path: root/man3/syslog.3

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.\" Updated 1999.12.19 by Karl M. Hegbloom <karlheg@debian.org>
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.\" Updated 13 Oct 2001, Michael Kerrisk <mtk-manpages@gmx.net>
.\"	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@gmx.net>
.\"
.TH SYSLOG 3 2002-01-03 "Linux" "Linux Programmer's Manual"
.SH NAME
closelog, openlog, syslog \- send messages to the system logger
.SH SYNOPSIS
.B #include <syslog.h>
.sp
.BI "void openlog(const char *" ident ", int " option ", int " facility );
.br
.BI "void syslog(int " priority ", const char *" format ", ...);"
.br
.BI "void closelog(void);"
.sp
.B #include <stdarg.h>
.sp
.BI "void vsyslog(int " priority ", const char *" format ", va_list " ap );
.br
.SH DESCRIPTION
.BR closelog ()
closes the descriptor being used to write to the system logger.  The use of
.BR closelog ()
is optional.
.sp
.BR openlog ()
opens a connection to the system logger for a program.  The string pointed
to by
.I ident
is prepended to every message, and is typically set to the program name.
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 ().
Values for
.I option
and
.I facility
are given below.  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.
.sp
.BR syslog ()
generates a log message, which will be distributed by 
.BR syslogd (8).
The
.I priority
argument is formed by ORing the
.I facility
and the 
.I level
values (explained below).  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 %m will be replaced by
the error message string 
.IR strerror ( errno ).
A trailing newline is added when needed.

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.
.SH PARAMETERS
This section lists the parameters used to set the values of 
.IR option , " facility" ", and " priority .
.SS option
The
.I option
argument to 
.BR openlog ()
is an OR of any of these:
.TP
.B LOG_CONS
Write directly to system console if there is an error while sending to
system logger.
.TP
.B LOG_NDELAY
Open the connection immediately (normally, the connection is opened when
the first message is logged).
.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.) Print to stderr as well.
.TP
.B LOG_PID
Include PID with each message.
.SS facility 
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
.B LOG_AUTH
security/authorization messages (DEPRECATED Use 
.B LOG_AUTHPRIV 
instead)
.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
.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
.B syslogd 
.TP
.BR LOG_USER " (default)"
generic user-level messages 
.TP
.B LOG_UUCP
UUCP subsystem

.SS level
This determines the importance of the message.  The levels are, in order
of decreasing importance:
.TP
.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
.LP
The function
.BR setlogmask (3)
can be used to restrict logging to specified levels only.
.SH "CONFORMING TO"
The functions
.BR openlog (),
.BR closelog (),
and
.BR syslog ()
(but not
.BR vsyslog ())
are specified in SUSv2 and POSIX.1-2001.
POSIX.1-2001 specifies only the
.B LOG_USER
and
.BR LOG_LOCAL*
values for
.IR facility .
However, with the exception of
.BR LOG_AUTHPRIV 
and
.BR LOG_FTP ,
the other
.I facility
values appear on most Unix systems.
The
.B LOG_PERROR
value for
.I option
is not specified by POSIX.1-2001, but is available
in most versions of Unix.
.SH HISTORY
A
.BR syslog ()
function call appeared in 4.2BSD.
4.3BSD documents
.BR openlog (),
.BR syslog (),
.BR closelog (),
and
.BR setlogmask ().
4.3BSD-Reno also documents
.BR vsyslog ().
Of course early v* functions used the
.I <varargs.h>
mechanism, which is not compatible with
.IR <stdarg.h> .
.SH NOTES
The parameter
.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.
.LP
Never pass a string with user-supplied data as a format, use
.RS
syslog(priority, "%s", string);
.RE
instead.
.SH "SEE ALSO"
.BR logger (1),
.BR setlogmask (3),
.BR syslog.conf (5),
.BR syslogd (8)