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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 "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 .