SYSLOG

Section: C Library Functions (3)
Updated: May 15, 1986
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NAME

syslog, openlog, closelog, setlogmask - control system log

SYNOPSIS

#include <syslog.h>

openlog(ident, logopt, facility)
char *ident;

syslog(priority, message, parameters ... )
char *message;

closelog()

setlogmask(maskpri)

DESCRIPTION

Syslog arranges to write message onto the system log maintained by syslogd(8). The message is tagged with priority. The message looks like a printf(3) string except that %m is replaced by the current error message (collected from errno). A trailing newline is added if needed. This message will be read by syslogd(8) and written to the system console, log files, or forwarded to syslogd on another host as appropriate.

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, e.g., hard device errors.
LOG_ERR: Errors.
LOG_WARNING: Warning messages.
LOG_NOTICE: Conditions that are not error conditions, but should possibly be handled specially.
LOG_INFO: Informational messages.
LOG_DEBUG: Messages that contain information normally of use only when debugging a program.

If syslog cannot pass the message to syslogd, it will attempt to write the message on /dev/console if the LOG_CONS option is set (see below).

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: useful for identifying instantiations of daemons.
LOG_CONS: Force writing messages to the console if unable to send it to syslogd. This option is safe to use in daemon processes that have no controlling terminal since syslog will fork before opening the console.
LOG_NDELAY: Open the connection to syslogd immediately. Normally the open is delayed until the first message is logged. Useful for programs that need to manage the order in which file descriptors are allocated.
LOG_NOWAIT: Don't wait for children forked to log messages on the console. This option should be used by processes that enable notification of child termination via SIGCHLD, as 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 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(8), routed(8), etc.
LOG_AUTH: The authorization system: login(1), su(1), getty(8), etc.
LOG_LPR: The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.
LOG_LOCAL0: Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.

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.

EXAMPLES

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

openlog("ftpd", LOG_PID, LOG_DAEMON);
setlogmask(LOG_UPTO(LOG_ERR));
syslog(LOG_INFO, "Connection from host %d", CallingHost);

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