Thread: Basic pointer question

Hi all,

I have what i think should be a very basic question about pointers I just can't seem to get my head around it at the moment! The code below illustrates the problem on a simpler scale:

Code:

#include <stdlib.h>
#include <stdio.h>

struct node {
	int i;
	char *s;
};

void f1(struct node *ptr) {
	ptr = malloc(sizeof(struct node));
	ptr->i = 2;
	printf("%s\n", ptr);

}

int main() {
	
	struct node *ptr=NULL;
	
	f1(ptr);
	
	printf("%s\n", ptr);
	
	return 0;
}

Basically printf within 'f1' doesn't show the ptr as being NULL yet the printf in main shows as '(null)'. Why is this - shouldn't changing the ptr in f1 reflect in main?

I am compiling using -Wall -Wextra -pedantic -ansi using gcc on windows if this helps?

Cheers,
James

First of all, you really shouldn't be printing the pointer as a character string. Use the %p specifier, or similar. Anyway, the issue is that in C all things are passed to functions by value - pointers included. So, from main, the pointer itself (the variable) is located at address 0xdeadbeef and contains the value of 0. When you pass it to the function, the *value* of the pointer (eg: 0) is copied, but the address of the local variable within the function is *not* 0xdeadbeef, so the changes made to it will obviously not be reflected in the variable within main. Hope that makes sense. Anyway, the solution is simply one more level of indirection - in other words, pass the *address* of the pointer:

Code:

#include <stdlib.h>
#include <stdio.h>

struct node {
	int i;
	char *s;
};

void f1(struct node **ptr) {
	(*ptr) = malloc(sizeof(struct node));
	(*ptr)->i = 2;
	printf("%p\n", ptr);

}

int main() {
	
	struct node *ptr=NULL;
	
	f1(&ptr);
	
	printf("%p\n", &ptr);
	
	return 0;
}

wt type of output u getting?

Thanks to everyone who has replied - I went for Sebastiani's solution because in my real code the function already returns an int value so returning is not an option - This is what I have done in the past hence why I was stumped when this didn't work!

Anyway the problem is sorted now and more importantly I understand why it works!

Thanks,
James