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

6       closelog, openlog, syslog, vsyslog - send messages to the system logger
7

SYNOPSIS

9       #include <syslog.h>
10
11       void openlog(const char *ident, int option, int facility);
12       void syslog(int priority, const char *format, ...);
13       void closelog(void);
14
15       void vsyslog(int priority, const char *format, va_list ap);
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       vsyslog():
20           Since glibc 2.19:
21               _DEFAULT_SOURCE
22           Glibc 2.19 and earlier:
23               _BSD_SOURCE
24

DESCRIPTION

26   openlog()
27       openlog() opens a connection to the system logger for a program.
28
29       The  string  pointed  to by ident is prepended to every message, and is
30       typically set to the program name.  If ident is NULL, the program  name
31       is  used.   (POSIX.1-2008  does  not specify the behavior when ident is
32       NULL.)
33
34       The option argument specifies flags  which  control  the  operation  of
35       openlog()  and subsequent calls to syslog().  The facility argument es‐
36       tablishes a default to be used if none is specified in subsequent calls
37       to  syslog().  The values that may be specified for option and facility
38       are described below.
39
40       The use of openlog() is optional; it will automatically  be  called  by
41       syslog() if necessary, in which case ident will default to NULL.
42
43   syslog() and vsyslog()
44       syslog()  generates  a  log  message, which will be distributed by sys‐
45       logd(8).
46
47       The priority argument is formed by ORing together a facility value  and
48       a  level  value  (described  below).  If no facility value is ORed into
49       priority, then the default value set by openlog() is used, or, if there
50       was no preceding openlog() call, a default of LOG_USER is employed.
51
52       The  remaining  arguments  are a format, as in printf(3), and any argu‐
53       ments required by the format, except that the two-character sequence %m
54       will be replaced by the error message string strerror(errno).  The for‐
55       mat string need not include a terminating newline character.
56
57       The function vsyslog() performs the same task as syslog() with the dif‐
58       ference that it takes a set of arguments which have been obtained using
59       the stdarg(3) variable argument list macros.
60
61   closelog()
62       closelog() closes the file descriptor being used to write to the system
63       logger.  The use of closelog() is optional.
64
65   Values for option
66       The option argument to openlog() is a bit mask constructed by ORing to‐
67       gether any of the following values:
68
69       LOG_CONS       Write directly to the system console if there is an  er‐
70                      ror while sending to the system logger.
71
72       LOG_NDELAY     Open  the  connection immediately (normally, the connec‐
73                      tion is opened when the first message is logged).   This
74                      may  be  useful,  for example, if a subsequent chroot(2)
75                      would make the pathname used internally by  the  logging
76                      facility unreachable.
77
78       LOG_NOWAIT     Don't  wait  for child processes that may have been cre‐
79                      ated while logging the message.  (The GNU C library does
80                      not create a child process, so this option has no effect
81                      on Linux.)
82
83       LOG_ODELAY     The converse of LOG_NDELAY; opening of the connection is
84                      delayed until syslog() is called.  (This is the default,
85                      and need not be specified.)
86
87       LOG_PERROR     (Not in POSIX.1-2001 or  POSIX.1-2008.)   Also  log  the
88                      message to stderr.
89
90       LOG_PID        Include the caller's PID with each message.
91
92   Values for facility
93       The  facility  argument is used to specify what type of program is log‐
94       ging the message.  This lets the configuration file specify  that  mes‐
95       sages from different facilities will be handled differently.
96
97       LOG_AUTH       security/authorization messages
98
99       LOG_AUTHPRIV   security/authorization messages (private)
100
101       LOG_CRON       clock daemon (cron and at)
102
103       LOG_DAEMON     system daemons without separate facility value
104
105       LOG_FTP        ftp daemon
106
107       LOG_KERN       kernel messages (these can't be generated from user pro‐
108                      cesses)
109
110       LOG_LOCAL0 through LOG_LOCAL7
111                      reserved for local use
112
113       LOG_LPR        line printer subsystem
114
115       LOG_MAIL       mail subsystem
116
117       LOG_NEWS       USENET news subsystem
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119       LOG_SYSLOG     messages generated internally by syslogd(8)
120
121       LOG_USER (default)
122                      generic user-level messages
123
124       LOG_UUCP       UUCP subsystem
125
126   Values for level
127       This determines the importance of the message.  The levels are, in  or‐
128       der of decreasing importance:
129
130       LOG_EMERG      system is unusable
131
132       LOG_ALERT      action must be taken immediately
133
134       LOG_CRIT       critical conditions
135
136       LOG_ERR        error conditions
137
138       LOG_WARNING    warning conditions
139
140       LOG_NOTICE     normal, but significant, condition
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142       LOG_INFO       informational message
143
144       LOG_DEBUG      debug-level message
145
146       The function setlogmask(3) can be used to restrict logging to specified
147       levels only.
148

ATTRIBUTES

150       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
151       tributes(7).
152
153       ┌─────────────────────────────────┬───────────────┬────────────────────┐
154Interface                        Attribute     Value              
155       ├─────────────────────────────────┼───────────────┼────────────────────┤
156openlog(), closelog()            │ Thread safety │ MT-Safe            │
157       ├─────────────────────────────────┼───────────────┼────────────────────┤
158syslog(), vsyslog()              │ Thread safety │ MT-Safe env locale │
159       └─────────────────────────────────┴───────────────┴────────────────────┘
160

CONFORMING TO

162       The  functions  openlog(), closelog(), and syslog() (but not vsyslog())
163       are specified in SUSv2, POSIX.1-2001, and POSIX.1-2008.
164
165       POSIX.1-2001 specifies only the LOG_USER and LOG_LOCAL* values for  fa‐
166       cility.   However,  with the exception of LOG_AUTHPRIV and LOG_FTP, the
167       other facility values appear on most UNIX systems.
168
169       The LOG_PERROR value for option is not  specified  by  POSIX.1-2001  or
170       POSIX.1-2008, but is available in most versions of UNIX.
171

NOTES

173       The  argument  ident in the call of openlog() is probably stored as-is.
174       Thus, if the string  it  points  to  is  changed,  syslog()  may  start
175       prepending the changed string, and if the string it points to ceases to
176       exist, the results are undefined.  Most portable is  to  use  a  string
177       constant.
178
179       Never  pass  a string with user-supplied data as a format, use the fol‐
180       lowing instead:
181
182           syslog(priority, "%s", string);
183

SEE ALSO

185       journalctl(1), logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
186

COLOPHON

188       This page is part of release 5.13 of the Linux  man-pages  project.   A
189       description  of  the project, information about reporting bugs, and the
190       latest    version    of    this    page,    can     be     found     at
191       https://www.kernel.org/doc/man-pages/.
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195Linux                             2021-03-22                         SYSLOG(3)
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