Access violation reading location exception

[ycinar]

A quick question:

Why doesn't the following code catch the Access violation reading location exception?

it crashes on line if ( xyz ) with an Access violation reading location exception. Is not catch supposed to catch it? Practically, it doesnt.
[code=cpp]
try
{
if ( xyz ) // crashes here
{
abc = xyz->ID;
}
}

catch (...)
{
//exception
}
[/code]
Any suggestion on how to get it caught?

Thanks!

[Banfa]

The C++ try{...} catch {...} methodology catches software exceptions. Accessing an invalid memory location is a hardware exception (that is it is not obviously wrong in the software it is only when you look at the location and find no hardware that the error is realised.

You could try this sort of exception using a signal but that is not going to help very much as following signal you would have to peform some major reset operation of your program (or exit it).

It would be much better not to create the situation in the first place, you can do this by always ensuring that your pointers either point to a valid location or have been set to NULL.

This should not be hard to do, intiialise them to NULL, then you can assign to them to point them to valid memory. When that memory is no longer valid (it has been deleted or gone out of scope) reset there value back to NULL.


It is dangerous to leave apointer floating with a random value.

[weaknessforcats]

try
{
if ( xyz ) // crashes here
{
abc = xyz->ID;
}
}

catch (...)
{
//exception
}

This code shouild not crash on the if statement if xyz is a pointer. The if statement does a test for true or false only and does not access the location in the pointer.

What can happen is that you never initialized xyz so that it is non-zero (hence true) but contains a garbage value. That will bring you down.

[ycinar]

This code shouild not crash on the if statement if xyz is a pointer. The if statement does a test for true or false only and does not access the location in the pointer.

What can happen is that you never initialized xyz so that it is non-zero (hence true) but contains a garbage value. That will bring you down.

I am surprised too that it crashes at if statement.

it is more understandable crashing at xyz->ID ...

but it crashes at if statement for some reason. It is definitely initialized by the way.

[weaknessforcats]

Post a little more code around this area. Also the struct/class you are using.

[ycinar]

the code is really all over the place.. but here is the most relevant part from the header file.

class APerson {
public:
APerson(Person* thePerson);
virtual ~APerson();

void SetPerson(Person* thePerson) 
                   { 
                    xyz = thePerson;
                   }

private:
	Person* xyz;

xyz must be get deleted somewhere, and that must be the reason why it is crashing.

[ycinar]

How to check an address of a pointer?

Can anyone advise on this please?

I have got a pointer like this

APerson* xyz = NULL;

xyz = abc->Caller();

abc is initialized somewhere publicly. Sometimes abc->Caller() returns 0xfeeefeee value and which will cause xyz to crush a later line.

So what I would like to is, I want to

if (xyz = 0xfeeefeee) 
// do something

so what is the syntax for the if above? How can I check the address of a pointer?

[ycinar]

is there a way to check the address value of a pointer?

if (xyz == 0xfeeefeee)
// do something

i wonder if there is such a thing, if so, what would the syntax for the if statement above?

thanks!

[ycinar]

Its probably just a memory dump which means that you are going to need a good text editor to see the hex information. Try Notepad++ or TextPad.

Thanks RedSon,

another quick question

is there a way to check the address of a pointer? something like:

if (xyz == 0xfeeefeee)
// do something

is there a special meaning for the address 0xfeeefeee as it crashes the program for me there?

[sicarie]

You know, if that was a new question, I'd suggest a new thread, but as that is already in a thread, I would recommend waiting for someone to answer it instead of being impatient and posting it all over, so I'm going to move it back over to your thread.

[weaknessforcats]

the code is really all over the place.. but here is the most relevant part from the header file.


Code: ( text )
class APerson {
public:
APerson(Person* thePerson);
virtual ~APerson();

void SetPerson(Perso n* thePerson)
{
xyz = thePerson;
}

private:
Person* xyz;


xyz must be get deleted somewhere, and that must be the reason why it is crashing.

If xyz is ever deleted then the APerson object is trash. This is one reason to not use pointers as data members. Use handles instead. There's an article in the C/C++ Articles forum on Handle Classes that addresses this issue.

If xyz is 0 or has a value outside your process address space (like it was never initialized) you will crash when you use it.

If xyz is deleted, the deleter has to notify all APerson objects that have the pointer that is going to be deleted. This is a big, big job. Again, a handle rather than a pointer will solve this.

Can anyone advise on this please?

I have got a pointer like this


Code: ( text )
APerson* xyz = NULL;

xyz = abc->Caller();


abc is initialized somewhere publicly. Sometimes abc->Caller() returns 0xfeeefeee value and which will cause xyz to crush a later line.

So what I would like to is, I want to


Code: ( text )
if (xyz = 0xfeeefeee)
// do something


so what is the syntax for the if above? How can I check the address of a pointer?

This looks like a case where abc->Caller() is returning an uninitialized pointer. Don't bother hard-coding an address value to check as the garbage address will be different from one execution to the next.

Your solution is to make sure that Caller() does not return garbage.

When it comes to using pointers in C++ classes:
1) you must initialize them. Period.
2) they should point to items on the heap so that you are assured that what they point to won't be deleted by the compiler when some function completes
3) they cannot be deleted unless the deleter knows it is deleting the last copy of the pointer in the program. A handle covers this condition.

Finally,
[quote=ycinar]

the code is really all over the place..

is a bad sign. Try to get all the code for a class into one source file and the class declaration into one header file. Use separate headers for different classes and separate source files for different classes.

[ycinar]

This looks like a case where abc->Caller() is returning an uninitialized pointer. Don't bother hard-coding an address value to check as the garbage address will be different from one execution to the next.

The address doesnt differentiate from one execution to the next. I know best way is not to return this value which seems non-trivial.

so im still interested in learning how to check the address of pointer?

if (xyz ==0xfeeefeee)
{
//do something
}

[weaknessforcats]

so im still interested in learning how to check the address of pointer?

Code: ( text )
if (xyz ==0xfeeefeee)
{
//do something
}

then you will need to type cast the pointer to an unisigned int to make the test.

Be advised, this is the wrong approach. The correct thing to do is to see why Caller() is returning garbage.

[Banfa]

You should not rely on the value 0xfeee. This is a value that the Microsoft Runtime Debug library assigns to memory as it is deallocated to aid in bug tacking.

This means that

1. On a non-Microsoft system the test may not work

2. On a Microsoft system built for production the test may not work

3. Using a future version the Microsoft Debug Library the test may not work