C language pointer array in Struct

Question

Code:

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

typedef struct{
    int *arr;
}example;

void Create(example var){
    var.arr = (int *)malloc(sizeof(int)*2);
}

int main(){
    example var1, var2;
    var1.arr = (int *)malloc(sizeof(int)*2);
    var1.arr[0] = 11;
    var1.arr[1] = 22;
    printf("%d %d\n",var1.arr[0],var1.arr[1]);
    Create(var2);
    var2.arr[0] = 111;
    var2.arr[1] = 222;
    printf("%d %d\n",var2.arr[0],var2.arr[1]);
    return 0;
}

OUT:
11 22
Segmentation Fault

My code is as above. I don't get any error when I do it manually as in var1. But if I do it inside a function as in var2, I get an error. How can I fix this. I want to do it inside the function.

EDIT:Thank you for your answers. It worked

Create has its argument passed by value. Calling Create(var2) does not modify var2 in any way, what's passed is a copy. You probably want Create to take a pointer as an argument, or return a new example. — Nate Eldredge
– Nate Eldredge, Commented Jul 13, 2021 at 17:49

Mridula daga · Accepted Answer · 2021-07-13 18:01:00Z

1

you have to pass reference of var2: Create(&var2);

answered Jul 13, 2021 at 18:01

Mridula daga

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Vlad from Moscow · Accepted Answer · 2021-07-13 17:52:44Z

0

The function parameter

void Create(example var){
    var.arr = (int *)malloc(sizeof(int)*2);
}

is its local variable that is initialized by the passed argument and is not alive after exiting the function.

That is this call

Create(var2);

did not change the variable var2 declared in main.

As a result in these statements

var2.arr[0] = 111;
var2.arr[1] = 222;

there is used uninitialized pointer arr that has an indeterminate value that invokes undefined behavior.

You need to pass the variable by reference to the function. For example

void Create(example *var){
    var->arr = (int *)malloc(sizeof(int)*2);
}

and the function is called like

Create( &var2 );

answered Jul 13, 2021 at 17:52

Vlad from Moscow

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Comments

paulsm4 · Accepted Answer · 2021-07-13 17:52:52Z

0

The problem is that Create() is making a COPY of your struct. The original struct is unchanged.

If your program was C++, you'd want to pass a "reference".

Here, you want to pass a pointer:

void Create(example * mystruct){
    mystruct->arr = (int *)malloc(sizeof(int)*2);
}

int main(){
    example var1, var2;
    ...
    Create(&var2);

answered Jul 13, 2021 at 17:52

paulsm4

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