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wiki/concepts/Checks-Effects-Interactions.md

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+---
+title: "Checks-Effects-Interactions Pattern"
+type: concept
+tags: [blockchain, security, smart-contract, reentrancy, best-practices]
+sources: [blockchain-security-auditor]
+last_updated: 2026-05-30
+---
+
+## Aliases
+- Checks-Effects-Interactions
+- CEI Pattern
+- Checks-Effects-Interactions Pattern
+
+## Definition
+
+Checks-Effects-Interactions（CEI，检查-效果-交互）模式是智能合约安全编程的核心防御模式，用于防止重入攻击和其他逻辑漏洞。其核心原则是：**在执行任何外部调用之前，先完成所有的状态检查和状态更新**。
+
+## Pattern
+
+```
+function withdraw() external {
+    // 1. CHECKS — 验证前置条件
+    require(balances[msg.sender] > 0, "No balance");
+    
+    // 2. EFFECTS — 更新合约状态（在外部调用之前）
+    balances[msg.sender] = 0;
+    
+    // 3. INTERACTIONS — 执行外部调用（最后一步）
+    (bool success,) = msg.sender.call{value: amount}("");
+    require(success, "Transfer failed");
+}
+```
+
+## Why It Works
+
+传统重入攻击利用的漏洞链：
+```
+外部调用（ETH转账）→ 攻击者receive()被触发 
+→ 攻击者 re-enter withdraw() 
+→ balances[msg.sender] 仍 > 0（未清零）
+→ 重复提取资金
+```
+
+CEI 模式切断漏洞链：**状态在外部调用之前清零**，即使攻击者 re-enter，条件检查阶段 `require(balances[msg.sender] > 0)` 已失败。
+
+## Common Mistakes
+
+### Wrong Order (Vulnerable)
+```solidity
+// VULNERABLE: 外部调用在状态更新之前
+(bool success,) = msg.sender.call{value: amount}("");
+require(success);
+balances[msg.sender] = 0; // 攻击者可在此之前 re-enter
+```
+
+### Missing Update (Vulnerable)
+```solidity
+// VULNERABLE: 状态根本没有更新
+require(balances[msg.sender] > 0);
+(bool success,) = msg.sender.call{value: amount}("");
+require(success);
+// balances[msg.sender] = 0; ← 忘记更新！
+```
+
+### ERC-20 Transfer Before Update
+```solidity
+// VULNERABLE: ERC-20 transfer 触发外部调用
+IERC20(token).transfer(msg.sender, amount); // 外部调用
+balances[msg.sender] -= amount; // 状态更新太晚
+```
+
+## Relationship to Other Defenses
+
+- **CEI is the primary defense** — should always be applied first
+- **ReentrancyGuard is additional defense** — use when CEI is difficult to apply (complex logic, callbacks)
+- CEI + ReentrancyGuard together provide defense in depth
+
+## Connections
+- [[Reentrancy]] ← prevents ← [[Checks-Effects-Interactions]]
+- [[Blockchain-Security-Auditor]] ← enforces ← [[Checks-Effects-Interactions]]
+- [[ReentrancyGuard]] ← supplements ← [[Checks-Effects-Interactions]]