Thread: NULL Pointer Access?

I am having some problems with the following code...

I believe it should pass through without any major issues. Am I doing enough checking for NULL pointers in here.

Within the Netware environment where this is executed, I had this chunk of code error on 2 different servers whilst processing some data.

Code:

if ( pItem -> pItemLevel2 == NULL )
{
sprintf ( MiscBuff, "Internal error code 9007 line %i" , __LINE__ ) ;
Exception ( EX_CRITICAL, 7, FBS_INTDBASE, MiscBuff ) ;
break ;
}

for ( pLoop = pItem->pItemLevel2->pItemLevel3 ; 
       pLoop != NULL ;
       pLoop = pLoop -> pNext )
            if ( pLoop -> cStatus != CHECK )
	break ;

Does anyone have any ideas for why this might fail - am I just unlucky?

Cheers in advance.

DS

>Am I doing enough checking for NULL pointers in here.
>Does anyone have any ideas for why this might fail

Do you check to see whether pItem is NULL before dereferencing it using the -> operator?

Yes, pItem is checked before that section.

Here are the potential unsafe parts looking only at what you have given:

>if ( pItem -> pItemLevel2 == NULL )
pItem may be NULL, thus dereferencing it would result in some form of access violation.

>sprintf ( MiscBuff, "Internal error code 9007 line %i" , __LINE__ ) ;
MiscBuff may not have memory allocated to it if it is a pointer and may not be large enough to hold the string and a decimal value if it is an array.

>Exception ( EX_CRITICAL, 7, FBS_INTDBASE, MiscBuff ) ;
Theoretically this could be unsafe, but I can't be more specific without the code for the Exception routine. I'll assume that it works correctly and all of the arguments are valid.

>for ( pLoop = pItem->pItemLevel2->pItemLevel3 ;
pItem may be NULL, pItemLevel2 may be NULL. Either of these cases would result in some form of access violation.

>pLoop = pLoop -> pNext )
pLoop may not be a valid pointer. See the next comment.

>if ( pLoop -> cStatus != CHECK )
If pLoop is an invalid pointer but not NULL then dereferencing it would be a Bad Thing. This can occur if your list structure is corrupted somewhere along the line.

The for could loop terminate (if nothing caused it to break) with pLoop equal to NULL. Any subsequent code that dereferences pLoop without first checking to see that it is not NULL would also be a problem.