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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |ATTRIBUTES |STANDARDS |HISTORY |NOTES |SEE ALSO |COLOPHON | |
syslog(3) Library Functions Manualsyslog(3)closelog, openlog, syslog, vsyslog - send messages to the system logger
Standard C library (libc,-lc)
#include <syslog.h>void openlog(const char *ident, intoption, intfacility);void syslog(intpriority, const char *format, ...);void closelog(void);void vsyslog(intpriority, const char *format, va_listap); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):vsyslog(): Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE
openlog()openlog() opens a connection to the system logger for a program. The string pointed to byident is prepended to every message, and is typically set to the program name. Ifident is NULL, the program name is used. (POSIX.1-2008 does not specify the behavior whenident is NULL.) Theoption argument specifies flags which control the operation ofopenlog() and subsequent calls tosyslog(). Thefacility argument establishes a default to be used if none is specified in subsequent calls tosyslog(). The values that may be specified foroption andfacility are described below. The use ofopenlog() is optional; it will automatically be called bysyslog() if necessary, in which caseident will default to NULL.syslog() and vsyslog()syslog() generates a log message, which will be distributed bysyslogd(8). Thepriority argument is formed by ORing together afacility value and alevel value (described below). If nofacility value is ORed intopriority, then the default value set byopenlog() is used, or, if there was no precedingopenlog() call, a default ofLOG_USERis employed. The remaining arguments are aformat, as inprintf(3), and any arguments required by theformat, except that the two-character sequence%mwill be replaced by the error message stringstrerror(errno). The format string need not include a terminating newline character. The functionvsyslog() performs the same task assyslog() with the difference that it takes a set of arguments which have been obtained using thestdarg(3) variable argument list macros.closelog()closelog() closes the file descriptor being used to write to the system logger. The use ofcloselog() is optional. [1mValues foroption Theoption argument toopenlog() is a bit mask constructed by ORing together any of the following values:LOG_CONS Write directly to the system console if there is an error while sending to the system logger.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 subsequentchroot(2) would make the pathname used internally by the logging facility unreachable.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.)LOG_ODELAY The converse ofLOG_NDELAY; opening of the connection is delayed untilsyslog() is called. (This is the default, and need not be specified.)LOG_PERROR (Not in POSIX.1-2001 or POSIX.1-2008.) Also log the message tostderr.LOG_PID Include the caller's PID with each message. [1mValues forfacility Thefacility 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.LOG_AUTH security/authorization messagesLOG_AUTHPRIV security/authorization messages (private)LOG_CRON clock daemon (cronandat)LOG_DAEMON system daemons without separate facility valueLOG_FTP ftp daemonLOG_KERN kernel messages (these can't be generated from user processes)LOG_LOCAL0throughLOG_LOCAL7 reserved for local useLOG_LPR line printer subsystemLOG_MAIL mail subsystemLOG_NEWS USENET news subsystemLOG_SYSLOG messages generated internally bysyslogd(8)LOG_USER(default) generic user-level messagesLOG_UUCP UUCP subsystem [1mValues forlevel This determines the importance of the message. The levels are, in order of decreasing importance:LOG_EMERG system is unusableLOG_ALERT action must be taken immediatelyLOG_CRIT critical conditionsLOG_ERR error conditionsLOG_WARNING warning conditionsLOG_NOTICE normal, but significant, conditionLOG_INFO informational messageLOG_DEBUG debug-level message The functionsetlogmask(3) can be used to restrict logging to specified levels only.
For an explanation of the terms used in this section, seeattributes(7). ┌───────────────────────────┬───────────────┬────────────────────┐ │Interface│Attribute│Value│ ├───────────────────────────┼───────────────┼────────────────────┤ │openlog(),closelog() │ Thread safety │ MT-Safe │ ├───────────────────────────┼───────────────┼────────────────────┤ │syslog(),vsyslog() │ Thread safety │ MT-Safe env locale │ └───────────────────────────┴───────────────┴────────────────────┘
syslog()openlog()closelog() POSIX.1-2008.vsyslog() None.
syslog() 4.2BSD, SUSv2, POSIX.1-2001.openlog()closelog() 4.3BSD, SUSv2, POSIX.1-2001.vsyslog() 4.3BSD-Reno. POSIX.1-2001 specifies only theLOG_USERandLOG_LOCAL*values forfacility. However, with the exception ofLOG_AUTHPRIVandLOG_FTP, the otherfacility values appear on most UNIX systems. TheLOG_PERRORvalue foroption is not specified by POSIX.1-2001 or POSIX.1-2008, but is available in most versions of UNIX.
The argumentident in the call ofopenlog() is probably stored as- is. Thus, if the string it points to is changed,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. Never pass a string with user-supplied data as a format, use the following instead: syslog(priority, "%s", string);
journalctl(1),logger(1),setlogmask(3),syslog.conf(5),syslogd(8)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17syslog(3)Pages that refer to this page:homectl(1), importctl(1), inotifywait(1), journalctl(1), localectl(1), logger(1), loginctl(1), machinectl(1), portablectl(1), systemctl(1), systemd(1), systemd-analyze(1), systemd-cat(1), systemd-inhibit(1), systemd-nspawn(1), systemd-vmspawn(1), timedatectl(1), updatectl(1), userdbctl(1), syslog(2), pam_syslog(3), pmnotifyerr(3), sd-daemon(3), sd_journal_print(3), sd_journal_stream_fd(3), selinux_restorecon(3), setlogmask(3), stdarg(3), faillock.conf(5), limits.conf(5), org.freedesktop.LogControl1(5), pam.conf(5), pwhistory.conf(5), rsyslog.conf(5), systemd.exec(5), time.conf(5), file-hierarchy(7), systemd.generator(7), agetty(8), cmirrord(8), cron(8), lloadd(8), PAM(8), pam_access(8), pam_env(8), pam_keyinit(8), pam_mkhomedir(8), pam_pwhistory(8), pam_selinux(8), pam_sepermit(8), pam_time(8), pam_timestamp(8), pam_unix(8), pam_warn(8), rsyslogd(8), slapd(8), systemd-journald.service(8), systemd-tmpfiles(8)
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