Undefined behavior in C when modifying a variable twice without sequencing: What will this program print? #include int main() { int i = 2; printf("%d, %d ", ++i, ++i); return 0; }

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Undefined behavior in C when modifying a variable twice without sequencing: What will this program print?  #include<stdio.h> int main() {  int i = 2;  printf("%d, %d ", ++i, ++i);  return 0; }

Accepted Answer

Introduction / Context: This question highlights a key rule in C: modifying a scalar more than once between sequence points (in C11 and earlier terminology) or without a defined sequencing (C11/C17/C18), produces undefined behavior. The expression uses ++i twice as arguments to the same printf call, without a sequencing guarantee between those increments. Given Data / Assumptions: Both arguments are ++i, which modifies i before each value is used. The C standard does not define the order of evaluation of function arguments (prior to C23 changes); compilers may choose different orders. Undefined behavior means any result can occur, including seemingly consistent numbers or surprising outputs. Concept / Approach: Because there is no guaranteed sequence between the two ++i evaluations, the compiler is free to evaluate either argument first, interleave increments, or even optimize in ways that make the outcome unpredictable. Therefore, predicting a specific pair such as "3, 4" or "4, 3" is incorrect in a standards-compliant sense. Step-by-Step Solution: Recognize that both arguments mutate the same variable i.Note there is no sequence point between the two arguments.Conclude that the program exhibits undefined behavior; any output (or crash) is permitted. Verification / Alternative check: Test on multiple compilers or optimization levels to see different outputs. The lack of a defined evaluation order often yields different results across environments. Why Other Options Are Wrong: (a), (b), (c), and (e) claim specific outputs, contradicting the undefined nature of the expression. Even if one compiler prints such a pair, it is not portable or guaranteed. Common Pitfalls: Assuming left-to-right evaluation of function arguments or believing that printf enforces sequencing between its arguments. It does not; sequencing occurs only at the call boundary. Final Answer: Output may vary from compiler to compiler

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