syslog(S)

syslog, openlog, closelog, setlogmask -- control system log

Synopsis

   cc [flag . . . ] file . . . -lgen [library] . . .
   
   #include <syslog.h>
   

   void openlog(const char ident, int logopt, int facility);
   

   void syslog(int priority, const char message, . . . /* parameters */);
   

   void closelog();
   

   int setlogmask(int maskpri);

Description

syslog passes message to syslogd(ADM), which logs it in an appropriate system log, writes it to the system console, forwards it to a list of users, or forwards it to the syslogd on another host over the network. The message is tagged with a priority of priority. The message looks like a printf [see fprintf(S)] string except that %m is replaced by the current error message (collected from errno). A trailing NEWLINE is added if needed.

Priorities are encoded as a facility and a level. The facility describes the part of the system generating the message. The level is selected from an ordered list:

LOG_EMERG: A panic condition. This is normally broadcast to all users.
LOG_ALERT: A condition that should be corrected immediately, such as a corrupted system database.
LOG_CRIT: Critical conditions, such as hard device errors.
LOG_ERR: Errors.
LOG_WARNING: Warning messages.
LOG_NOTICE: Conditions that are not error conditions, but that may require special handling.
LOG_INFO: Informational messages.
LOG_DEBUG: Messages that contain information normally of use only when debugging a program.

If special processing is needed, openlog can be called to initialize the log file. The parameter ident is a string that is prepended to every message. logopt is a bit field indicating logging options. Current values for logopt are:

LOG_PID: Log the process ID with each message. This is useful for identifying specific daemon processes (for daemons that fork).
LOG_CONS: Write messages to the system console if they cannot be sent to syslogd. This option is safe to use in daemon processes that have no controlling terminal, since syslog forks before opening the console.
LOG_NDELAY: Open the connection to syslogd immediately. Normally the open is delayed until the first message is logged. This is useful for programs that need to manage the order in which file descriptors are allocated.
LOG_ODELAY: Delay open until syslog() is called.
LOG_NOWAIT: Do not wait for child processes that have been forked to log messages onto the console. This option should be used by processes that enable notification of child termination using SIGCHLD, since syslog may otherwise block waiting for a child whose exit status has already been collected.

The facility parameter encodes a default facility to be assigned to all messages that do not have an explicit facility already encoded:

LOG_KERN: Messages generated by the kernel. These cannot be generated by any user processes.
LOG_USER: Messages generated by random user processes. This is the default facility identifier if none is specified.
LOG_MAIL: The mail system.
LOG_DAEMON: System daemons, such as ftpd(ADMN), routed(ADMN), etc.
LOG_AUTH: The authorization system: login(C), su(ADM), getty(ADM), etc.
LOG_SYSLOG: Messages generated internally by syslogd.
LOG_LPR: The line printer spooling system: lpr(C), lpc(1Mbsd), etc.
LOG_NEWS: Reserved for the USENET network news system.
LOG_UUCP: Reserved for the UUCP system; it does not currently use syslog.
LOG_LFMT: The log alert facility.
LOG_CRON: The cron/at facility; crontab(C), at(C), cron(ADM), etc.
LOG_LOCAL0-7: Reserved for local use.

closelog can be used to close the log file.

setlogmask sets the log priority mask to maskpri and returns the previous mask. Calls to syslog with a priority not set in maskpri are rejected. The mask for an individual priority pri is calculated by the macro LOG_MASK (pri); the mask for all priorities up to and including toppri is given by the macro LOG_UPTO (toppri). The default allows all priorities to be logged.

References

at(C), cron(ADM), crontab(C), fprintf(S), ftpd(ADMN), getty(ADM), logger(C), login(C), lpc(1Mbsd), lpr(C), routed(ADMN), su(ADM), syslogd(ADM)

Examples

This call logs a message at priority LOG_ALERT:

   syslog(LOG_ALERT, "who: internal error 23");

The FTP daemon, ftpd, would make this call to openlog to indicate that all messages it logs should have an identifying string of ftpd, should be treated by syslogd as other messages from system daemons are, and should include the process ID of the process logging the message:

   openlog("ftpd", LOG_PID, LOG_DAEMON);

Then it would make the following call to setlogmask to indicate that messages at priorities from LOG_EMERG through LOG_ERR should be logged, but that no messages at any other priority should be logged:

   setlogmask(LOG_UPTO(LOG_ERR));

Then, to log a message at priority LOG_INFO, it would make the following call to syslog:

   syslog(LOG_INFO, "Connection from host %d", CallingHost);

A locally-written utility could use the following call to syslog to log a message at priority LOG_INFO, to be treated by syslogd as other messages to the facility LOG_LOCAL2 are treated:

   syslog(LOG_INFO|LOG_LOCAL2, "error: %m");