默认的日志记录级别应该是WARN，它只在已经阻止了请求的情况下在日志文件中写消息。

The default log level should be WARN which will write a message to the log file only if a request has been blocked.

youdao

最后一个文件描述符是只写的，并且指向 /var/run/sendmail.pid。

The final file descriptor is write-only and refers to /var/run/sendmail.pid.

youdao

它们在连接的两端使用两个标准的文件描述符—一个只执行读操作，另一个只执行写操作。

Both use two standard file descriptors on each end of the connection-one exclusive to read and another exclusive to write operations.

youdao

All you have to include in your own programs is the so-called header file.

你们自己写的程序只需要包括,头文件就可以了。

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