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NAME

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

SYNOPSIS

       #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

   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

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────┬───────────────┬────────────────────┐
       │InterfaceAttributeValue              │
       ├──────────────────────┼───────────────┼────────────────────┤
       │openlog(), closelog() │ Thread safety │ MT-Safe            │
       ├──────────────────────┼───────────────┼────────────────────┤
       │syslog(), vsyslog()   │ Thread safety │ MT-Safe env locale │
       └──────────────────────┴───────────────┴────────────────────┘

CONFORMING TO

       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

       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

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

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.