In the context of the C and C++ language standards, what kind of execution ordering of side effects does a sequence point guarantee?

Introduction / Context:
Sequence points are a classical concept in the C and C++ language standards that describe well defined moments in program execution when all side effects of previous evaluations are guaranteed to be complete. This concept is important for understanding undefined behaviour, expression evaluation, and safe use of increment or decrement operators. The question asks what kind of execution ordering is allowed at a sequence point and what guarantee the language gives to programmers.

Given Data / Assumptions:

• We are working with C or C++ source code as defined by the language standards.
• Expressions may have side effects, such as modifying variables, performing input or output, or calling functions.
• The term sequence point refers to specific points in the abstract execution model, such as after the evaluation of a full expression.
• We assume that undefined behaviour occurs if the language does not define an ordering for conflicting side effects.

Concept / Approach:
The key idea behind a sequence point is non overlapping completion of side effects. All side effects of evaluations that happen before the sequence point must be complete before evaluations that follow the sequence point begin to produce new side effects. This does not mean that the compiler cannot optimise or reorder instructions internally, but from the point of view of the abstract machine, no side effect that is conceptually after the sequence point can start before all previous side effects finish. This rule allows compilers to be aggressive while still providing a clear mental model for programmers about when variable values are well defined.

Step-by-Step Solution:
Step 1: Recall examples of sequence points, such as the end of a full expression at a semicolon, logical operators like && and ||, and the comma operator. Step 2: Understand that at each sequence point, all side effects of expressions to the left are guaranteed to have taken place. Step 3: Recognise that this guarantee implies non overlapping execution of side effects across that boundary. Step 4: Relate this to common rules, such as avoiding undefined behaviour by not modifying a variable more than once between sequence points without an intervening sequence point. Step 5: Conclude that the correct description is a non overlapping ordering of side effects, as described in option a.

Verification / Alternative check:
Consider an expression such as i = 0; followed by i = i + 1;. There is a sequence point at the semicolon, so the side effect of setting i to 0 is complete before the increment statement begins. By contrast, in older C and C++ standards, an expression like i = i++ + i++ had no defined sequence point between the increments and the use of i, which resulted in undefined behaviour because side effects overlapped. These examples show how sequence points separate non overlapping regions of side effects.

Why Other Options Are Wrong:
Option b describes overlapping execution where side effects interleave unpredictably across the boundary, which is exactly what sequence points are designed to avoid. Option c suggests fully concurrent execution with no ordering guarantees, which would destroy the usefulness of sequence points. Option d refers to parallel execution controlled only by the operating system scheduler, but sequence points are specified by the language abstract machine model, not by operating system level threading rules.

Common Pitfalls:
A common mistake is to assume that the presence of a sequence point means that every subexpression is strictly evaluated left to right, which is not required. The compiler still has freedom to reorder as long as it respects non overlapping side effects around each sequence point. Another pitfall is ignoring sequence point rules and writing expressions with multiple side effects on the same variable between sequence points, which can lead to undefined behaviour and unpredictable results. Understanding that sequence points enforce non overlapping completion of side effects helps developers write safer and more portable C and C++ code.

Final Answer:
A sequence point allows non overlapping execution in which all side effects of previous evaluations are completed before the next evaluations begin.