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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 *“This*is*a*message” 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 .

Author: dt

Created: 2022-02-20 Sun 09:20