SYSLOG(3) BSD Library Functions Manual SYSLOG(3)
closelog, openlog, setlogmask, syslog, vsyslog -- control system log
Standard C Library (libc, -lc)
openlog(const char *ident, int logopt, int facility);
syslog(int priority, const char *message, ...);
vsyslog(int priority, const char *message, va_list args);
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.
Newlines and other non-printable characters embedded in the message string are printed in an alternate format. This prevents someone from
using non-printable characters to construct misleading log messages in an output file. Newlines are printed as "
", tabs are printed as
" ". Other control characters are printed using a caret ("^") representation, for example "^M" for carriage return.
The vsyslog() function is an alternate form in which the arguments have already been captured using the variable-length argument facilities
The message is tagged with priority. Priorities are encoded as a facility and a level. The facility describes the part of the system gener-
ating 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_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 (``/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 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 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: cups-lpd(8), cupsd(8), etc.
LOG_MAIL The mail system.
LOG_NEWS The network news 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.
The routines closelog(), openlog(), syslog(), and vsyslog() return no value.
The routine setlogmask() always returns the previous log mask level.
syslog(LOG_ALERT, "who: internal error 23");
openlog("ftpd", LOG_PID | LOG_NDELAY, LOG_FTP);
syslog(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
These include files are necessary for all functions.
asl(3), logger(1), compat(5), syslogd(8)
These functions appeared in 4.2BSD.
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(LOG_ERR, "%s", string);
June 4, 1993 BSD