Value Vs Pointer

When we pass parameters to a function, we can do it in two ways: by value or by pointer. Let's take a simple example to understand what we mean by value and pointer:

#include <stdio.h>

void sum(int a) {
   a = a + 1;
   printf("sum function: %d\n", a);
}

int main(void) {
   int a = 10;
   sum(a);
   printf("main function: %d\n", a);
   return 0;
}

The output:

sum function: 11 
main function: 10 

As you can see in this example, we use the value, so the variable a's value didn't change. However, if we need to access the variable and change its value, we can use a pointer as a parameter.

#include <stdio.h>

void sum(int *a) {
   *a = *a + 1;
   printf("sum function: %d\n", *a);
}

int main(void) {
   int a = 10;
   sum(&a);
   printf("main function: %d\n", a);
   return 0;
}

Since the function now accepts a pointer as the parameter, when it is called, you must pass the address of the variable:

sum(&a);

The output:

sum function: 11 
main function: 11