.\" $OpenBSD: syslog.3,v 1.37 2022/03/31 17:27:15 naddy Exp $ .\" .\" Copyright (c) 1985, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. 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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd $Mdocdate: March 31 2022 $ .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 points to a string that will be prepended to every message; its storage must persist until .Fn closelog or the corresponding .Fn closelog_r . If the content of the string is changed, behaviour is unspecified. .Pp The .Fa logopt argument is a bit field specifying logging options, which is formed by OR'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 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 STANDARDS The functions .Fn syslog , .Fn openlog , .Fn closelog , and .Fn setlogmask conform to the X/Open Systems Interfaces option of .St -p1003.1-2008 . .Pp The facilities .Dv LOG_AUTHPRIV , .Dv LOG_FTP , and .Dv LOG_SYSLOG , the option .Dv LOG_PERROR , and the macro .Fn LOG_UPTO are extensions to that standard. .Pp The standard option .Dv LOG_NOWAIT is deprecated in .Ox and has no effect. .Sh HISTORY The functions .Fn syslog , .Fn openlog , and .Fn closelog appeared in .Bx 4.2 , .Fn setlogmask in .Bx 4.3 , and .Fn vsyslog in .Bx 4.3 Net/1 . .Pp The functions .Fn syslog_r , .Fn vsyslog_r , .Fn openlog_r , .Fn closelog_r , and .Fn setlogmask_r appeared in .Bx 386 0.1 and have been available since .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 .