C struct pointers [duplicate]

Question

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

typedef struct node* Node;

struct node{
int a;
struct node* next;
};

void insertAtBeginning(Node head){
Node temp = malloc(sizeof(struct node));
temp->a = 10;
temp->next = head;
head = temp;
}

int main(){
Node one = malloc(sizeof(struct node));
one->a = 11;
Node head = one;
insertAtBeginning(head);
printf("%d",head->a);

}

Expected : 10

Received : 11

I'm passing pointer(struct node* head) to function why is it not reflecting back in main method?

Passing a pointer to a function provides a way for the function to modify the thing the pointer points to. It does not provide a way for the function to modify the caller's copy of the pointer itself. If you want that, you can have the function receive a pointer to the pointer instead. — John Bollinger
– John Bollinger, Commented Feb 20 at 14:22
Also, do not hide pointer nature behind a typedef (i.e. your Node), except possibly in very special circumstances. Disguising the number of levels of indirection is confusing, much more than expressing object pointer types explicitly creates any kind of trouble. — John Bollinger
– John Bollinger, Commented Feb 20 at 14:26

PkDrew · Accepted Answer · 2025-02-20 14:13:01Z

0

You need to pass in pointer of pinter:

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

typedef struct node* Node;

struct node{
    int a;
    struct node* next;
};

void insertAtBeginning(Node *head){
    Node temp = malloc(sizeof(struct node));
    temp->a = 10;
    temp->next = head;
    *head = temp;
}

int main(){
    Node one = malloc(sizeof(struct node));
    one->a = 11;
    Node head = one;
    insertAtBeginning(&head);
    printf("%d",head->a);

}

answered Feb 20 at 14:13

PkDrew

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2 Comments

PkDrew Feb 20 at 14:16

C does not support reference, for C++ you may also pass the pointer by reference. Also see stackoverflow.com/questions/55401073/…

ad absurdum Feb 20 at 17:01

Better to avoid explicit types in malloc calls, e.g., Node one = malloc(sizeof *one);. Also better to avoid typedefs of pointers unless you are creating an opaque data type, e.g., typedef struct node Better; /*...*/ Better *two = malloc(sizeof *two);.

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