Thread: How To Detect Pointers In Structure

Hi!

I want to write a function that will be able to detect pointers in a structure and if necessary, free them. It will work like this. If the pointer is found in a structure than check if it was freed. If it was not freed, than free it.

Example:

Code:

...
typedef struct _TEMP
{
    short *Number;
    char **Text, Temp;
    int i;
} TEMP;
...
CheckStruct (TEMP);
...

Is this possible? If anyone of you guys/girls have any idea, please post it.

Thanks!

>Then there's the problem of what to do with say a linked list ...

I don't need such a complex function. I'll skip that .

>Then for something like this
char **Text,
How do you know what the value of 'n' was in this call
Text = malloc( n * sizeof(char*) );
which allocated it in the first place

That's the biggest problem.

How can I distinct pointers from other types?

>Only the compiler can do that
>You can't for example tell simply by looking at an anonymous array of bytes, and decide whether any consecutive sequence of them are a valid pointer to malloc'ed memory

Damn!

>This is what a garbage collector does. Do a search:c "garbage collector code"

I don't need that. Thanks anyway.

Instead that I write the code for freeing the pointers in a structure I wanted that the function will do that instead of me.

Positive sides:
- less writing
- the function will never forget to free a pointer

Negative sides:
- I don't know how to write it

Mad idea: switch to C++ and use an auto_ptr

That's a good one. I'll switch to C++ as soon as I can. I'm learning it now for about a week.

What if you don't want to free the pointer? Can you ensure that the function gets called? If so, the function probably does something useful aside from silently killing the structure variable. I've found that you'll usually have more reliable code if you leave memory management to the highest reasonable level. That way your other functions don't have to worry about problematic memory management and you save yourself the trouble of making sure that memory management spanning multiple functions/nesting levels works properly. For example:

Code:

main()
{
    allocate memory A
    call f1(A)
    release memory A
    call f2()
}

f1(A)
{
    use A
}

f2()
{
    allocate memory B
    use B
    release memory B
}

Notice that the allocation and release is performed in the same function. You don't have to make sure that it was silently released elsewhere, or hunt for the point of release...It's in the same function, so finding it is easier.

The same goes for specialized memory routines:

Code:

alloc()
release(mem)

main()
{
    A = alloc()
    call f1(A)
    release(A)
    call f2()
}

f1(A)
{
    use A
}

f2()
{
    B = alloc()
    use B
    release(B)
}

Standard garbage collector:

Code:

/* this code has not been compiled. use at your own risk */
struct _memory {
void ** marshal, * pointer;
} memory; 
void init(memory * m)
{
 m->pointer = 0;
 m->marshal = &m->pointer;
}
int track(memory * m, void ** pointer)
/* returns 1 on success, 0 if the memory is still tied up */
{
 if(collect(m)) {
 m->marhal = pointer;
 m->pointer = *pointer;
 } else return 0;
 return 1;
}
int collect(memory * m)
/* returns 1 on success, even if no collection was necessary, 
   0 if the memory is still tied up */
{
 if(collectable(m))free(m->pointer), m->marshal = &m->pointer;
 else return 0;
 return 1;
}
int collectable(memory * m)
/* returns 1 if the memory is invalid, 0 if still good  */
{
 if(m->pointer != *m->marshal  // trash
 || m->marshal == &m->pointer) // neutral
  return 1;
 return 0;
}
int alloc(memory * m, void ** pointer, unsigned amount)
/* returns number of bytes allocated on success, 
   0 on bad alloc, -1 if the memory is still tied up */ 
{
 *pointer = malloc(amount);
 if(!*pointer)
  return 0; 
 if(!track(m, pointer)) 
  return -1;
 return amount;
}


int main()
{
const int size = 10240;
memory monitor, * m = &monitor;
char * buffer = 0;

init(m);

alloc(m, &buffer, size); // ignore result for now.

buffer = "- memory leak?"; 

collect(m);
}

Store the structures in a linked list or similar.
Launching the process in its own, low priority thread would be a good idea, too.

Naturally, C++ would better suited for this task, IMHO;