Question

A friend told me there is a way to check with printf without modifying the original void function.

But I can’t see how you can do it.

#include<stdio.h>

void test()
{
  int a = 1;
  int b = a;
}

main()
{
    printf("%d",test());    
}

I kept getting

error: invalid use of void expression

Is there a way to do it?

Answer 1

I see from your edit that you want to do this without modifying the function. You can employ the comma operator to make the function call in a printf:

#include <stdio.h>

void test()
{
    int a = 1;
    int b = a;
}

main()
{
    printf("%d\n", (test(), 0));

    // or even this, to be more explicit:
    printf("%s\n", (test(), "test() was called."));
}

The comma operator evaluates the first operand – in this case your function call – and discards the result, then evaluates the second operand and the result is the result of the entire expression.