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syslog.3 revision 1.24 
 $NetBSD: syslog.3,v 1.24 2006/11/22 17:23:25 christos Exp $
 $OpenBSD: syslog.3,v 1.25 2005/07/22 03:16:58 jaredy Exp $

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 @(#)syslog.3 8.1 (Berkeley) 6/4/93

.Dd November 22, 2006
.Dt SYSLOG 3
.Os
.Sh NAME
.Nm syslog ,
.Nm syslog_r ,
.Nm vsyslog ,
.Nm vsyslog_r ,
.Nm openlog ,
.Nm openlog_r ,
.Nm closelog ,
.Nm closelog_r ,
.Nm setlogmask ,
.Nm setlogmask_r
.Nd control system log
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
n syslog.h .Ft void
.Fn syslog "int priority" "const char *message" "..."
.Ft void
.Fn syslog_r "int priority" "struct syslog_data *data" "const char *message" "..."
 .Ft void
 .Fn syslog_ss "int priority" "struct syslog_data *data" "const char *message" "..."
.Ft void
.Fn openlog "const char *ident" "int logopt" "int facility"
.Ft void
.Fn openlog_r "const char *ident" "int logopt" "int facility" "struct syslog_data *data"
.Ft void
.Fn closelog void
.Ft void
.Fn closelog_r "struct syslog_data *data"
.Ft int
.Fn setlogmask "int maskpri"
.Ft int
.Fn setlogmask_r "int maskpri" "struct syslog_data *data"
n stdarg.h .Ft void
.Fn vsyslog "int priority" "const char *message" "va_list args"
.Ft void
.Fn vsyslog_r "int priority" "struct syslog_data *data" "const char *message" "va_list args"
 .Ft void
 .Fn vsyslog_ss "int priority" "struct syslog_data *data" "const char *message" "va_list args"
d -literal 
struct syslog_data {
 int log_file;
 int connected;
 int opened;
 int log_stat;
 const char *log_tag;
 int log_fac;
 int log_mask;
};

#define SYSLOG_DATA_INIT { \e
 .log_file = -1, \e
 .log_fac = LOG_USER, \e
 .log_mask = 0xff, \e
}
.Ed
.Sh DESCRIPTION
The
.Fn syslog
function
writes
.Fa 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
.Xr syslogd 8 ) .

p
The message is identical to a
.Xr printf 3
format string, except that
.Ql %m
is replaced by the current error
message.
(As denoted by the global variable
.Va errno ;
see
.Xr strerror 3 . )
A trailing newline is added if none is present.
The
.Fn syslog_r
function is a multithread-safe version of the
.Fn syslog
function.
It takes a pointer to a
.Fa syslog_data
structure which is used to store
information.
This parameter must be initialized before
.Fn syslog_r
is called.
The
.Dv SYSLOG_DATA_INIT
constant is used for this purpose.
The
.Fa syslog_data
structure is composed of the following elements:
l -tag -width connected t Dv log_file contains the file descriptor of the file where the message is logged
t Dv connected indicates if connect has been done
t Dv opened indicates if
.Fn openlog_r
has been called
t Dv log_stat status bits, set by
.Fn openlog_r
t Dv log_tag string to tag the entry with
t Dv log_fac facility code
t Dv log_mask mask of priorities to be logged
.El
 .Pp
 The
 .Fn syslog_ss
 is the async-signal-safe version of
 .Fn syslog_r
 and is also multithread-safe.
 It has the following limitations:
 .Bl -enum -offset indent
 .It
 The format string cannot contain multi-byte character sequences.
 .It
 Floating point formats are not supported and print
 .Dq UNK .
 .It
 The time of the event is not sent to
 .Xr syslogd 8 .
 .It
 The error string in the %m format is not printed symbolically but as
 .Dq Error %d .
 .El
 .Pp
 For more information about async-signal-safe functions and signal handlers, see
 .Xr signal 7 .
p
The
.Fn vsyslog
function
is an alternative form in which the arguments have already been captured
using the variable-length argument facilities of
.Xr varargs 3 .

p
The message is tagged with
.Fa priority .
Priorities are encoded as a
.Fa facility
and a
.Em level .
The facility describes the part of the system
generating the message.
The level is selected from the following
.Em ordered
(high to low) list:
l -tag -width LOG_AUTHPRIV t Dv LOG_EMERG A panic condition.
This is normally broadcast to all users.
t Dv LOG_ALERT A condition that should be corrected immediately, such as a corrupted
system database.
t Dv LOG_CRIT Critical conditions, e.g., hard device errors.
t Dv LOG_ERR Errors.
t Dv LOG_WARNING Warning messages.
t Dv LOG_NOTICE Conditions that are not error conditions,
but should possibly be handled specially.
t Dv LOG_INFO Informational messages.
t Dv LOG_DEBUG Messages that contain information
normally of use only when debugging a program.
.El

p
The
.Fn vsyslog_r
is used the same way as
.Fn vsyslog
except that it takes an additional pointer to a
.Fa syslog_data
structure.
It is a multithread-safe version of the
.Fn vsyslog
function described above.
 The
 .Fn vsyslog_ss
 is the async-signal-safe version of
 .Fn vsyslog_r ,
 is also multithread-safe, and has the same limitations as
 .Fn syslog_ss .
p
The
.Fn openlog
function
provides for more specialized processing of the messages sent
by
.Fn syslog
and
.Fn vsyslog .
The parameter
.Fa ident
is a string that will be prepended to every message.
The
.Fa logopt
argument
is a bit field specifying logging options, which is formed by
.Tn OR Ns 'ing
one or more of the following values:
l -tag -width LOG_AUTHPRIV t Dv LOG_CONS If
.Fn syslog
cannot pass the message to
.Xr syslogd 8
it will attempt to write the message to the console

q Dq Pa /dev/console .
t Dv LOG_NDELAY Open the connection to
.Xr 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.
t Dv LOG_PERROR Write the message to standard error output as well to the system log.
t Dv LOG_PID Log the process id with each message: useful for identifying
instantiations of daemons.
(This PID is placed within brackets
between the ident and the message.)
.El

p
The
.Fa facility
parameter encodes a default facility to be assigned to all messages
that do not have an explicit facility encoded:
l -tag -width LOG_AUTHPRIV t Dv LOG_AUTH The authorization system:
.Xr login 1 ,
.Xr su 1 ,
.Xr getty 8 ,
etc.
t Dv LOG_AUTHPRIV The same as
.Dv LOG_AUTH ,
but logged to a file readable only by
selected individuals.
t Dv LOG_CRON The cron daemon:
.Xr cron 8 .
t Dv LOG_DAEMON System daemons, such as
.Xr routed 8 ,
that are not provided for explicitly by other facilities.
t Dv LOG_FTP The file transfer protocol daemon:
.Xr ftpd 8 .
t Dv LOG_KERN Messages generated by the kernel.
These cannot be generated by any user processes.
t Dv LOG_LPR The line printer spooling system:
.Xr lpr 1 ,
.Xr lpc 8 ,
.Xr lpd 8 ,
etc.
t Dv LOG_MAIL The mail system.
t Dv LOG_NEWS The network news system.
t Dv LOG_SYSLOG Messages generated internally by
.Xr syslogd 8 .
t Dv LOG_USER Messages generated by random user processes.
This is the default facility identifier if none is specified.
t Dv LOG_UUCP The uucp system.
t Dv LOG_LOCAL0 Reserved for local use.
Similarly for
.Dv LOG_LOCAL1
through
.Dv LOG_LOCAL7 .
.El

p
The
.Fn openlog_r
function is the multithread-safe version of the
.Fn openlog
function.
It takes an additional pointer to a
.Fa syslog_data
structure.
This function must be used in conjunction with the other
multithread-safe functions.

p
The
.Fn closelog
function
can be used to close the log file.

p
The
.Fn closelog_r
does the same thing as
.Xr closelog 3
but in a multithread-safe way and takes an additional
pointer to a
.Fa syslog_data
structure.

p
The
.Fn setlogmask
function
sets the log priority mask to
.Fa maskpri
and returns the previous mask.
Calls to
.Fn syslog
with a priority not set in
.Fa maskpri
are rejected.
The mask for an individual priority
.Fa pri
is calculated by the macro
.Fn LOG_MASK pri ;
the mask for all priorities up to and including
.Fa toppri
is given by the macro
.Fn LOG_UPTO toppri .
The default allows all priorities to be logged.

p
The
.Fn setlogmask_r
function is the multithread-safe version of
.Fn setlogmask .
It takes an additional pointer to a
.Fa syslog_data
structure.
.Sh RETURN VALUES
The routines
.Fn closelog ,
.Fn closelog_r ,
.Fn openlog ,
.Fn openlog_r ,
.Fn syslog ,
.Fn syslog_r ,
.Fn vsyslog ,
and
.Fn vsyslog_r
return no value.

p
The routines
.Fn setlogmask
and
.Fn setlogmask_r
always return the previous log mask level.
.Sh EXAMPLES
d -literal -offset indent -compact 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");
.Ed

p
For the multithread-safe functions:
d -literal -offset indent struct syslog_data sdata = SYSLOG_DATA_INIT;

syslog_r(LOG_INFO|LOG_LOCAL2, \*[Am]sdata, "foobar error: %m");
.Ed
.Sh SEE ALSO
.Xr logger 1 ,
.Xr syslogd 8
.Sh HISTORY
These non-multithread-safe functions appeared in
x 4.2 . The multithread-safe functions appeared in
.Ox 3.1
and then in
.Nx 4.0 .
The async-signal-safe functions appeared in
.Nx 4.0 .
.Sh CAVEATS
It is important never to pass a string with user-supplied data as a
format without using
.Ql %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 you have built the string
.Dq by hand
using a function like
.Fn snprintf ,
as the resulting string may still contain user-supplied conversion specifiers
for later interpolation by
.Fn syslog .

p
Always be sure to use the proper secure idiom:
d -literal -offset indent syslog(priority, "%s", string);
.Ed