SYSLOG(3C) Standard C Library Functions SYSLOG(3C)
NAME
syslog, openlog, closelog, setlogmask - control system log
SYNOPSIS
#include <syslog.h>
void openlog(
const char *ident,
int logopt,
int facility);
void syslog(
int priority,
const char *message,
.../* arguments */);
void closelog(
void);
int setlogmask(
int maskpri);
DESCRIPTION
The
syslog() function sends a message to
syslogd(8), which, depending on
the configuration of
/etc/syslog.conf, logs it in an appropriate system
log, writes it to the system console, forwards it to a list of users, or
forwards it to
syslogd on another host over the network. The logged
message includes a message header and a message body. The message header
consists of a facility indicator, a severity level indicator, a
timestamp, a tag string, and optionally the process ID.
The message body is generated from the
message and following arguments in
the same manner as if these were arguments to
printf(3C), except that
occurrences of
%m in the format string pointed to by the
message argument
are replaced by the error message string associated with the current
value of
errno. A trailing
NEWLINE character is added if needed.
Symbolic constants for use as values of the
logopt,
facility,
priority,
and
maskpri arguments are defined in the <
syslog.h> header.
Values of the
priority argument are formed by ORing together a
severity level value and an optional
facility value. If no facility value is
specified, the current default facility value is used.
Possible values of severity level include, in decreasing order:
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, such as hard device errors.
LOG_ERR Errors.
LOG_WARNING Warning messages.
LOG_NOTICE Conditions that are not error conditions, but that may
require special handling.
LOG_INFO Informational messages.
LOG_DEBUG Messages that contain information normally of use only
when debugging a program.
The facility indicates the application or system component generating the
message. Possible facility values include:
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.
LOG_AUTH The authentication / security / authorization system:
login(1),
su(8),
getty(8).
LOG_LPR The line printer spooling system:
lpr(1B),
lpc(1B).
LOG_NEWS Designated for the
USENET network news system.
LOG_UUCP Designated for the
UUCP system; it does not currently use
syslog().
LOG_CRON The
cron/
at facility;
crontab(1),
at(1),
cron(8).
LOG_AUDIT The audit facility, for example,
auditd(8).
LOG_LOCAL0 Designated for local use.
LOG_LOCAL1 Designated for local use.
LOG_LOCAL2 Designated for local use.
LOG_LOCAL3 Designated for local use.
LOG_LOCAL4 Designated for local use.
LOG_LOCAL5 Designated for local use.
LOG_LOCAL6 Designated for local use.
LOG_LOCAL7 Designated for local use.
The
openlog() function sets process attributes that affect subsequent
calls to
syslog(). The
ident argument is a string that is prepended to
every message. The
openlog() function uses the passed-in
ident argument
directly, rather than making a private copy of it. The
logopt argument
indicates logging options. Values for
logopt are constructed by a
bitwise-inclusive OR of zero or more of the following:
LOG_PID Log the process
ID with each message. This is useful for
identifying specific daemon processes (for daemons that
fork).
LOG_CONS Write messages to the system console if they cannot be sent
to
syslogd(8). This option is safe to use in daemon
processes that have no controlling terminal, since
syslog() forks before opening the console.
LOG_NDELAY Open the connection to
syslogd(8) immediately. Normally
the open is delayed until the first message is logged. This
is useful for programs that need to manage the order in
which file descriptors are allocated.
LOG_ODELAY Delay open until
syslog() is called.
LOG_NOWAIT Do not wait for child processes that have been forked to
log messages onto the console. This option should be used
by processes that enable notification of child termination
using
SIGCHLD, since
syslog() may otherwise block waiting
for a child whose exit status has already been collected.
The
facility argument encodes a default facility to be assigned to all
messages that do not have an explicit facility already encoded. The
initial default facility is
LOG_USER.
The
openlog() and
syslog() functions may allocate a file descriptor. It
is not necessary to call
openlog() prior to calling
syslog().
The
closelog() function closes any open file descriptors allocated by
previous calls to
openlog() or
syslog().
The
setlogmask() function sets the log priority mask for the current
process to
maskpri and returns the previous mask. If the
maskpri argument is 0, the current log mask is not modified. Calls by the
current process 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 log mask
allows all priorities to be logged.
RETURN VALUES
The
setlogmask() function returns the previous log priority mask. The
closelog(),
openlog() and
syslog() functions return no value.
ERRORS
No errors are defined.
EXAMPLES
Example 1: Example of LOG_ALERT message.
This call logs a message at priority
LOG_ALERT:
syslog(LOG_ALERT, "who: internal error 23");
The
FTP daemon
ftpd would make this call to
openlog() to indicate that
all messages it logs should have an identifying string of
ftpd, should be
treated by
syslogd(8) as other messages from system daemons are, should
include the process
ID of the process logging the message:
openlog("ftpd", LOG_PID, LOG_DAEMON);
Then it would make the following call to
setlogmask() to indicate that
messages at priorities from
LOG_EMERG through
LOG_ERR should be logged,
but that no messages at any other priority should be logged:
setlogmask(LOG_UPTO(LOG_ERR));
Then, to log a message at priority
LOG_INFO, it would make the following
call to
syslog:
syslog(LOG_INFO, "Connection from host %d", CallingHost);
A locally-written utility could use the following call to
syslog() to log
a message at priority
LOG_INFO to be treated by
syslogd(8) as other
messages to the facility
LOG_LOCAL2 are:
syslog(LOG_INFO|LOG_LOCAL2, "error: %m");
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Safe |
+--------------------+-------------------+
|Standard | See
standards(7). |
+--------------------+-------------------+
SEE ALSO
at(1),
crontab(1),
logger(1),
login(1),
lpc(1B),
lpr(1B),
printf(3C),
syslog.conf(5),
attributes(7),
standards(7),
auditd(8),
cron(8),
getty(8),
su(8),
syslogd(8) May 13, 2017
SYSLOG(3C)