How do i convert an integer to an address for a function pointer

[rodrigo.gloria]

I am trying to convert a integer to an address of a function pointer.

I want to encrypt the pointer and then do some validation, afterwards i
will decrpyt the pointer back to an address. Can this be done. Below
is some of my code.


typedef void (*funcptr)(void);
void tryme(void){};

void main()
{

char *buffer = new char[30];
// save address of tryme fuction into a string
itoa ( (unsigned int)(&tryme),buffer,16);
Encrypt(buffer);

//some code here

// retreive address tryme function
decryptbuffer = Decrypt(buffer);
unsigned int x = (unsigned int)atoi(decryptbuffer );
funcptr restoreFuncPtr = (funcptr)x; // convert int to funcptr
address

}



}

 

[benben]

Can this be done.

Compile and run your code and you'd find out.

Ben

 

[Ramki]

Hi,
Your code should work.
But, using reinterpret_cast<> is safe

Checkout the following code snippet

typedef int (*FP)(void);
int add()
{
return 10;
}

main()
{
FP funPtr;
unsigned int num = reinterpret_cast<unsigned int>(add);
funPtr = reinterpret_cast<FP>(num);
printf("%d", funPtr());
}


Thanx,
Rama

 

[Jacek Dziedzic]

I am trying to convert a integer to an address of a function pointer.

I want to encrypt the pointer and then do some validation, afterwards i
will decrpyt the pointer back to an address. Can this be done. Below
is some of my code.


typedef void (*funcptr)(void);
void tryme(void){};

void main()
{

char *buffer = new char[30];
// save address of tryme fuction into a string
itoa ( (unsigned int)(&tryme),buffer,16);
Encrypt(buffer);

//some code here

// retreive address tryme function
decryptbuffer = Decrypt(buffer);
unsigned int x = (unsigned int)atoi(decryptbuffer );
funcptr restoreFuncPtr = (funcptr)x; // convert int to funcptr
address

What makes you think the value of a pointer will FIT
inside an unsigned integer?

You might have 64-bit pointers and 32-bit unsigned ints,
what then?

HTH,
- J.

 

[Tomás]

posted:

I am trying to convert a integer to an address of a function pointer.

typedef unsigned long PointerNumericValue;


void* Encrypt( void* p )
{
PointerNumericValue value = reinterpret_cast<unsigned long>(p);

//Now encrypt it:

p << 2;

p += 3;

//Now return it:

return reinterpret_cast<void*>(p);
}

 

[Ramki]

Then u can take ,

unsigned _int64 (or)
long long unsigned int
........ here 64-bit pointers may fit.


Thanx,
Rama

 

[Jacek Dziedzic]

Then u can take ,

unsigned _int64 (or)
long long unsigned int
........ here 64-bit pointers may fit.

Yes, if you have stuff like "_int64". That, however,
raises the question of what happens if pointers are
bigger still, or ints are 16-bit?

What I meant to say was, you can't assume a 1:1
relationship between the size of a pointer and
sizeof(unsigned int).

- J.

 

[Jacek Dziedzic]

posted:





typedef unsigned long PointerNumericValue;


void* Encrypt( void* p )
{
PointerNumericValue value = reinterpret_cast<unsigned long>(p);

//Now encrypt it:

p << 2;

p += 3;

//Now return it:

return reinterpret_cast<void*>(p);
}

Still, what if it doesn't fit inside an unsigned long?,
although it's the OP's problem.

- J.

 

[Jack Klein]

Hi,
Your code should work.
But, using reinterpret_cast<> is safe

Checkout the following code snippet

typedef int (*FP)(void);
int add()
{
return 10;
}

main()

Your program is ill-formed, implicit int is no valid in C++, all
function definitions and declarations must specify a return type.

{
FP funPtr;
unsigned int num = reinterpret_cast<unsigned int>(add);
funPtr = reinterpret_cast<FP>(num);
printf("%d", funPtr());
}


Thanx,
Rama

Your compiler is non conforming, or you are not invoking it in
conforming mode, if it accepts this program without issuing a
diagnostic for the implicit int in the definition of main().

It is also non-conforming if it accepts the application of
reinterpret_cast on a function pointer type to any scalar type. This
is just plain not valid C++.

 

[Me]

I am trying to convert a integer to an address of a function pointer.

I want to encrypt the pointer and then do some validation, afterwards i
will decrpyt the pointer back to an address. Can this be done. Below
is some of my code.

Not guaranteed to work at all (see J.5.7 in the C standard)

typedef void (*funcptr)(void);
void tryme(void){}

<snip>

funcptr fn = tryme;
unsigned char buf[sizeof(fn)];
memcpy(buf, &fn, sizeof(fn));
encrypt_inplace(buf, sizeof(fn));

....

decrypt_inplace(buf, sizeof(fn));
memcpy(&fn, buf, sizeof(fn));
fn();

 

[rodrigo.gloria]

Thanks alot the method below worked.

Hi,
Your code should work.
But, using reinterpret_cast<> is safe

Checkout the following code snippet


typedef int (*FP)(void);
int add()
{
return 10;



}


main()
{
FP funPtr;
unsigned int num = reinterpret_cast<unsigned int>(add);
funPtr = reinterpret_cast<FP>(num);
printf("%d", funPtr());


}

Thank you,

Rodrigo