syslog(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

SYSLOG(3)               Linux Programmer's Manual              SYSLOG(3)

NAME         top

       closelog, openlog, syslog, vsyslog - send messages to the system
       logger

SYNOPSIS         top

       #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

DESCRIPTION         top

   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.

   Values 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.

   Values 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

   Values 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.

ATTRIBUTES         top

       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 │
       └───────────────────────────┴───────────────┴────────────────────┘

CONFORMING TO         top

       The functions openlog(), closelog(), and syslog() (but not
       vsyslog()) are specified in SUSv2, POSIX.1-2001, and
       POSIX.1-2008.

       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.

NOTES         top

       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);

SEE ALSO         top

       journalctl(1), logger(1), setlogmask(3), syslog.conf(5),
       syslogd(8)

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                      SYSLOG(3)

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