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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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syslog(3) Library Functions Manual syslog(3)
closelog, openlog, syslog, vsyslog - send messages to the system
logger
Standard C library (libc, -lc)
#include <syslog.h>
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);
void vsyslog(int priority, const char *format, va_list ap);
Feature Test Macro Requirements for glibc (see
feature_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 by ident is prepended to every message, and
is typically set to the program name. If ident is NULL, the
program name is used. (POSIX.1-2008 does not specify the behavior
when ident is NULL.)
The option argument specifies flags which control the operation of
openlog() and subsequent calls to syslog(). The facility argument
establishes a default to be used if none is specified in
subsequent calls to syslog(). The values that may be specified
for option and facility are described below.
The use of openlog() is optional; it will automatically be called
by syslog() if necessary, in which case ident will default to
NULL.
syslog() and vsyslog()
syslog() generates a log message, which will be distributed by
syslogd(8).
The priority argument is formed by ORing together a facility value
and a level value (described below). If no facility value is ORed
into priority, then the default value set by openlog() is used,
or, if there was no preceding openlog() call, a default of
LOG_USER is employed.
The remaining arguments are a format, as in printf(3), and any
arguments required by the format, except that the two-character
sequence %m will be replaced by the error message string
strerror(errno). The format string need not include a terminating
newline character.
The function vsyslog() performs the same task as syslog() with the
difference that it takes a set of arguments which have been
obtained using the stdarg(3) variable argument list macros.
closelog()
closelog() closes the file descriptor being used to write to the
system logger. The use of closelog() is optional.
[1mValues for option
The option argument to openlog() 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 subsequent chroot(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 of LOG_NDELAY; opening of the connection is
delayed until syslog() 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 to stderr.
LOG_PID
Include the caller's PID with each message.
[1mValues for facility
The 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.
LOG_AUTH
security/authorization messages
LOG_AUTHPRIV
security/authorization messages (private)
LOG_CRON
clock daemon (cron and at)
LOG_DAEMON
system daemons without separate facility value
LOG_FTP
ftp daemon
LOG_KERN
kernel messages (these can't be generated from user
processes)
LOG_LOCAL0 through LOG_LOCAL7
reserved for local use
LOG_LPR
line printer subsystem
LOG_MAIL
mail subsystem
LOG_NEWS
USENET news subsystem
LOG_SYSLOG
messages generated internally by syslogd(8)
LOG_USER (default)
generic user-level messages
LOG_UUCP
UUCP subsystem
[1mValues for level
This determines the importance of the message. The levels are, in
order of decreasing importance:
LOG_EMERG
system is unusable
LOG_ALERT
action must be taken immediately
LOG_CRIT
critical conditions
LOG_ERR
error conditions
LOG_WARNING
warning conditions
LOG_NOTICE
normal, but significant, condition
LOG_INFO
informational message
LOG_DEBUG
debug-level message
The function setlogmask(3) can be used to restrict logging to
specified levels only.
For an explanation of the terms used in this section, see
attributes(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 the LOG_USER and LOG_LOCAL* values for
facility. However, with the exception of LOG_AUTHPRIV and
LOG_FTP, the other facility values appear on most UNIX systems.
The LOG_PERROR value for option is not specified by POSIX.1-2001
or POSIX.1-2008, but is available in most versions of UNIX.
The argument ident in the call of openlog() 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 the man-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 is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 syslog(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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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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