RB Smissaert wrote:
OK, I understand better now. This though doesn't seem quite right to
me:
0, //sqlite3_libversion,
It looks sqlite3_libversion should be a string.
No, this is a list of function pointers. C has no notion of the *name*
of the function, only its address. Functions are called by writing that
address with a following "()". If you used a string containing the name
of the function you end up trying to execute ASCII text. This is never
going to work, though it is the basis of the buffer overflow technique
that our friends the virus writers use. :(
Would it matter that I make it 0, rather than say ""? If it should be
an empty string then how would I do that in C?
You're confusing 0 the string terminator with 0 the NULL pointer. "" is
an empty string but it has an address and at that address is a 0 which
(as defined by the C standard) terminates the string. 0 in a pointer
context tells the compiler to generate a special (NULL) pointer which
cannot be dereferenced. The C standard defines that 0 in source code
means NULL pointer but the implementation value is not defined. That is,
a NULL pointer may or may not be numerically 0 in the binary object but
usually is.
It matters because Dr Hipp has set it up that way. Wherever it is that
these functions are dispatched, there will be a NULL pointer check. If
the pointer is NULL then no attempt is made to run the function. As I
said above, "" has an address so it will pass the not NULL test and an
attempt will be made to execute whatever that string pointer points at
(0 followed by ???) and...crash.
The real trouble with this is that execution might proceed for a while
or even end up at a random but legitimate location within the program
code, meaning the crash occurs some time later and far away from the
actual bug location. M$ have something called DEP in XP which is meant
to prevent this kind of thing, but it needs a reasonably up to date CPU.
We might be in danger of getting coughed <off topic> soon. ;)
Martin
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