SYSLOG(3)

NAME
syslog, vsyslog, syslogmt, vsyslogmt, openlog, closelog, setlogmask - control system log

SYNOPSIS
#include <stdarg.h>
#include <syslog.h>

void syslog (int priority, const char *message, ...);
void vsyslog (int priority, const char *message, va_list args);
void syslogmt (int priority, const char *message, ...);
void vsyslogmt (int priority, const char *message, va_list args);
void openlog (const char *ident, int logopt, int facility);
void closelog (void);
int setlogmask (int maskpri);

DESCRIPTION
The syslog function writes message to the system message logger. The message is then written to the system console, log files, logged-in users, or forwarded to other machines as appropriate. (See syslogd(8).)

The message is identical to a printf(3) format string, except that %m is replaced by the current error message. (As denoted by the global variable errno; see strerror(3).) A trailing newline is added if none is present.

The vsyslog function is an alternate form in which the arguments have already been captured using the variable-length argument facilities of varargs(3).

The syslogmt and vsyslogmt are multithread-safe variants of the syslog and vsyslog routines. Note that the permissable list of format specifiers for these two routines is much smaller than those for syslog or vsyslog. The full list of format specifiers is given in the sprintmt(3) manual page. Note that syslogmt and vsyslogmt will not log to syslogd(8) unless openlog has been called. Since openlog is not itself thread safe, the call to openlog must be made before the process forks if any children will be using these routines. These two routines are GNO-specific and therefore non-portable.

The message is tagged with priority. 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 the following ordered (high to low) 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.

The openlog function provides for more specialized processing of the messages sent by syslog and vsyslog. The parameter ident is a string that will be prepended to every message. The logopt argument is a bit field specifying logging options, which is formed by OR'ing one or more of the following values:

    LOG_CONS
        If syslog cannot pass the message to syslogd(8) it will attempt to write the message to the console .ttyco.

    LOG_NDELAY
        Open the connection to syslogd(8) 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_PERROR
        Write the message to standard error output as well to the system log.

    LOG_PID
        Log the process id with each message: useful for identifying instantiations of daemons.

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

    LOG_AUTH
        The authorization system: login(1), su(1), getty(8), etc.

    LOG_AUTHPRIV
        The same as LOG_AUTH, but logged to a file readable only by selected individuals.

    LOG_CRON
        The cron daemon: cron(8).

    LOG_DAEMON
        System daemons, such as routed(8), that are not provided for explicitly by other facilities.

    LOG_FTP
        The file transfer protocol daemon: ftpd(8).

    LOG_KERN
        Messages generated by the kernel. These cannot be generated by any user processes.

    LOG_LPR
        The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.

    LOG_MAIL
        The mail system.

    LOG_NEWS
        The network news system.

    LOG_SYSLOG
        Messages generated internally by syslogd(8).

    LOG_USER
        Messages generated by random user processes. This is the default facility identifier if none is specified.

    LOG_UUCP
        The uucp system.

    LOG_LOCAL0
        Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.

The closelog function can be used to close the log file.

The setlogmask function 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.

RETURN VALUES
The routines closelog, openlog, syslog and vsyslog return no value.

The routine setlogmask always returns the previous log mask level.

EXAMPLES
syslog(LOG_ALERT, "who: internal error 23");
openlog("ftpd", LOG_PID | LOG_NDELAY, LOG_FTP);
setlogmask(LOG_UPTO(LOG_ERR));
syslog(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");

MT STATUS
syslogmt and vsyslogmt are thread safe; the remainder are not. openlog should be called in the parent thread before creating the child threads. This will ensure that any generated messages are properly tagged with ident.

BUGS
The current implementation does not take into account the possibility that syslogd(8) may be killed and restarted. If this happens, all logging from the current process will stop until a call is made to closelog followed by openlog, which is of course not thread safe. This could be handled better.

SEE ALSO
logger(1), sprintmt(3), syslogd(8)

HISTORY
These functions appeared in 4.2BSD.