very simple

hi all...

just getting into c. trying to understand pointers. reading the C programming language book. i have a few very simple lines of code that produce:

Bus error (core dumped)

here is the code:

Code:

---

int main () {
        int k, *p, *m,l;
        k = 10;
        p = &k;
        *p = 23;  //  line 5
        l = *p;
        *m = l;  // problem line
   
        printf("%d\n",*m);

        return 0;
}

---

the cc -Wall doesnt complain. but if i run it i get the above bus error.
if i coment out the 'problem line' it works fine. don't understand since 'line 5' is pretty much the same statement except instead of actually assigning a number i assign the 'l' variable that holds that number.
i know that if i change 'problem line' to m = &l; it works fine. i just dont understand why it wouldn't work the other way around. like in line 5.

is it because *m doesnt even have an address yet?

i'd appreciate an explanation. thanks....

http://cpwiki.sourceforge.net/A_pointer_on_pointers
Pointers are not storage units.

> is it because *m doesnt even have an address yet?
Yes, you need to do m = something, before you do *m = somethingElse

Just like you need to set var to something before adding to it, say
var = 0;
var = var + 1;

The only difference with pointers is that it's more likely to blow up with a bus error / seg fault if you do it wrong. Most uninitialised values just produce garbage results.

Quote:

---

Originally Posted by toshog

the cc -Wall doesnt complain.

---

That is interesting. I would expect that the compiler can see that m is being used while not initialized.

I just verified: gcc 3.4.3 indeed doesn't complain, Visual Studio 2005 gives a warning: "uninitialized local variable 'm' used".

Does anyone have a gcc 4 version around to test this? I'd be interested to know whether that one warns about this.

--
Computer Programming: An Introduction for the Scientifically Inclined

Detection of uninitialized variable use in gcc is relies on the flow-graph-analyzis, and it's only done if you turn on certain levels/forms of optimization (not sure which level - I know -O2 will do it). I doubt it changes in gcc 4, but rather gcc 3.x can do that already, but not without enabling optimization.

--
Mats

Which address "m" should get?

Typically you use malloc and free to get an allocated memory block to keep in your pointer.