Sequence Points 序列点

1. Definition of a Sequence Point

A Sequence Point is a specific point in the program's execution where: * All side effects of previous evaluations must be complete. * No side effects of subsequent evaluations may have started.

It acts as a "synchronization barrier" for memory visibility.

2. The Golden Rule

The C Standard states:

"Between two sequence points, the value of a scalar object shall be modified only once by the evaluation of an expression."

3. List of Standard Sequence Points

1. Statement termination: The semicolon ;.
2. Logical Operators: && and ||.
   • Note: These enforce Short-Circuit Evaluation. The left side is fully evaluated (including side effects) before the right side is touched.
3. Comma Operator: , (e.g., func(), do_something()).
   • Trap: The comma used to separate function arguments (func(a, b)) is a separator, NOT a sequence point.
4. Ternary Operator: ? : (The condition is fully evaluated before the branch，which means the ? is a sequence point).
5. Function Call: Before the execution of the function body begins (after all arguments are evaluated).

4. The "Unsequenced" Hazards

If there is no sequence point between two operations on the same variable, the behavior is Undefined.

• Example 1: i = i++ + 1;
  • Conflict: The side effect of i++ (increment) and the assignment = happen in the same window without a sequence point.
• Example 2: func(i++, i++);
  • Conflict: Function arguments are unsequenced relative to each other. The compiler may increment the first i, the second i, or both in any order.
• Example 3: a[i] = i++;
  • Conflict: The read of i (for array indexing) and the write to i (post-increment) are unsequenced. The index could be the old i or the new i.

5. C11 Standard Update

The C11 standard officially deprecated the term "Sequence Point" in favor of more precise relationships to handle multi-threading and complex expressions: * Sequenced Before: Strict ordering (A happens before B). * Unsequenced: Two actions can happen in any order or arguably simultaneously (interleaved instructions). This causes UB if data races occur. * Indeterminately Sequenced: One must finish before the other starts, but the choice of which goes first is up to the compiler (e.g., function arguments func(a(), b())).