man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

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(1M), 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 (1M), getty(1M).

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(1M).

LOG_AUDIT

The audit facility, for example, auditd(1M).

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(1M). 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(1M) 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(1M) 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(1M) as other messages to the facility LOG_LOCAL2 are:

syslog(LOG_INFO|LOG_LOCAL2, "error: %m");

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Safe
Standard

See Also

at(1), crontab(1), logger(1), login(1), auditd(1M), cron(1M), getty(1M), su(1M), syslogd(1M), printf(3C), syslog.conf(4), attributes(5), standards(5)