syslog(3) — Linux manual page


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

NAME         top

       closelog, openlog, syslog, vsyslog - send messages to the system log‐

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)):

           Since glibc 2.19:
           Glibc 2.19 and earlier:

DESCRIPTION         top

       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

       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

       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() 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

       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

       │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 5.09 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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