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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
     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  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
     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  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
     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
     allbox; lbw21 lb lb l l  l.   Interface Attribute Value  T{  openlog(),
     closelog()   T}   Thread   safety  MT-Safe   T{   syslog(),   vsyslog()
     T}   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
     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
     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)


     This  page  is  part of release 4.16 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

Linux 2017-09-15 SYSLOG(3)

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