Initializing structure

[sakura]

I have this code which does not print "3" as it is supposed to

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


typedef struct Node
{
int i;
}Node, *NodePtr;


void Init(NodePtr mynode)
{
mynode=(NodePtr)malloc(sizeof(Node));
}


void Insert(NodePtr mynode, int i)
{
mynode->i=i;
}


void Retrieve(NodePtr mynode)
{
printf("%d",mynode->i);
}


void UnInit(NodePtr mynode)
{
free(mynode);
}


int main()
{
NodePtr mynode;
Init(mynode);
Insert(mynode,3);
Retrieve(mynode);
UnInit(mynode);


return 0;
}

But when I initialize the structure in main, it works(check the following code)

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


typedef struct Node
{
int i;
}Node, *NodePtr;


void Insert(NodePtr mynode, int i)
{
mynode->i=i;
}


void Retrieve(NodePtr mynode)
{
printf("%d",mynode->i);
}


void UnInit(NodePtr mynode)
{
free(mynode);
}


int main()
{
NodePtr mynode;
mynode=(NodePtr)malloc(sizeof(Node));
Insert(mynode,3);
Retrieve(mynode);
UnInit(mynode);


return 0;
}

Why does the first code not work? Is there a way to initialize structure by calling a function?
Thanks!

[Salem]

Do you understand why this doesn't print 3

void foo ( int bar ) {
  bar = 3;
}
int main ( ) {
  int x = 0;
  foo(x);
  printf("%d\n", x);
  return 0;
}

When you understand that you need x=foo(); or foo(&x); to make this work, you'll know how to fix your code.

[sakura]

I know why your code doesn't print. But I am passing the structure address only. So why wouldn't it print the value?

[Salem]

> But I am passing the structure address only. So why wouldn't it print the value?
Ah, but you aren't are you.

First thing to do is to stop typedef'ing pointer types.
It really doesn't help to clear things up for you.

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

typedef struct Node
{
    int i;
}Node;

void Init(Node *mynode)
{
    mynode=malloc(sizeof(Node));
}

int main()
{
    Node *mynode = NULL;
    Init(mynode);
}

It may be a pointer, but you're still passing it "by value", so changing it inside Init won't change it in main.

[iMalc]

It does not matter at all what type the thing you want to modify is, if you want to modify it through a function parameter, you need to pass a pointer to it. Even if that thing is itself a pointer!

As it is, your function could only modify what mynode already points at, but cannot be made to point at anything else; and as it currently points to nothing, it shall stay pointing at nothing.

[sakura]

So how do I pass it by reference?

[sakura]

Okay! I got it!