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.
closelog, openlog, setlogmask, syslog — control system log
void openlog(const char *ident, int logopt, int facility);
int setlogmask(int maskpri);
void syslog(int priority, const char *message, ... /* arguments */);
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_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.
The setlogmask() function shall return the previous log priority mask.
The closelog(), openlog(), and syslog() functions shall not return a
No errors are defined.
The following sections are informative.
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.
char *ident = "Process demo";
int logopt = LOG_PID | LOG_CONS;
int facility = LOG_USER;
openlog(ident, logopt, facility);
The following example causes subsequent calls to syslog() to accept
error messages, and to reject all other messages.
int mask = LOG_MASK (LOG_ERR);
result = setlogmask(mask);
The following example sends the message "Thisisamessage" to the default
logging facility, marking the message as an error message generated by
char *message = "This is a message";
int priority = LOG_ERR | LOG_USER;
The Base Definitions volume of POSIX.1‐2008, <syslog.h>
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
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IEEE/The Open Group 2013 CLOSELOG(3P)