libc/gen/syslog.3

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.Dd July 3, 2023
.Dt SYSLOG 3
.Os
.Sh NAME
.Nm syslog ,
.Nm vsyslog ,
.Nm openlog ,
.Nm closelog ,
.Nm setlogmask
.Nd control system log
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In syslog.h
.Ft void
.Fn syslog "int priority" "const char *message" "..."
.Ft void
.Fn openlog "const char *ident" "int logopt" "int facility"
.Ft void
.Fn closelog void
.Ft int
.Fn setlogmask "int maskpri"
.In syslog.h
.In stdarg.h
.Ft void
.Fn vsyslog "int priority" "const char *message" "va_list args"
.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 vsyslog
function
is an alternate form in which the arguments have already been captured
using the variable-length argument facilities of
.Xr stdarg 3 .
.Pp
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:
.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 openlog
function
provides for more specialized processing of the messages sent
by
.Fn syslog
and
.Fn vsyslog .
The
.Fa ident
argument
is a string that will be prepended to every message.
It may be formatted as
.Fa ident[N]
in which case decimal number
.Fa N
replaces the process id within messages.
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 Dq 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.
.It Dv LOG_PERROR
Write the message to standard error output as well to the system log.
.It Dv LOG_PID
Log the process id with each message: useful for identifying
instantiations of daemons.
On
.Fx ,
this option is enabled by default and cannot be disabled.
.El
.Pp
The
.Fa facility
argument encodes a default facility to be assigned to all messages
that do not have an explicit facility encoded:
.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_CONSOLE
Messages written to
.Pa /dev/console
by the kernel console output driver.
.It Dv LOG_CRON
The cron daemon:
.Xr cron 8 .
.It Dv LOG_DAEMON
System daemons, such as
.Xr routed 8 ,
that are not provided for explicitly by other facilities.
.It Dv LOG_FTP
The file transfer protocol daemons:
.Xr ftpd 8 ,
.Xr tftpd 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_NTP
The network time protocol system.
.It Dv LOG_SECURITY
Security subsystems, such as
.Xr ipfw 4 .
.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
.Fn closelog
function
can be used to close the log file.
.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.
.Sh RETURN VALUES
The routines
.Fn closelog ,
.Fn openlog ,
.Fn syslog
and
.Fn vsyslog
return no value.
.Pp
The routine
.Fn setlogmask
always returns the previous log mask level.
.Sh EXAMPLES
.Bd -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_ERR|LOG_LOCAL2, "foobar error: %m");
.Ed
.Sh SEE ALSO
.Xr logger 1 ,
.Xr syslogd 8
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
.Sh BUGS
Never 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 the string was built 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 use the proper secure idiom:
.Pp
.Dl syslog(priority, \*q%s\*q, string);