1、void openlog(const char *ident, int option, int facility);
第一个参数ident将是一个标记，ident所表示的字符串将固定地加在每行日志的前面以标识这个日志，通常就写成当前程序的名称以作标记。
第二个参数option是下列值取与运算的结果：LOG_CONS，LOG_NDELAY， LOG_NOWAIT， LOG_ODELAY， LOG_PERROR，LOG_PID，各值意义请参考man openlog手册：
LOG_CONS
Writedirectly to system console if there is an error while sending tosystem logger.
LOG_NDELAY
Openthe connection immediately (normally, the connection is opened whenthe first message is logged).
LOG_NOWAIT
Don’t wait for childprocesses that may have been created while logging themessage. (The GNU C library does not createa child process, so this option has no effect onLinux.)
LOG_ODELAY
The converseof LOG_NDELAY; opening of the connection is delayed until syslog()is called. (This is the default, and need not be specified.)
LOG_PERROR
(Notin SUSv3.) Print to stderr as well.
LOG_PID
IncludePID with each message.
第三个参数facility指明记录日志的程序的类型。
The facility argument is used to specify what type ofprogram is logging the message.
This lets the configuration file specify thatmessages from different facilities will be
handled differently.
LOG_AUTH security/authorization messages (DEPRECATED Use LOG_AUTHPRIVinstead)
LOG_AUTHPRIV security/authorization messages (private)
LOG_CRON clock daemon (cron and at)
LOG_DAEMON system daemons without separate facility value
LOG_FTP ftp daemon
LOG_KERN kernel messages (these can't be generage from user processes)
LOG_LOCAL0 through LOG_LOCAL7
reserved for local use
LOG_LPR line printer subsystem
LOG_MAIL mail subsystem
LOG_NEWS USENET news subsystem
LOG_SYSLOG messages generated internally by syslogd(8)
LOG_USER (default)
generic user-level messages
LOG_UUCP UUCP subsystem
2、void syslog(int priority, const char *format, ...);
This determines the importance of the message. The levels are, in order of decreasing
importance:
LOG_EMERG system is unusable
LOG_ALERT action must be taken immediately
LOG_CRIT critical conditions
LOG_ERR error conditions
LOG_WARNING warning conditions
LOG_NOTICE normal, but significant, condition
LOG_INFO informational message
LOG_DEBUG debug-level message
The function setlogmask(3) can be used to restrict logging tospecified levels only.
3、int setlogmask(int maskpri);
4、调用openlog是可选择的。如果不调用openlog，则在第一次调用syslog时，自动调用openlog。调用closelog也是可选择的，它只是关闭被用于与syslog守护进程通信的描述符。调用openlog使我们可以指定一个ident，以后， 此ident 将被加至每则记录消息中。
5、代码示例
#include <syslog.h> int main(int argc, char **argv) { openlog("MyMsgMARK", LOG_CONS | LOG_PID, 0); syslog(LOG_DEBUG, "This is a syslog test message generated by program '%s'\n", argv[0]); closelog(); return0; } 编译生成可执行程序后，运行一次程序将向/var/log/message文件添加一行信息如下： Feb 12 08:48:38 localhost MyMsgMARK[7085]: This is a syslog testmessage generated by program './a.out'
