Syslog Howto

8 July 2011

1 min read

Syslog is a standard for logging program messages. It allows separation of the software that generates messages from the system that stores them.

Format

The traditional use of a syslog daemon is to store events in log files, in a format like this:

Jan 1 12:12:12 10.245.3.99 foo[421]: this is a message from foo

All the log files are in the /var/log/ directory. You can explore the files on your own machine an see the log messages for yourself.

Writing your own messages

Now, how can you log something with syslog? There is a command-line utility suited just for the job — logger . For instance:

$ logger -t "dude" "What's up New York?"

will add something like

Jul 8 10:45:53 astro-desktop dude: What's up New York? into `/var/log/messages` .

Logger is most useful for bash scripts, but while in C, you'd probably like to use something else. So let's have look at some syscalls. Here is a very simple program, that will do the same as the logger command shown a couple lines back.

/* Example of using syslog. */
#include <syslog.h>

int main()
{
    openlog("dude", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_USER);
    syslog(LOG_NOTICE, "What's up %s?", "New York\0");
    closelog();
}

Function openlog() will establish a connection to the logging system. As soon as it's ready, you can start using syslog() to save messages and in the end you'll need to close the connection by closelog(). Fairly simple 🎉.

There are some parameters to the functions which are described in man syslog or somewhere on the interwebz.

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