Exception handler

Question

There is this code:

char text[] = "zim";
int x = 777;

If I look on stack where x and text are placed there output is:

09 03 00 00 7a 69 6d 00

Where:

• 09 03 00 00 = 0x309 = 777 <- int x = 777
• 7a 69 6d 00 = char text[] = "zim" (ASCII code)

There is now code with try..catch:

char text[] = "zim";
try{
  int x = 777;
}
catch(int){
}

Stack:

09 03 00 00 **97 85 04 08** 7a 69 6d 00

Now between text and x is placed new 4 byte value. If I add another catch, then there will be something like:

09 03 00 00 **97 85 04 08** **xx xx xx xx** 7a 69 6d 00

and so on. I think that this is some value connected with exception handling and it is used during stack unwinding to find appropriate catch when exception is thrown in try block. However question is, what is exactly this 4-byte value (maybe some address to excception handler structure or some id)?

I use g++ 4.6 on 32 bit Linux machine.

Answer 1

AFAICT, that's a pointer to an "unwind table". Per the the Itanium ABI implementation suggestions, the process "[uses] an unwind table, [to] find information on how to handle exceptions that occur at that PC, and in particular, get the address of the personality routine for that address range. "

The idea behind unwind tables is that the data needed for stack unwinding is rarely used. Therefore, it's more efficient to put a pointer on the stack, and store the reast of the data in another page. In the best cases, that page can remain on disk and doesn't even need to be loaded in RAM. In comparison, C style error handling often ends up in the L1 cache because it's all inline.