Dereferencing a char pointer in C/C++?

Question

I am working in a program using the DWARF and ELF info. I am hooked to another program using a tool called Pin. I have the addresses from the global variables declared in "Y" program and I hook it into my Pin module which I will call "X".

I am getting a bunch of addresses for these global variables. Unfortunately I am having troubles when I try to dereference them. For example (right now I am doing things manually to see if it is doing what is supposed to be doing):

char * limit, * address1;

for(address1 = (char *) 0x804A040, limit = address1 + bytesize; address1 < limit; address1++)
      cout <<  *(address1) << "\n";

I should be getting the variable that is stored in that address which is a char * to a word. Do I have to dereference two pointers in this case? The address and then the char * stored in that address?

This works perfectly fine when I want to dereference an int variable, but whenever I try to dereference a char pointer or variable I get non-ASCII values...

Answer 1

Think like this: if you want to dereference an int , you use int * :

int *addr = (int *)0x12345678;
printf("%d\n", *addr);

But a string is already a pointer, so if you do this:

char *addr = (char *)0x12345678;

You don't need to dereference it to print it out, so you get it this way:

printf("%s\n", addr);

---

Additionally, suppose you have this memory:

00000001:  0x12345678
...
12345678:  'A'
12345679:  'B'
1234567A:  '\0'
...

Let's access the address 0x00000001 and see what we can do:

unsigned int *addr = (unsigned int *)0x00000001;

Now *addr holds an integer which is the address of the first character of our string. Functions that take a string argument, usually ask for the string address, that is, a pointer to it, so *addr is also the string we should print on cout or printf for example.

// This should print "AB"
printf("%s\n", (char *)(*addr));

Now if you dereference our value ( 0x12345678 ), we'd get the string's first character, and only it, as a char . So if you misuse it and dereference it to a char * , printf would try to search for the memory 'A', which is 0x00000041 , and would probably get a segmentation fault.

So, the right way to dereference its value, to get the first character would be this:

// This should print "A"
printf("%c\n", (char)(**addr));

So, as a conclusion, if you are still getting non-ASCII values, that's probably an issue with this kind of hierarchy between addresses, and I would recommend you to strictly debug the memory and see if creating a pointer to a pointer is the right path to your string.

Answer 2

Yes, you need to dereference two pointers. First to get the address of the variable that holds the char*, and then to dereference the char*.