libc/gen/syslog.3

.\"	$OpenBSD: syslog.3,v 1.15 2001/11/08 11:12:09 mpech Exp $
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.Dd June 4, 1993
.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
.Fd #include <syslog.h>
.Fd #include <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"
.Bd -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 {-1, 0, 0, 0, NULL, LOG_USER, 0xff}
.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 ) .
.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
.Xr syslog 3
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 SYSLOG_DATA_INIT constant is used for this purpose.
The
.Fa syslog_data
structure is composed of the following elements:
.Bl -tag -width connected
.It Dv log_file
contains the file descriptor of the file where the message is logged.
.It Dv connected
indicates if connect has been done
.It Dv opened
indicates if
.Xr openlog_r 3
has been called.
.It Dv log_stat
status bits, set by
.Xr openlog_r 3
.It Dv log_tag
string to tag the entry with
.It Dv log_fac
facility code
.It Dv log_mask
mask of priorities to be logged
.El
.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 varargs 3 .
.Pp
The message is tagged with
.Fa priority .
Priorities are encoded as a
.Fa facility
and a
.Dq 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 vsyslog_r
is used the same way than
.Fn vsyslog
except than it takes an additional pointer on 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.
.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.
.El
.Pp
The
.Fa facility
parameter 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_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 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
.Tn UUCP
system.
.It Dv LOG_LOCAL0
Reserved for local use.
Similarly for
.Dv LOG_LOCAL1
through
.Dv LOG_LOCAL7 .
.El
.Pp
The
.Fn openlog_r
function is the reentrant version of the
.Fn openlog
function.
It takes an additional pointer on 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
do the same thing but in a reentrant way and takes an additional
pointer on 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.
.Pp
The
.Fn setlogmask_r
function is the reentrant version of
.Fn setlogmask .
It takes an additional pointer on 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 -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");
.Pp
.Ed
For the reentrant functions:
.Bd -literal -offset indent -compact
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 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 .
.Pp
Always be sure to use the proper secure idiom:
.Bd -literal -offset indent
syslog("%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 .