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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.1‐2008, <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 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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .