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 sys-
logd(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 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()
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-
cesses)
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).
+----------------------+---------------+--------------------+
|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 fol-
lowing instead:
syslog(priority, "%s", string);
SEE ALSO
journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
COLOPHON
This page is part of release 4.15 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/.
Linux 2017-09-15 SYSLOG(3)
CLOSELOG(3P) POSIX Programmer's Manual CLOSELOG(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
closelog, openlog, setlogmask, syslog -- control system log
SYNOPSIS
#include <syslog.h>
void closelog(void);
void openlog(const char *ident, int logopt, int facility);
int setlogmask(int maskpri);
void syslog(int priority, const char *message, ... /* arguments */);
DESCRIPTION
The syslog() function shall send a message to an implementation-defined
logging facility, which may log it in an implementation-defined system
log, write it to the system console, forward it to a list of users, or
forward it to the logging facility on another host over the network.
The logged message shall include a message header and a message body.
The message header contains at least a timestamp and a tag string.
The message body is generated from the message and following arguments
in the same manner as if these were arguments to printf(), except that
the additional conversion specification %m shall be recognized; it
shall convert no arguments, shall cause the output of the error message
string associated with the value of errno on entry to syslog(), and may
be mixed with argument specifications of the "%n$" form. If a complete
conversion specification with the m conversion specifier character is
not just %m, the behavior is undefined. A trailing <newline> may be
added if needed.
Values of the priority argument are formed by OR'ing together a sever-
ity-level value and an optional facility value. If no facility value is
specified, the current default facility value is used.
Possible values of severity level include:
LOG_EMERG A panic condition.
LOG_ALERT A condition that should be corrected immediately, such as a
corrupted system database.
LOG_CRIT Critical conditions, such as hard device errors.
LOG_ERR Errors.
LOG_WARNING
Warning messages.
LOG_NOTICE Conditions that are not error conditions, but that may
require special handling.
LOG_INFO Informational messages.
LOG_DEBUG Messages that contain information normally of use only when
debugging a program.
The facility indicates the application or system component generating
the message. Possible facility values include:
LOG_USER Messages generated by arbitrary processes. This is the
default facility identifier if none is specified.
LOG_LOCAL0 Reserved for local use.
LOG_LOCAL1 Reserved for local use.
LOG_LOCAL2 Reserved for local use.
LOG_LOCAL3 Reserved for local use.
LOG_LOCAL4 Reserved for local use.
LOG_LOCAL5 Reserved for local use.
LOG_LOCAL6 Reserved for local use.
LOG_LOCAL7 Reserved for local use.
The openlog() function shall set process attributes that affect subse-
quent calls to syslog(). The ident argument is a string that is
prepended to every message. The logopt argument indicates logging
options. Values for logopt are constructed by a bitwise-inclusive OR of
zero or more of the following:
LOG_PID Log the process ID with each message. This is useful for
identifying specific processes.
LOG_CONS Write messages to the system console if they cannot be sent
to the logging facility. The syslog() function ensures that
the process does not acquire the console as a controlling
terminal in the process of writing the message.
LOG_NDELAY Open the connection to the logging facility immediately.
Normally the open is delayed until the first message is
logged. This is useful for programs that need to manage the
order in which file descriptors are allocated.
LOG_ODELAY Delay open until syslog() is called.
LOG_NOWAIT Do not wait for child processes that may have been created
during the course of logging the message. This option
should be used by processes that enable notification of
child termination using SIGCHLD, since syslog() may other-
wise block waiting for a child whose exit status has
already been collected.
The facility argument encodes a default facility to be assigned to all
messages that do not have an explicit facility already encoded. The
initial default facility is LOG_USER.
The openlog() and syslog() functions may allocate a file descriptor. It
is not necessary to call openlog() prior to calling syslog().
The closelog() function shall close any open file descriptors allocated
by previous calls to openlog() or syslog().
The setlogmask() function shall set the log priority mask for the cur-
rent process to maskpri and return the previous mask. If the maskpri
argument is 0, the current log mask is not modified. Calls by the cur-
rent process to syslog() with a priority not set in maskpri shall be
rejected. The default log mask allows all priorities to be logged. A
call to openlog() is not required prior to calling setlogmask().
Symbolic constants for use as values of the logopt, facility, priority,
and maskpri arguments are defined in the <syslog.h> header.
RETURN VALUE
The setlogmask() function shall return the previous log priority mask.
The closelog(), openlog(), and syslog() functions shall not return a
value.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Using openlog()
The following example causes subsequent calls to syslog() to log the
process ID with each message, and to write messages to the system con-
sole if they cannot be sent to the logging facility.
#include <syslog.h>
char *ident = "Process demo";
int logopt = LOG_PID | LOG_CONS;
int facility = LOG_USER;
...
openlog(ident, logopt, facility);
Using setlogmask()
The following example causes subsequent calls to syslog() to accept
error messages, and to reject all other messages.
#include <syslog.h>
int result;
int mask = LOG_MASK (LOG_ERR);
...
result = setlogmask(mask);
Using syslog
The following example sends the message "Thisisamessage" to the default
logging facility, marking the message as an error message generated by
random processes.
#include <syslog.h>
char *message = "This is a message";
int priority = LOG_ERR | LOG_USER;
...
syslog(priority, message);
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
fprintf()
The Base Definitions volume of POSIX.1-2008, <syslog.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 CLOSELOG(3P)
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