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cpointerssyntax
John Smith
John Smith

Reputation: 633

What are the semantics of passing pointers as function arguments?

There are some things that I still don't really understand with pointers when you pass them into functions. If declare a function like 

void help (const int *p)

can I modify the argument p within the function help? Can I change what the pointer is pointing too?

Thanks for the clarification.

Upvotes: 0

Views: 118

Answers (3)

Robert Kolner
Robert Kolner

Reputation: 1572

The easiest way to understand const notation in C is to say the declaration out loud. Start with the name and go left:

const int *p; // 'p' is a pointer to an int that is const

This is by the way equal to:

int const *p;

This suggests that you can change the what p points at, but you can't change whatever it points at. So you have read-only access. However if you'd have something like this:

int * const p; // 'p' is a const pointer to an int

...then you could change the memory pointed to by p as much as you want, but you couldn't change p.

Upvotes: 1

Adam Mihalcin
Adam Mihalcin

Reputation: 14458

In this case, since p is declared as a const int *, an attempt to modify p will be disallowed by the compiler.

However, if p were a plain old int *, you could modify the thing that p is pointing to, and the caller would notice. Say you wrote:

void foo(void) {
    int n = 100;

    help1(&n);
    printf("n = %d", n);

    n = 100;
    help2(&n);
    printf("n = %d", n);
}

void help1(int *p) {
    *p = 50;
}

void help2(int *p) {
    p = (int *)malloc(sizeof(int));
    *p = 50;
    free(p);
}

Then calling foo() would cause your program to print

n = 50
n = 100

In this program, help1 changes the thing that p points to, and the caller can see it. On the other hand, help2 makes p point to a different place in memory, and anything help2 does to modify that other location in memory is not visible to the caller.

Upvotes: 1

Carl Norum
Carl Norum

Reputation: 224844

Yes, you can modify p. However, it won't change in the caller. C is a pass-by-value language. Check out the C FAQ, which has a question about exactly this situation.

Upvotes: 4

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