GATE2025_CS1_Q34

The output of the given C program is ______. (Answer in integer)

#include 
void foo(int *p, int x){
    *p = x;
}
int main(){
    int *z;
    int a = 20, b = 25;
    z = &a;
    foo(z, b);
    printf("%d", a);
    return 0;
}

✅ Final Answer:

The correct output of the program is 25.

🔹 Short Answer:

The program demonstrates the concept of "pass-by-reference" using pointers. The pointer z is set to the address of variable a. This address is passed to the function foo. Inside foo, the statement *p = x dereferences the pointer to access the original variable a and changes its value to 25. Therefore, when printf is called in main, it prints the modified value of a, which is 25.

🔸 Long Answer:

To understand the program's output, let's trace its execution step-by-step and visualize the memory.

Execution Trace:

1. int main() execution begins.
2. int a = 20, b = 25;
   Two integer variables are declared and initialized. a holds the value 20, and b holds the value 25.
3. int *z;
   An integer pointer z is declared. It currently points to nothing (or holds a garbage value).
4. z = &a;
   The pointer z is assigned the memory address of variable a. Now, z "points to" a. Any operation using *z will directly affect the variable a.
5. foo(z, b);
   The program calls the function foo.
   • The value of z (which is the address of a) is passed to the pointer parameter p. So, inside foo, p also points to a.
   • The value of b (which is 25) is passed to the integer parameter x. This is a pass-by-value, so x inside _foo_ is a copy of b.
6. Inside foo(int *p, int x):
   • The statement *p = x; is executed.
   • *p is the dereferencing operator. It means "the value at the address stored in p". Since p holds the address of a, *p refers to the variable a itself.
   • The statement effectively becomes: "Set the value of a to the value of x".
   • Since x is 25, the value of a in the main function is changed from 20 to 25.
7. printf("%d", a);
   The function foo returns, and the program continues in main. The printf statement prints the current value of the variable a. Since a was modified by the foo function, its current value is 25.

The final output printed to the console is 25.

Bibliography:
- Kernighan, B. W., & Ritchie, D. M. (1988). The C Programming Language. Prentice Hall. (Chapter 5: Pointers and Arrays).

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