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

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 de‐
       fault 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) vari‐
       able 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 sub‐
                      sequent 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  fa‐
       cilities 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).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                  β”‚ Attribute     β”‚ Value              β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚openlog(), closelog()                                                                                                      β”‚ Thread safety β”‚ MT-Safe            β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚syslog(), vsyslog()                                                                                                        β”‚ Thread safety β”‚ MT-Safe env locale β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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)

Linux                                                                          2021-03-22                                                                      SYSLOG(3)