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.Dd $Mdocdate: August 30 2019 $
.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 SYNOPSIS
.In syslog.h
.In stdarg.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 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 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"
.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 ) .
.Pp
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.
.Pp
The
.Fn syslog_r
function is a reentrant 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.
.Pp
The
.Fn vsyslog
function is an alternate form in which the arguments have already been captured
using the variable-length argument facilities of
.Xr va_start 3 .
.Pp
The message is tagged with
.Fa priority .
Priorities are encoded as a
.Fa facility
and a
.Fa level .
The
.Fa facility
describes the part of the system
generating the message:
.Bl -tag -width LOG_AUTHPRIV
.It Dv LOG_AUTH
The authorization system:
.Xr login 1 ,
.Xr su 1 ,
.Xr getty 8 ,
etc.
.It Dv LOG_AUTHPRIV
The same as
.Dv LOG_AUTH ,
but logged to a file readable only by
selected individuals.
.It Dv LOG_CRON
The cron daemon,
.Xr cron 8 .
.It Dv LOG_DAEMON
System daemons, such as
.Xr dhcpd 8 ,
that are not provided for explicitly by other facilities.
.It Dv LOG_FTP
The file transfer protocol daemon,
.Xr ftpd 8 .
.It Dv LOG_KERN
Messages generated by the kernel.
These cannot be generated by any user processes.
.It Dv LOG_LPR
The line printer spooling system:
.Xr lpr 1 ,
.Xr lpc 8 ,
.Xr lpd 8 ,
etc.
.It Dv LOG_MAIL
The mail system.
.It Dv LOG_NEWS
The network news system.
.It Dv LOG_SYSLOG
Messages generated internally by
.Xr syslogd 8 .
.It Dv LOG_USER
Messages generated by random user processes.
This is the default facility identifier if none is specified.
.It Dv LOG_UUCP
The UUCP system.
.It Dv LOG_LOCAL0
Reserved for local use.
Similarly for
.Dv LOG_LOCAL1
through
.Dv LOG_LOCAL7 .
.El
.Pp
The
.Fa level
(ORed with the
.Fa facility )
is selected from the following list, ordered by decreasing importance:
.Bl -tag -width LOG_AUTHPRIV
.It Dv LOG_EMERG
A panic condition.
This is normally broadcast to all users.
.It Dv LOG_ALERT
A condition that should be corrected immediately, such as a corrupted
system database.
.It Dv LOG_CRIT
Critical conditions, e.g., hard device errors.
.It Dv LOG_ERR
Errors.
.It Dv LOG_WARNING
Warning messages.
.It Dv LOG_NOTICE
Conditions that are not error conditions,
but should possibly be handled specially.
.It Dv LOG_INFO
Informational messages.
.It Dv LOG_DEBUG
Messages that contain information
normally of use only when debugging a program.
.El
.Pp
The
.Fn vsyslog_r
function is used the same way as
.Fn vsyslog
except that it takes an additional pointer to a
.Fa syslog_data
structure.
It is a reentrant version of the
.Fn vsyslog
function described above.
.Pp
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:
.Bl -tag -width LOG_AUTHPRIV
.It Dv LOG_CONS
If
.Fn syslog
cannot pass the message to
.Xr syslogd 8
it will attempt to write the message to the console
.Pq Pa /dev/console .
.It 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.
This option must be used in programs that call
.Xr chroot 2
where the new root does not have its own log socket.
.It Dv LOG_ODELAY
Delay opening the connection to
.Xr syslogd 8
until the first message is logged.
This is the opposite of
.Dv LOG_NDELAY
and is the default behaviour when neither option is specified.
.It Dv LOG_PERROR
Write the message to standard error output as well as to the system log.
.It Dv LOG_PID
Log the process ID with each message; useful for identifying
instantiations of daemons.
.El
.Pp
The
.Fa facility
parameter encodes a default facility to be assigned to all messages
that do not have an explicit facility encoded.
.Pp
The
.Fn openlog_r
function is the reentrant 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
reentrant functions.
.Pp
The
.Fn closelog
function can be used to close the log file.
.Fn closelog_r
does the same thing but in a reentrant way and takes an additional
pointer to a
.Fa syslog_data
structure.
.Pp
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, which
corresponds to
.Li setlogmask(LOG_UPTO(LOG_DEBUG)) .
.Pp
The
.Fn setlogmask_r
function is the reentrant version of
.Fn setlogmask .
It takes an additional pointer to a
.Fa syslog_data
structure.
.Sh RETURN VALUES
The
.Fn closelog ,
.Fn closelog_r ,
.Fn openlog ,
.Fn openlog_r ,
.Fn syslog ,
.Fn syslog_r ,
.Fn vsyslog ,
and
.Fn vsyslog_r
functions return no value.
.Pp
The routines
.Fn setlogmask
and
.Fn setlogmask_r
always return the previous log mask level.
.Sh EXAMPLES
.Bd -literal -offset indent
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
.Pp
For the reentrant functions:
.Bd -literal -offset indent
struct syslog_data sdata = SYSLOG_DATA_INIT;

syslog_r(LOG_INFO|LOG_LOCAL2, &sdata, "foobar error: %m");
.Ed
.Sh SEE ALSO
.Xr logger 1 ,
.Xr syslogd 8
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
The reentrant functions appeared in
.Ox 3.1 .
.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 the stack,
leading to a possible security hole.
This holds true even if the string has been built
.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 .
.Pp
Always be sure to use the proper secure idiom:
.Bd -literal -offset indent
syslog(priority, "%s", string);
.Ed
.Pp
.Fn syslog_r
and the other reentrant functions should only be used where
reentrancy is required (for instance, in a signal handler).
.Fn syslog
being not reentrant, only
.Fn syslog_r
should be used here.
For more information about reentrancy and signal handlers, see
.Xr signal 3 .