closelog, openlog, syslog – send messages to the system logger

closelog() closes the descriptor being used to write to the system logger. The use of closelog() is optional.
openlog() opens a connection to the system logger for a program. The string pointed to by ident is prepended to every message, and is typically set to the program name. The option argument specifies flags which control the operation of openlog() and subsequent calls to syslog(). The facility argument establishes a default to be used if none is specified in subsequent calls to syslog(). Values for option and facility are given below. The use of openlog() is optional; it will automatically be called by syslog() if necessary, in which case ident will default to NULL.

syslog() generates a log message, which will be distributed by syslogd(8). The priority argument is formed by ORing the facility and the level values (explained below). The remaining arguments are a format, as in printf(3) and any arguments required by the format, except that the two character sequence %m will be replaced by the error message string strerror(errno). A trailing newline is added when needed.

The function vsyslog() performs the same task as syslog() with the difference that it takes a set of arguments which have been obtained using the stdarg(3) variable argument list macros.