SYSLOG(3)

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

LIBRARY
libc

SYNOPSIS
#include <syslog.h>
#include <stdarg.h>
void
syslog(int priority, const char *message, ...);
void
vsyslog(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 stdarg(3).

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 ident argument 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 (“/dev/console”).

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 argument 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_CONSOLE
    Messages written to /dev/console by the kernel console output driver.

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 daemons: ftpd(8), tftpd(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_NTP
    The network time protocol system.

LOG_SECURITY
    Security subsystems, such as ipfw(4).

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");

SEE ALSO
logger(1), syslogd(8)

HISTORY
These functions appeared in 4.2BSD.

BUGS
Never pass a string with user-supplied data as a format without using ‘%s’. An attacker can put format specifiers in the string to mangle your stack, leading to a possible security hole. This holds true even if the string was built using a function like snprintf(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by syslog().

Always use the proper secure idiom:

    syslog("%s", string);