'\" t .\" Written Feb 1994 by Steve Greenland (stevegr@neosoft.com) .\" and Copyright 2001, 2017 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Updated 1999.12.19 by Karl M. Hegbloom .\" .\" Updated 13 Oct 2001, Michael Kerrisk .\" Added description of vsyslog .\" Added descriptions of LOG_ODELAY and LOG_NOWAIT .\" Added brief description of facility and option arguments .\" Added CONFORMING TO section .\" 2001-10-13, aeb, minor changes .\" Modified 13 Dec 2001, Martin Schulze .\" Modified 3 Jan 2002, Michael Kerrisk .\" .TH syslog 3 2024-05-02 "Linux man-pages 6.9.1" .SH NAME closelog, openlog, syslog, vsyslog \- send messages to the system logger .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .P .BI "void openlog(const char *" ident ", int " option ", int " facility ); .BI "void syslog(int " priority ", const char *" format ", ...);" .B "void closelog(void);" .P .BI "void vsyslog(int " priority ", const char *" format ", va_list " ap ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR vsyslog (): .nf Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE .fi .SH DESCRIPTION .SS openlog() .BR openlog () opens a connection to the system logger for a program. .P The string pointed to by .I ident is prepended to every message, and is typically set to the program name. If .I ident is NULL, the program name is used. (POSIX.1-2008 does not specify the behavior when .I ident is NULL.) .P The .I option argument specifies flags which control the operation of .BR openlog () and subsequent calls to .BR syslog (). The .I facility argument establishes a default to be used if none is specified in subsequent calls to .BR syslog (). The values that may be specified for .I option and .I facility are described below. .P The use of .BR openlog () is optional; it will automatically be called by .BR syslog () if necessary, in which case .I ident will default to NULL. .\" .SS syslog() and vsyslog() .BR syslog () generates a log message, which will be distributed by .BR syslogd (8). .P The .I priority argument is formed by ORing together a .I facility value and a .I level value (described below). If no .I facility value is ORed into .IR priority , then the default value set by .BR openlog () is used, or, if there was no preceding .BR openlog () call, a default of .B LOG_USER is employed. .P The remaining arguments are a .IR format , as in .BR printf (3), and any arguments required by the .IR format , except that the two-character sequence .B %m will be replaced by the error message string .IR strerror ( errno ). The format string need not include a terminating newline character. .P The function .BR vsyslog () performs the same task as .BR syslog () with the difference that it takes a set of arguments which have been obtained using the .BR stdarg (3) variable argument list macros. .\" .SS closelog() .BR closelog () closes the file descriptor being used to write to the system logger. The use of .BR closelog () is optional. .\" .SS Values for \fIoption\fP The .I option argument to .BR openlog () is a bit mask constructed by ORing together any of the following values: .TP 15 .B LOG_CONS Write directly to the system console if there is an error while sending to the system logger. .TP .B LOG_NDELAY Open the connection immediately (normally, the connection is opened when the first message is logged). This may be useful, for example, if a subsequent .BR chroot (2) would make the pathname used internally by the logging facility unreachable. .TP .B LOG_NOWAIT Don't wait for child processes that may have been created while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.) .TP .B LOG_ODELAY The converse of .BR LOG_NDELAY ; opening of the connection is delayed until .BR syslog () is called. (This is the default, and need not be specified.) .TP .B LOG_PERROR (Not in POSIX.1-2001 or POSIX.1-2008.) Also log the message to .IR stderr . .TP .B LOG_PID Include the caller's PID with each message. .\" .SS Values for \fIfacility\fP The .I facility argument is used to specify what type of program is logging the message. This lets the configuration file specify that messages from different facilities will be handled differently. .TP 15 .B LOG_AUTH security/authorization messages .TP .B LOG_AUTHPRIV security/authorization messages (private) .TP .B LOG_CRON clock daemon .RB ( cron " and " at ) .TP .B LOG_DAEMON system daemons without separate facility value .TP .B LOG_FTP ftp daemon .TP .B LOG_KERN kernel messages (these can't be generated from user processes) .\" LOG_KERN has the value 0; if used as a facility, zero translates to: .\" "use the default facility". .TP .BR LOG_LOCAL0 " through " LOG_LOCAL7 reserved for local use .TP .B LOG_LPR line printer subsystem .TP .B LOG_MAIL mail subsystem .TP .B LOG_NEWS USENET news subsystem .TP .B LOG_SYSLOG messages generated internally by .BR syslogd (8) .TP .BR LOG_USER " (default)" generic user-level messages .TP .B LOG_UUCP UUCP subsystem .\" .SS Values for \fIlevel\fP This determines the importance of the message. The levels are, in order of decreasing importance: .TP 15 .B LOG_EMERG system is unusable .TP .B LOG_ALERT action must be taken immediately .TP .B LOG_CRIT critical conditions .TP .B LOG_ERR error conditions .TP .B LOG_WARNING warning conditions .TP .B LOG_NOTICE normal, but significant, condition .TP .B LOG_INFO informational message .TP .B LOG_DEBUG debug-level message .P The function .BR setlogmask (3) can be used to restrict logging to specified levels only. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR openlog (), .BR closelog () T} Thread safety MT-Safe T{ .na .nh .BR syslog (), .BR vsyslog () T} Thread safety MT-Safe env locale .TE .SH STANDARDS .TP .BR syslog () .TQ .BR openlog () .TQ .BR closelog () POSIX.1-2008. .TP .BR vsyslog () None. .SH HISTORY .TP .BR syslog () 4.2BSD, SUSv2, POSIX.1-2001. .TP .BR openlog () .TQ .BR closelog () 4.3BSD, SUSv2, POSIX.1-2001. .\" .SH HISTORY .\" 4.3BSD documents .\" .BR setlogmask (). .TP .BR vsyslog () 4.3BSD-Reno. .\" Of course early v* functions used the .\" .I .\" mechanism, which is not compatible with .\" .IR . .P POSIX.1-2001 specifies only the .B LOG_USER and .B LOG_LOCAL* values for .IR facility . However, with the exception of .B LOG_AUTHPRIV and .BR LOG_FTP , the other .I facility values appear on most UNIX systems. .P The .B LOG_PERROR value for .I option is not specified by POSIX.1-2001 or POSIX.1-2008, but is available in most versions of UNIX. .SH NOTES The argument .I ident in the call of .BR openlog () is probably stored as-is. Thus, if the string it points to is changed, .BR syslog () may start prepending the changed string, and if the string it points to ceases to exist, the results are undefined. Most portable is to use a string constant. .P Never pass a string with user-supplied data as a format, use the following instead: .P .in +4n .EX syslog(priority, "%s", string); .EE .in .SH SEE ALSO .BR journalctl (1), .BR logger (1), .BR setlogmask (3), .BR syslog.conf (5), .BR syslogd (8)