Passing void pointer by reference

Hi,
In the following code, I am passing a void pointer by reference assigning a value to it. But the value is not reflected in the main program.

Code:

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#include<stdio.h>


void function(void *p)
{
    int i=9;
    int *a=&i;
    p=(void *)a;
}


int main()
{
    int *p;
    p=(int *)malloc(sizeof(int));
    function((void *)p);
    printf("%d",*p);
    return 0;
}

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It's printing a garbage value. Please tell me what is the correct way of passing a void pointer by reference and getting a value in it from the function.
Thanks in advance! :)

You mean you want to access the value pointed to by p? You must make sure the types match. First, no need for all the casting to/from void *. Every pointer type can be cast to/from void *. Also, you must make sure the types of what you assign, etc match. For example, in function, p is pointer to int, so *p is the int it points to. If you want to assign the value of i to the value p points to, just do:

Code:

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*p = i;

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No need for the a variable.

Also, you don't necessarily need to malloc memory for this, you can just do:

Code:

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int x;
function(&x);  // pass address of (pointer to) x to function()

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Quote:

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Originally Posted by sakura

I am passing a void pointer by reference assigning a value to it.

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No, the pointer is passed by value. The fact that you assign a value to the pointer is a separate matter.

To pass a pointer by reference, you should pass a pointer to the pointer.

Quote:

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Originally Posted by sakura

In the following code, I am passing a void pointer by reference assigning a value to it. But the value is not reflected in the main program.

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Actually, you are passing a void pointer BY VALUE, simulating pass-by-reference semantics.

You never assign to *p in function, so how do you expect the value to be changed in main? Assigning to p just modifies the local value of the pointer.

Code:

---

#include<stdio.h>

void function(void *p) {
    int *ip = (int*)p;
    *ip = 9;
}

int main() {
    int p;
    function((void*)&p);
    printf("%d", p);
    return 0;
}

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