Official PYQ · 2006Q57Programming in CMCQ1 mark

GATE CS 2006 Q57 — Programming in C

Consider this C code to swap two integers and these five statements: the code

void swap (int *px, int *py) 
{ 
        *px = *px - *py;
        *py = *px + *py;
        *px = *py - *px;      
}

S1: will generate a compilation error

S2: may generate a segmentation fault at runtime depending on the arguments passed

S3: correctly implements the swap procedure for all input pointers referring to integers stored in memory locations accessible to the process

S4: implements the swap procedure correctly for some but not all valid input pointers

S5: may add or subtract integers and pointers

View worked solution

Answer: C

To determine which statements are correct, let us analyze each of them:

S1 (False): The code is syntactically valid in C. It dereferences integer pointers and performs standard integer arithmetic, so it will compile without errors.
S2 (True): If either px or py is a NULL pointer or points to an invalid/unmapped memory address, dereferencing them (e.g., *px) will cause a segmentation fault at runtime.
S3 (False) & S4 (True): Let us trace the code when both pointers point to the same memory location (aliasing), i.e., px == py. Let the value at this location be $V$ $V$ .
1. *px = *px - *py; $\implies *px = V - V = 0$
2. *py = *px + *py; $\implies *py = 0 + 0 = 0$
3. *px = *py - *px; $\implies *px = 0 - 0 = 0$
The value at the memory location becomes $0$ $0$ instead of remaining $V$ $V$ . Thus, the swap fails when the pointers are identical. It only works correctly when px != py. Hence, it works for some but not all valid pointers.
S5 (False): The operations are performed on *px and *py, which are of type int, not on the pointers px and py themselves. No pointer arithmetic is involved.

Thus, only statements S2 and S4 are correct, which corresponds to option C.

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Published 10 Oct 2026

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