enum LogLevel {
    DEBUG = 0,
    INFO,
    WARN,
    ERROR
};

LogLevel g_log_level = INFO; // 默认级别

void log(LogLevel level, const std::string& msg) {
    if (level >= g_log_level) {
        std::cout << "[" << toString(level) << "] " << msg << std::endl;
    }
}
</font>

<H3>运行时修改日志级别的实现方式</H3>
<p>要实现运行时修改，关键是提供一种外部触发机制来更新 <strong>g_log_level</strong>。以下是几种常见方法：</p>

<ul>
<li><strong>通过信号（Signal）响应</strong>：Linux下可使用 SIGUSR1 或 SIGUSR2 触发级别变更。例如，收到 SIGUSR1 提升为 DEBUG，SIGUSR2 降回 INFO。</li>
<li><strong>监听配置文件变化</strong>：定期检查配置文件（如 log.conf）中的 level 字段，发现更改后重新加载。</li>
<li><strong>提供HTTP或IPC接口</strong>：在后台服务中开放简单REST API或本地socket命令，接收新级别并更新。</li>
</ul>

<p>示例：使用信号机制切换到 DEBUG 级别：</p>
<font face="Courier New">
<pre class="brush:php;toolbar:false;">
#include <signal.h>

void signal_handler(int sig) {
    if (sig == SIGUSR1) {
        g_log_level = DEBUG;
        std::cout << "[LOG] Log level changed to DEBUG\n";
    } else if (sig == SIGUSR2) {
        g_log_level = INFO;
        std::cout << "[LOG] Log level changed to INFO\n";
    }
}

// 注册信号处理
signal(SIGUSR1, signal_handler);
signal(SIGUSR2, signal_handler);

log_level=DEBUG

void config_watcher() {
    std::string last_level;
    while (true) {
        std::ifstream f("log.conf");
        std::string line;
        if (std::getline(f, line) && line.find("log_level=") == 0) {
            std::string new_level_str = line.substr(10);
            LogLevel new_level;
            if (new_level_str == "DEBUG") new_level = DEBUG;
            else if (new_level_str == "INFO") new_level = INFO;
            // ... 其他映射
            if (new_level != g_log_level) {
                g_log_level = new_level;
                log(INFO, "Log level updated via config: " + new_level_str);
            }
        }
        std::this_thread::sleep_for(std::chrono::seconds(5));
    }
}