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

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(): _BSD_SOURCE

DESCRIPTION

closelog() closes the descriptor being used to write to the system logger. The use of closelog() is
optional.

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(). Values for option and facility are given 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() generates a log message, which will be distributed by syslogd(8). The priority argument is
formed by ORing the facility and the level values (explained below). 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). A trailing newline may be added if
needed.

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.

The subsections below list the parameters used to set the values of option, facility, and priority.

option
The option argument to openlog() is an OR of any of these:

LOG_CONS Write directly to system console if there is an error while sending to system logger.

LOG_NDELAY Open the connection immediately (normally, the connection is opened when the first message
is logged).

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.) Print to stderr as well.

LOG_PID Include PID with each message.

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

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).

       ┌──────────────────────┬───────────────┬────────────────────┐
       │Interface             │ Attribute     │ Value              │
       ├──────────────────────┼───────────────┼────────────────────┤
       │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);

COLOPHON

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