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

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

       #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)):

           Since glibc 2.19:
           Glibc 2.19 and earlier:

       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

       The option argument specifies flags  which  control  the  operation  of
       openlog()  and subsequent calls to syslog().  The facility argument es-
       tablishes 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 sys-

       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 argu-
       ments required by the format, except that the two-character sequence %m
       will be replaced by the error message string strerror(errno).  The for-
       mat string need not include a terminating newline character.

       The function vsyslog() performs the same task as syslog() with the dif-
       ference that it takes a set of arguments which have been obtained using
       the stdarg(3) variable argument list macros.

       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 to-
       gether any of the following values:

       LOG_CONS       Write directly to the system console if there is an  er-
                      ror while sending to the system logger.

       LOG_NDELAY     Open  the  connection immediately (normally, the connec-
                      tion 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 cre-
                      ated 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 log-
       ging the message.  This lets the configuration file specify  that  mes-
       sages 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 pro-

       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  or-
       der 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  at-

       |Interface             | Attribute     | Value              |
       |openlog(), closelog() | Thread safety | MT-Safe            |
       |syslog(), vsyslog()   | Thread safety | MT-Safe env locale |
       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  fa-
       cility.   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

       Never  pass  a string with user-supplied data as a format, use the fol-
       lowing instead:

           syslog(priority, "%s", string);

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

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Linux                             2017-09-15                         SYSLOG(3)
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