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
closelog() closes the file 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
 | Thread safety | MT-Safe
 | 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
logger(1), setlogmask(3), syslog.conf(5), syslogd(8)

COLOPHON
This page is part of release 4.05 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 https://www.kernel.org/doc/man-pages/.