How to pass application printf messages to /var/log/messages

Question

I want to pass application printf log messages to the /var/log/messages. Because kernel debug messages can be visible to /var/log/messages.But i am not getting how to pass application printf log messages to the /var/log/messages. Can anyone please suggest me how to do this.

Answer 1

Change from, for example:

printf("The error was %s\n", strerror(errno));

to:

syslog(LOG_ERR, "The error was %s", strerror(errno));

Note that syslog is always sent a complete line at a time (and no need for \n).

For more control over your logging options use the openlog() function before any calls to syslog(). See the openlog man page for more details.

Syslog routes messages based on the message priority -- the first argument to syslog (eg LOG_ERR in the sample above) -- and the facility. The facility either set in a call to openlog() or it defaults to LOG_USER. The basic syntax of the syslog configuration file is lines of the form:

selector[;selector]     destination

where selector is:

facility[,facility].priority

(facility and/or priority can be the wildcard *). A priority implicitly includes all higher priorities.

The destination can be a file, a remotehost, a program or a (list of) users. Some examples:

*.*                   ihateyou             -- every message sent to this user
*.debug               *                    -- to every logged in user!
*.emerg               root,bob,tom         -- emergencies to these three
*.err                 /var/log/all-errors  -- all LOG_ERR and above to this file
cron.info             |/some/program       -- pipe these to /some/program
user.*                @some.host.com       -- send these to this host
cron,user.crit        /this/file           -- an example with multiple facilities
lpr.err;ftp.warn      /other/file          -- an example with multiple selectors

There may be some additional options, (eg, "none" as priority, a specific priority like =warn, and negation like !=warn), see your syslog.conf manpage for details on those.

See your syslog configuration file (usually /etc/syslog.conf) for how your system is routing its syslog messages. Note: some systems run a variant of syslog like nsyslog or rsyslog -- these have more options and hence a potentially more complex configuration file.

The known facilities are: LOG_AUTH, LOG_AUTHPRIV, LOG_CRON, LOG_DAEMON, LOG_FTP, LOG_KERN, LOCAL_LOCAL0 .. LOG_LOCAL7, LOG_LPR, LOG_MAIL, LOG_NEWS, LOG_SYSLOG, LOG_USER, LOG_UUCP. (LOG_KERN is not available to user processes) Generally, one of LOG_DAEMON, LOG_LOCALn or LOG_USER is usually the best choice.

The known priorities (also called severities) are (highest to lowest): LOG_EMERG, LOG_ALERT, LOG_CRIT, LOG_ERR, LOG_WARNING, LOG_NOTICE, LOG_INFO, LOG_DEBUG.