13.3 syslog Function
Since a daemon does not have a controlling
terminal, it cannot just fprintf to stderr. The
common technique for logging messages from a daemon is to call the
syslog function.
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#include <syslog.h>
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void syslog(int priority, const char *message, ... );
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Although this function was originally developed
for BSD systems, it is provided by virtually all Unix vendors
today. The description of syslog in the POSIX
specification is consistent with what we describe here. RFC 3164
provides documentation of the BSD syslog protocol.
The priority
argument is a combination of a level and a facility, which we show in Figures 13.1 and 13.2. Additional detail on the priority may be found in RFC 3164. The
message is like a format string to
printf, with the addition of a %m specification,
which is replaced with the error message corresponding to the
current value of errno. A newline can appear at the end of
the message, but is not
mandatory.
Log messages have a level between 0 and 7, which we show in
Figure 13.1. These are
ordered values. If no level is
specified by the sender, LOG_NOTICE is the default.
Log messages also contain a facility to identify the type of process
sending the message. We show the different values in Figure 13.2. If no facility is specified, LOG_USER is
the default.
For example, the following call could be issued
by a daemon when a call to the rename function
unexpectedly fails:
syslog(LOG_INFO|LOG_LOCAL2, "rename(%s, %s): %m", file1, file2);
The purpose of facility and level is to allow all messages from a given
facility to be handled the same in the /etc/syslog.conf
file, or to allow all messages of a given level to be handled the
same. For example, the configuration file could contain the
lines
kern.* /dev/console
local7.debug /var/log/cisco.log
to specify that all kernel messages get logged
to the console and all debug messages from the
local7 facility get appended to the file
/var/log/cisco.log.
When the application calls syslog the
first time, it creates a Unix domain datagram socket and then calls
connect to the well-known pathname of the socket created
by the syslogd daemon (e.g., /var/run/log). This
socket remains open until the process terminates. Alternately, the
process can call openlog and closelog.
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#include <syslog.h>
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void openlog(const char *ident, int options, int facility);
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void closelog(void);
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openlog can be called before the first
call to syslog and closelog can be called when
the application is finished sending log messages.
ident is a
string that will be prepended to each log message by
syslog. Often this is the program name.
The options
argument is formed as the logical OR of one or more of the
constants in Figure
13.3.
Normally the Unix domain socket is not created
when openlog is called. Instead, it is opened during the
first call to syslog. The LOG_NDELAY option
causes the socket to be created when openlog is
called.
The facility
argument to openlog specifies a default facility for any
subsequent calls to syslog that do not specify a facility.
Some daemons call openlog and specify the facility (which
normally does not change for a given daemon). They then specify
only the level in each call to
syslog (since the level
can change depending on the error).
Log messages can also be generated by the
logger command. This can be used from within shell
scripts, for example, to send messages to syslogd.
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