GATE CS 2014 Set 1 Q.20 (CS Q.10): Pointers, Arrays and Strings

Consider the following program in C language:

#include <stdio.h>
main()
{
    int i;
    int *pi = &i;
    scanf("%d", pi);
    printf("%d\n", i + 5);
}

Which one of the following statements is TRUE?

Compilation fails.

Execution results in a run-time error.

On execution, the value printed is 5 more than the address of variable i.

On execution, the value printed is 5 more than the integer value entered.

Official answer and solution

Official answer: D
From: Official GATE 2014 answer key

Solution

Trace pointer initialization and argument passing to scanf in C.

In the program:

int i; declares an integer variable i.
int *pi = &i; initializes pointer pi to the address of i.
scanf("%d", pi); expects a pointer to an integer where the input should be stored. Since pi points to i, this correctly reads user input and stores it into i.
printf("%d\n", i + 5); outputs the value stored in i plus 5.

All operations are completely valid standard C code. Therefore, upon execution, the value printed is 5 more than the integer value entered (Statement D).

Final answer

Common trap

Mistakenly thinking scanf requires &pi, which would be an error since pi is already an int*.

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2014 Computer Science and Information Technology (CS) Session 1
Question: 20 of 65, printed as CS Q.10
Section: Computer Science and Information Technology

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