syslog(3)

1SYSLOG(3)                  Linux Programmer's Manual                 SYSLOG(3)
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NAME

6       closelog, openlog, syslog - send messages to the system logger
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SYNOPSIS

9       #include <syslog.h>
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11       void openlog(const char *ident, int option, int facility);
12       void syslog(int priority, const char *format, ...);
13       void closelog(void);
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15       #define _BSD_SOURCE
16       #include <stdarg.h>
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18       void vsyslog(int priority, const char *format, va_list ap);
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DESCRIPTION

21       closelog() closes the descriptor being used to write to the system log‐
22       ger.  The use of closelog() is optional.
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24       openlog() opens a connection to the system logger for a  program.   The
25       string  pointed to by ident is prepended to every message, and is typi‐
26       cally set to the program name.  The  option  argument  specifies  flags
27       which  control  the operation of openlog() and subsequent calls to sys‐
28       log().  The facility argument establishes a default to be used if  none
29       is  specified  in  subsequent calls to syslog().  Values for option and
30       facility are given below.  The use of openlog() is  optional;  it  will
31       automatically  be  called by syslog() if necessary, in which case ident
32       will default to NULL.
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34       syslog() generates a log message, which will  be  distributed  by  sys‐
35       logd(8).  The priority argument is formed by ORing the facility and the
36       level values (explained below).  The remaining arguments are a  format,
37       as  in  printf(3) and any arguments required by the format, except that
38       the two character sequence %m will be replaced  by  the  error  message
39       string strerror(errno).  A trailing newline may be added if needed.
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41       The function vsyslog() performs the same task as syslog() with the dif‐
42       ference that it takes a set of arguments which have been obtained using
43       the stdarg(3) variable argument list macros.
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PARAMETERS

46       This  section  lists  the  parameters used to set the values of option,
47       facility, and priority.
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49   option
50       The option argument to openlog() is an OR of any of these:
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52       LOG_CONS
53              Write directly to system console if  there  is  an  error  while
54              sending to system logger.
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56       LOG_NDELAY
57              Open  the  connection  immediately  (normally, the connection is
58              opened when the first message is logged).
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60       LOG_NOWAIT
61              Don't wait for child processes that may have been created  while
62              logging the message.  (The GNU C library does not create a child
63              process, so this option has no effect on Linux.)
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65       LOG_ODELAY
66              The converse of LOG_NDELAY; opening of the connection is delayed
67              until syslog() is called.  (This is the default, and need not be
68              specified.)
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70       LOG_PERROR
71              (Not in POSIX.1-2001.) Print to stderr as well.
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73       LOG_PID
74              Include PID with each message.
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76   facility
77       The facility argument is used to specify what type of program  is  log‐
78       ging  the  message.  This lets the configuration file specify that mes‐
79       sages from different facilities will be handled differently.
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81       LOG_AUTH
82              security/authorization  messages  (DEPRECATED  Use  LOG_AUTHPRIV
83              instead)
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85       LOG_AUTHPRIV
86              security/authorization messages (private)
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88       LOG_CRON
89              clock daemon (cron and at)
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91       LOG_DAEMON
92              system daemons without separate facility value
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94       LOG_FTP
95              ftp daemon
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97       LOG_KERN
98              kernel messages
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100       LOG_LOCAL0 through LOG_LOCAL7
101              reserved for local use
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103       LOG_LPR
104              line printer subsystem
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106       LOG_MAIL
107              mail subsystem
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109       LOG_NEWS
110              USENET news subsystem
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112       LOG_SYSLOG
113              messages generated internally by syslogd
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115       LOG_USER (default)
116              generic user-level messages
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118       LOG_UUCP
119              UUCP subsystem
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121
122   level
123       This  determines  the  importance  of  the message.  The levels are, in
124       order of decreasing importance:
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126       LOG_EMERG
127              system is unusable
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129       LOG_ALERT
130              action must be taken immediately
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132       LOG_CRIT
133              critical conditions
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135       LOG_ERR
136              error conditions
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138       LOG_WARNING
139              warning conditions
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141       LOG_NOTICE
142              normal, but significant, condition
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144       LOG_INFO
145              informational message
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147       LOG_DEBUG
148              debug-level message
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150       The function setlogmask(3) can be used to restrict logging to specified
151       levels only.
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CONFORMING TO

154       The  functions  openlog(), closelog(), and syslog() (but not vsyslog())
155       are specified in SUSv2 and POSIX.1-2001.  POSIX.1-2001  specifies  only
156       the  LOG_USER  and  LOG_LOCAL*  values for facility.  However, with the
157       exception of LOG_AUTHPRIV and LOG_FTP, the other facility values appear
158       on most Unix systems.  The LOG_PERROR value for option is not specified
159       by POSIX.1-2001, but is available in most versions of Unix.
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HISTORY

162       A syslog() function call appeared in 4.2BSD.   4.3BSD  documents  open‐
163       log(),  syslog(), closelog(), and setlogmask().  4.3BSD-Reno also docu‐
164       ments vsyslog().  Of course early v*  functions  used  the  <varargs.h>
165       mechanism, which is not compatible with <stdarg.h>.
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NOTES

168       The  parameter ident in the call of openlog() is probably stored as-is.
169       Thus, if the string  it  points  to  is  changed,  syslog()  may  start
170       prepending the changed string, and if the string it points to ceases to
171       exist, the results are undefined.  Most portable is  to  use  a  string
172       constant.
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174       Never pass a string with user-supplied data as a format, use
175              syslog(priority, "%s", string);
176       instead.
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