In the following C program, what is the correct description of its behaviour? int main() { int i = 2; printf(" %d%d", ++i, ++i); return 0; }

Introduction / Context:
This question tests your understanding of undefined behaviour in C, specifically related to modifying a variable multiple times without an intervening sequence point. It is a common trap in exams because some compilers appear to give a predictable result even though the language standard does not guarantee it.

Given Data / Assumptions:

• i is initialized to 2.
• The printf call is printf("%d%d", ++i, ++i);.
• The order of evaluation of function arguments in C is unspecified.

Concept / Approach:
In C, if you modify a scalar variable more than once without an intervening sequence point and then read its value, the behaviour is undefined. The call printf("%d%d", ++i, ++i); increments i twice in the same full expression, and the standard does not specify whether the left or right argument to printf is evaluated first. As a result, different compilers, or even different optimization levels of the same compiler, may produce different results, and some behaviours may be surprising.

Step-by-Step Solution:
Step 1: Recognize that both arguments ++i and ++i modify the same variable.Step 2: The C standard does not define the order in which these arguments are evaluated.Step 3: Because i is modified more than once without a sequence point between these modifications and is also read to produce the argument values, the expression exhibits undefined behaviour.Step 4: Undefined behaviour means that the program may print different numbers such as 34 or 43, or may exhibit other unexpected results.

Verification / Alternative check:
If you compile and run this program with different compilers, or with different optimization flags, you may observe different outputs. This variation confirms that there is no portable, well defined result.

Why Other Options Are Wrong:
Option A and option B both claim a specific fixed output, which is not guaranteed by the language.Option D is incorrect because the syntax is valid; the issue is semantic undefined behaviour, not a compilation error.

Common Pitfalls:
Many programmers assume that arguments are evaluated left to right, but C does not require this. It is best to avoid writing expressions that both modify and use a variable more than once in a single expression.

Final Answer:
The correct description is The behaviour is undefined, so the printed values may vary between compilers or runs.