something (e.g. via malloc() or by assigning to it the address of a struct my_T), then you can properly memset() it:

memset(p_my_t, 0, sizeof(struct my_T));

This will zero out the entire structure, setting all fields to zero.

Answer 5

Per your update, it seems to me that what you are really trying to do is guard access to a resource, which means you should use a read/write lock that is shared between the processes to guard the ptr to that resource, and test the ptr before using.

• Allocate the structure in shared memory.
• Allocate the ptr to the structure in shared memory.
• Guard access to the ptr to the structure with a Read/Write lock.
• Process A should acquire a WRITE lock to the ptr when initializing or invalidating the ptr and structure.
• Process B should acquire a READ lock to the ptr, and test that the ptr is valid before using the structure

Answer 6

The recommended code for setting a pointer to null is assigning 0 (zero). Bjarne Stroustrup does it :) Anyway it is just as expressive as NULL and does not depend on a macro definition.

Note that NULL is not a keyword, it is not reserved and while it would be confusing to redefine, nothing says that you should not (more than style). A coworker often jokes about defining NULL to something different than 0 in some header just to see how other people's code behave.

In the upcoming standard there will be a more expressive nullptr keyword to identify a null pointer.