SYSLOG(3) BSD Library Functions Manual SYSLOG(3)
NAME
closelog, openlog, setlogmask, syslog, vsyslog -- control system log
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <syslog.h>
void
closelog(void);
void
openlog(const char *ident, int logopt, int facility);
int
setlogmask(int maskpri);
void
syslog(int priority, const char *message, ...);
#include <syslog.h>
#include <stdarg.h>
void
vsyslog(int priority, const char *message, va_list args);
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.
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 "\n", tabs are printed as "\t". 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 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 parameter ident is a string that will be prepended to every message. The logopt argu-ment argument
ment 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 facili-ties. facilities.
ties.
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_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");
LEGACY SYNOPSIS
#include <syslog.h>
#include <stdarg.h>
These include files are necessary for all functions.
SEE ALSO
logger(1), compat(5), 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 con-tain contain
tain user-supplied conversion specifiers for later interpolation by syslog().
Always use the proper secure idiom:
syslog(LOG_ERR, %s, string);
BSD June 4, 1993 BSD
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