Provided by: manpages-posix-dev_2017a-2_all 
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 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_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 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 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 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 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 console 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.1‐2017, <syslog.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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.kernel.org/doc/man-pages/reporting_bugs.html .