Originální popis anglicky:
closelog, openlog, syslog - send messages to the system logger
Návod, kniha: Linux Programmer's Manual
#include <syslog.h>
void openlog(const char *ident, int option, int
facility);
void syslog(int priority, const char *format,
...);
void closelog(void);
#include <stdarg.h>
void vsyslog(int priority, const char *format,
va_list ap);
closelog() closes the descriptor being used to write to the system
logger. The use of
closelog() is optional.
openlog() opens a connection to the system logger for a program. The
string pointed to by
ident is prepended to every message, and is
typically set to the program name. The
option argument specifies flags
which control the operation of
openlog() and subsequent calls to
syslog(). The
facility argument establishes a default to be used
if none is specified in subsequent calls to
syslog(). Values for
option and
facility are given below. The use of
openlog()
is optional; it will automatically be called by
syslog() if necessary,
in which case
ident will default to NULL.
syslog() generates a log message, which will be distributed by
syslogd(8). The
priority argument is formed by ORing the
facility and the
level values (explained below). The remaining
arguments are a
format, as in
printf(3) and any arguments
required by the
format, except that the two character sequence %m will
be replaced by the error message string
strerror(
errno). A
trailing newline is added when needed.
The function
vsyslog() performs the same task as
syslog() with the
difference that it takes a set of arguments which have been obtained using the
stdarg(3) variable argument list macros.
This section lists the parameters used to set the values of
option,
facility, and
priority.
The
option argument to
openlog() is an OR of any of these:
- LOG_CONS
- Write directly to system console if there is an error while
sending to system logger.
- LOG_NDELAY
- Open the connection immediately (normally, the connection
is opened when the first message is logged).
- LOG_NOWAIT
- Don't wait for child processes that may have been created
while logging the message. (The GNU C library does not create a child
process, so this option has no effect on Linux.)
- LOG_ODELAY
- The converse of LOG_NDELAY; opening of the
connection is delayed until syslog() is called. (This is the
default, and need not be specified.)
- LOG_PERROR
- (Not in SUSv3.) Print to stderr as well.
- LOG_PID
- Include PID with each message.
The
facility argument is used to specify what type of program is logging
the message. This lets the configuration file specify that messages from
different facilities will be handled differently.
- LOG_AUTH
- security/authorization messages (DEPRECATED Use
LOG_AUTHPRIV instead)
- LOG_AUTHPRIV
- security/authorization messages (private)
- LOG_CRON
- clock daemon (cron and at)
- LOG_DAEMON
- system daemons without separate facility value
- LOG_FTP
- ftp daemon
- LOG_KERN
- kernel messages
- LOG_LOCAL0 through LOG_LOCAL7
- reserved for local use
- LOG_LPR
- line printer subsystem
- LOG_MAIL
- mail subsystem
- LOG_NEWS
- USENET news subsystem
- LOG_SYSLOG
- messages generated internally by syslogd
- LOG_USER (default)
- generic user-level messages
- LOG_UUCP
- UUCP subsystem
This determines the importance of the message. The levels are, in order of
decreasing importance:
- LOG_EMERG
- system is unusable
- LOG_ALERT
- action must be taken immediately
- LOG_CRIT
- critical conditions
- LOG_ERR
- error conditions
- LOG_WARNING
- warning conditions
- LOG_NOTICE
- normal, but significant, condition
- LOG_INFO
- informational message
- LOG_DEBUG
- debug-level message
The function
setlogmask(3) can be used to restrict logging to specified
levels only.
The functions
openlog(),
closelog(), and
syslog() (but not
vsyslog()) are specified in SUSv2 and POSIX 1003.1-2001. POSIX
1003.1-2001 specifies only the
LOG_USER and
LOG_LOCAL* values
for
facility. However, with the exception of
LOG_AUTHPRIV and
LOG_FTP, the other
facility values appear on most Unix systems.
The
LOG_PERROR value for
option is not specified by POSIX
1003.1-2001, but is available in most versions of Unix.
A
syslog function call appeared in BSD 4.2. BSD 4.3 documents
openlog(),
syslog(),
closelog(), and
setlogmask().
4.3BSD-Reno also documents
vsyslog(). Of course early v* functions used
the
<varargs.h> mechanism, which is not compatible with
<stdarg.h>.
The parameter
ident in the call of
openlog() is probably stored
as-is. Thus, if the string it points to is changed,
syslog() may start
prepending the changed string, and if the string it points to ceases to exist,
the results are undefined. Most portable is to use a string constant.
Never pass a string with user-supplied data as a format, use
syslog(priority, "%s",
string);
instead.
logger(1),
setlogmask(3),
syslog.conf(5),
syslogd(8)
