In a language like C, for example, if a routine receives a pointer, is there any system call or other test that can be applied to the pointer to check that it is a valid memory location, other than catching SIGSEGV or equivalent?
A:
Well, if you knew where the memory being pointed to was being stored (on the stack, for instance), you could check to see if it's in a certain 'range' that is the approximate address range of the stack. That could also work for something on the heap, if you have an idea of how big your heap "should" be. It's definitely not a fail-safe approach, but I'm unaware of any sure-fire methods for checking the 'validity' of a pointer.
hatorade
2010-07-16 09:53:26
+1
A:
No, you can't for sure check whether the address is invalid. Even if you used some operating system function to test if teh address is mapped into the address space you still can't be sure if the address is of some service data that you should not read or modify.
One good example. If your program uses Microsoft RPC to accept calls from another program you have to implement a set of callback functions to server the requests. Those callback functions will be run on separated threads started by RPC. You don't know when those thereads start and what their stack size is, so you can't detect whether a buffer overrun occurs if you write through an address that is meant to be of a stack variable but accidentially is to the stack of another thread.
sharptooth
2010-07-16 10:07:59
A:
If you mean purely within your own application you can establish a convention that any memory allocated by your code is initialized in a way you can recognize. E.g. in one project I saw they wrote an eyecatcher in the first few bytes. In some products I know they write a unique id at the start and end and each time it's accessed they check the 2 ids still match to show it's not been corrupted. E.g CICS on z/Series does the latter.