Very Basic C Question

Question

Can someone check my understanding and correct me if i am wrong?

int p = 5; //create an int holding 5
int *ptr; //create a pointer that can point to an int
*ptr = &p; // not sure - does this mean that my pointer now points to memory address   five, or that the memory address my pointer points at contains 5?

Sorry for the basic question - i have an assignmnet soon that requires the use of pointers and i really want to crack the basics before its set.

Answer 1

Almost there - change it to:

int p = 5; // create an int holding 5
int *ptr; // create a pointer that can point to an int
ptr = &p; // ptr now points at p

Answer 2

You want

ptr = &p; // set ptr to point to the location holding p

Answer 3

Your ptr now points to the memory address that has 5 stored in it.

Also, I don't believe that code compiles. You probably want:

int p = 5; //create an int holding 5
int *ptr; //create a pointer that can point to an int
ptr = &p; // not sure - does this mean that my pointer now points to memory address   five, or that the memory address my pointer points at contains 5?

Answer 4

Your program is wrong. ptr is not initialized. Assigning to *ptr creates a memory violation most likely. You can't assign an int* (which &p is) to an int (which *ptr is).

Correct is:

ptr = &p;

Answer 5

Others already explained how to fix your code. For understanding, let me tell you what your wrong line does:

*ptr = &p;

*ptr dereferences the pointer, i.e., *ptr = ... assigns something into the memory slot to which ptr is pointing. Since ptr has not been initialized, it points to some unknown location in memory and *ptr = ... will probably fail with a Segmentation Fault or something similar.

Since ptr is a pointer to int, the right-hand side of *ptr = ... expects an int as well, but you pass to it the address of an int (&p), which results in a compiler warning.

Answer 6

A pointer is, basically, an address. Think about a pointer as an address label, and the value as the actual house. You can use the label to find the house, but the label isn't a house.

int *ptr; // declares a pointer to an int

So ptr is, essentially, a memory address. (It's possible that the C spec doesn't actually specify that it's an address, but bear with me).

int i = 5; // create a local int.

Declares an integer on the stack, and sets the value of it to 5. The address of i is somewhere in the stack space.

Let's look at one intermediate step before we go on. This probably wouldn't compile, and if it did, wouldn't actually do anything.

&i;

What this expression does is return the address of the variable i. It's the location of i in memory - the address of an integer.

And one last one...

*ptr;

Again, this probably wouldn't actually do anything, it's just an expression. But, what it does is dereference the pointer - it refers to the actual int located at the address contained in ptr.

Okay, so let's take a look at a few things that we can do.

ptr = i;

This doesn't do anything, at least anything we want to do. Probably won't compile, but I haven't checked it out. It doesn't do anything because it assigns an integer to the address of an integer. That's like sending a box through the post office to an address label - you actually want it to go to the house!

i = ptr;

Okay, this is the same thing as the last one, but in reverse. Following our analogy, this is trying to turn a house into a label!

*ptr = i;

Here we've dereferenced the pointer, and assigned the value of i to it. Dereferencing a pointer is essentially like using the label to drive to the house. Once we're there, we can do things to the actual house. This works because a dereferenced int pointer is an int, and an int is also an int.

ptr = &i;

This works too. &i basically makes a new label for the house 'i'. Since we have a pointer on both sides, we can assign one to the other. This is basically copying the address label for i to the address label called ptr.

*ptr = &i;

This doesn't make sense. We've started with two different things, and converted each into the other! Now we're trying to assign a label to a house, whereas before we were assigning a house to an address label.

Answer 7

I know this doesn't directly answer your immediate question (which others have already done), but I would recommend reading the section on pointers from the c-faq.

Lot's of good info.