path: root/man3/syslog.3

'\" t
.\" Written  Feb 1994 by Steve Greenland (stevegr@neosoft.com)
.\" and Copyright 2001, 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Updated 1999.12.19 by Karl M. Hegbloom <karlheg@debian.org>
.\"
.\" Updated 13 Oct 2001, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added description of vsyslog
.\"	Added descriptions of LOG_ODELAY and LOG_NOWAIT
.\"	Added brief description of facility and option arguments
.\"	Added CONFORMING TO section
.\" 2001-10-13, aeb, minor changes
.\" Modified 13 Dec 2001, Martin Schulze <joey@infodrom.org>
.\" Modified 3 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH syslog 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
closelog, openlog, syslog, vsyslog \- send messages to the system logger
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <syslog.h>
.PP
.BI "void openlog(const char *" ident ", int " option ", int " facility );
.BI "void syslog(int " priority ", const char *" format ", ...);"
.B "void closelog(void);"
.PP
.BI "void vsyslog(int " priority ", const char *" format ", va_list " ap );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR vsyslog ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE
.fi
.SH DESCRIPTION
.SS openlog()
.BR openlog ()
opens a connection to the system logger for a program.
.PP
The string pointed to by
.I ident
is prepended to every message, and is typically set to the program name.
If
.I ident
is NULL, the program name is used.
(POSIX.1-2008 does not specify the behavior when
.I ident
is NULL.)
.PP
The
.I option
argument specifies flags which control the operation of
.BR openlog ()
and subsequent calls to
.BR syslog ().
The
.I facility
argument establishes a default to be used if
none is specified in subsequent calls to
.BR syslog ().
The values that may be specified for
.I option
and
.I facility
are described below.
.PP
The use of
.BR openlog ()
is optional; it will automatically be called by
.BR syslog ()
if necessary, in which case
.I ident
will default to NULL.
.\"
.SS syslog() and vsyslog()
.BR syslog ()
generates a log message, which will be distributed by
.BR syslogd (8).
.PP
The
.I priority
argument is formed by ORing together a
.I facility
value and a
.I level
value (described below).
If no
.I facility
value is ORed into
.IR priority ,
then the default value set by
.BR openlog ()
is used, or, if there was no preceding
.BR openlog ()
call, a default of
.B LOG_USER
is employed.
.PP
The remaining arguments are a
.IR format ,
as in
.BR printf (3),
and any arguments required by the
.IR format ,
except that the two-character sequence
.B %m
will be replaced by
the error message string
.IR strerror ( errno ).
The format string need not include a terminating newline character.
.PP
The function
.BR vsyslog ()
performs the same task as
.BR syslog ()
with the difference that it takes a set of arguments which have
been obtained using the
.BR stdarg (3)
variable argument list macros.
.\"
.SS closelog()
.BR closelog ()
closes the file descriptor being used to write to the system logger.
The use of
.BR closelog ()
is optional.
.\"
.SS Values for \fIoption\fP
The
.I option
argument to
.BR openlog ()
is a bit mask constructed by ORing together any of the following values:
.TP 15
.B LOG_CONS
Write directly to the system console if there is an error while sending to
the system logger.
.TP
.B LOG_NDELAY
Open the connection immediately (normally, the connection is opened when
the first message is logged).
This may be useful, for example, if a subsequent
.BR chroot (2)
would make the pathname used internally by the logging facility unreachable.
.TP
.B LOG_NOWAIT
Don't wait for child processes that may have been created while logging
the message.
(The GNU C library does not create a child process, so this
option has no effect on Linux.)
.TP
.B LOG_ODELAY
The converse of
.BR LOG_NDELAY ;
opening of the connection is delayed until
.BR syslog ()
is called.
(This is the default, and need not be specified.)
.TP
.B LOG_PERROR
(Not in POSIX.1-2001 or POSIX.1-2008.)
Also log the message to
.IR stderr .
.TP
.B LOG_PID
Include the caller's PID with each message.
.\"
.SS Values for \fIfacility\fP
The
.I facility
argument is used to specify what type of program is logging the message.
This lets the configuration file specify that messages from different
facilities will be handled differently.
.TP 15
.B LOG_AUTH
security/authorization messages
.TP
.B LOG_AUTHPRIV
security/authorization messages (private)
.TP
.B LOG_CRON
clock daemon
.RB ( cron " and " at )
.TP
.B LOG_DAEMON
system daemons without separate facility value
.TP
.B LOG_FTP
ftp daemon
.TP
.B LOG_KERN
kernel messages (these can't be generated from user processes)
.\" LOG_KERN has the value 0; if used as a facility, zero translates to:
.\" "use the default facility".
.TP
.BR LOG_LOCAL0 " through " LOG_LOCAL7
reserved for local use
.TP
.B LOG_LPR
line printer subsystem
.TP
.B LOG_MAIL
mail subsystem
.TP
.B LOG_NEWS
USENET news subsystem
.TP
.B LOG_SYSLOG
messages generated internally by
.BR syslogd (8)
.TP
.BR LOG_USER " (default)"
generic user-level messages
.TP
.B LOG_UUCP
UUCP subsystem
.\"
.SS Values for \fIlevel\fP
This determines the importance of the message.
The levels are, in order of decreasing importance:
.TP 15
.B LOG_EMERG
system is unusable
.TP
.B LOG_ALERT
action must be taken immediately
.TP
.B LOG_CRIT
critical conditions
.TP
.B LOG_ERR
error conditions
.TP
.B LOG_WARNING
warning conditions
.TP
.B LOG_NOTICE
normal, but significant, condition
.TP
.B LOG_INFO
informational message
.TP
.B LOG_DEBUG
debug-level message
.PP
The function
.BR setlogmask (3)
can be used to restrict logging to specified levels only.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR openlog (),
.BR closelog ()
T}	Thread safety	MT-Safe
T{
.na
.nh
.BR syslog (),
.BR vsyslog ()
T}	Thread safety	MT-Safe env locale
.TE
.sp 1
.SH STANDARDS
.TP
.BR syslog ()
.TQ
.BR openlog ()
.TQ
.BR closelog ()
POSIX.1-2008.
.TP
.BR vsyslog ()
None.
.SH HISTORY
.TP
.BR syslog ()
4.2BSD, SUSv2, POSIX.1-2001.
.TP
.BR openlog ()
.TQ
.BR closelog ()
4.3BSD, SUSv2, POSIX.1-2001.
.\" .SH HISTORY
.\" 4.3BSD documents
.\" .BR setlogmask ().
.TP
.BR vsyslog ()
4.3BSD-Reno.
.\" Of course early v* functions used the
.\" .I <varargs.h>
.\" mechanism, which is not compatible with
.\" .IR <stdarg.h> .
.PP
POSIX.1-2001 specifies only the
.B LOG_USER
and
.B LOG_LOCAL*
values for
.IR facility .
However, with the exception of
.B LOG_AUTHPRIV
and
.BR LOG_FTP ,
the other
.I facility
values appear on most UNIX systems.
.PP
The
.B LOG_PERROR
value for
.I option
is not specified by POSIX.1-2001 or POSIX.1-2008, but is available
in most versions of UNIX.
.SH NOTES
The argument
.I ident
in the call of
.BR openlog ()
is probably stored as-is.
Thus, if the string it points to
is changed,
.BR syslog ()
may start prepending the changed string, and if the string
it points to ceases to exist, the results are undefined.
Most portable is to use a string constant.
.PP
Never pass a string with user-supplied data as a format,
use the following instead:
.PP
.in +4n
.EX
syslog(priority, "%s", string);
.EE
.in
.SH SEE ALSO
.BR journalctl (1),
.BR logger (1),
.BR setlogmask (3),
.BR syslog.conf (5),
.BR syslogd (8)