how to not use GNU "C" Library

astropirhana · 08-24-2006, 09:59 AM

I am reading about the way Linux Systems Access a null pointer. The GNU "C" implementation allows you to read it, but not write to it.

Code:

char *tst = (char*) 0;
printf("Reading a null pointer gives you %s.\n", tst);
sprintf(tst, "Write to null pointer");

//so you get:
$./tst
Reading a null pointer gives you (null).
Segmentation fault.

If you read it as a string you get "(null)\0". If you are not using the GNU "C" library, you can do neither and you get your segmentation fault on the read.

//so you get
$./tst
Segmentation fault.

How do I eliminate GNU "C" Library to execute this code?

astro

cupubboy · 08-24-2006, 10:25 AM

I'm more of a c++ guy .. than a c ..

but this line

Code:

char *tst = (char*) 0;

Seems terribly wrong to me
so you have 0 wich is an literal constant and you cast that to a pointer is this legal??

wouldn't it be correct to have

Code:

Whatever * myVar = NULL; // or 0

??

Quigi · 08-24-2006, 10:28 AM

No, you cannot read it. The function printf is cautious enough to check whether you passed a null pointer (which you probably shouldn't), and in that case does NOT dereference it but instead prints "(null)".

What are you trying to do?

aluser · 08-24-2006, 10:31 AM

The printf function happens to check if the pointer you give it is null, and prints "(null)" if so. There's nothing fundamental in C or linux that makes reading a null pointer give you "(null)". In fact, if printf() had actually used the null pointer, it would have segfaulted.

edit: oops, quigi answered first

astropirhana · 08-24-2006, 10:36 AM

I am reading about abusing memory and the NULL pointer.

The code is from a book. And the book says the GNU "C" Lib allows you to read the pointer but not write to the memory that it points to.

Other implementations of the "C" Library dont allow you to read the null pointer.

So how do I swap out GNU C Lib for some other using gcc?

Thanks for all your responses

astro

astropirhana · 08-24-2006, 10:54 AM

Quote:

Originally Posted by Quigi

No, you cannot read it. The function printf is cautious enough to check whether you passed a null pointer (which you probably shouldn't), and in that case does NOT dereference it but instead prints "(null)".

What are you trying to do?

I think this is a great answer!

How do I use a different C Library from GNU C Lib?

ahoodin

Quigi · 08-24-2006, 12:10 PM

Thanks. Aluser's reply is more accurate than your book.

Normally you'd strive to write portable code, that doesn't rely on a particular implementation's quirks. So I assume you are into experimenting and just want to see that SEGV. (You could get one easily with "char oops = *tst;" -- that illustrates that you cannot read through the null pointer.)

To answer your question, read "man gcc", particularly about the linker option -nostdlib; you'll have to supply a replacement library. Disclaimer: I haven't done that.

astropirhana · 08-24-2006, 12:14 PM

Thanks alot guys!

astro

xhi · 08-24-2006, 12:18 PM

> The code is from a book. And the book says the GNU "C" Lib allows you to read the pointer but not write to the memory that it points to.

your book must be leaving out an important illustration that would show you why it is could be possible to read or write to a NULL ptr. a NULL ptr points to memory that is just as usable as any other memory if the OS would let it. when memory is laid out for your program at runtime, the lowest memory 0-?? (usually a couple of addresses above 0) are set aside for a NULL area.

anyone correct me if im wrong but i think the runtime address space looks like so

--- high address

heap

---
...
... extra space for heap to grow
...
---

stack

---
NULL address
---

so in theory it would be possible to read or write to null, but it is typically not accessible.

what i am not sure about is if it is read only or not. i thought it was unreadable, completely unreadable. that is why you segfault, because you cant read that adderess. but i have not had a reason to dig into that.