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.
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
severity-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
subsequent 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
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 otherwise 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
current process to maskpri and return the previous mask. If the
maskpri argument is 0, the current log mask is not modified. Calls by
the current 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
Symbolic constants for use as values of the logopt, facility,
priority, and maskpri arguments are defined in the <syslog.h> header.
The following example causes subsequent calls to syslog() to log the
process ID with each message, and to write messages to the system
console 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 random processes.
char *message = "This is a message";
int priority = LOG_ERR | LOG_USER;
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 Electrical 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
IEEE/The Open Group 2013 CLOSELOG(3P)