writing type of error in log file with syslog (linux)

Question

have a question about using syslog library in writing the error messages to the log file.

Below is the code function I am trying to use

syslog(int priority, const char_message)

and I want to print priority of massage in the log file. for example:

Mar 23 17:56:37 mypc slog[3597]: this is log massage "ERR"

but now it is only shows:

Mar 23 17:56:37 mypc slog[3597]: this is log massage

is there anyway i can write to the log file the type of priority too? (syslog accept only string literal)

You can try writing complete output log to file. Say you have executable file name prog, then it will be ./prog > log.txt — svtag
– svtag, Commented Mar 23, 2017 at 17:52
@Sma You can try writing complete output log to file. Say you have executable file name prog, then it will be ./prog > log.txt Suppose prog is a long-running, critical production process that can't be stopped or restarted, and log.txt fills up the file system? Logging by redirecting stdout/stderr is a BAD IDEA as it ties a process to a specific file - that file can't be deleted, nor can it reliably be truncated to release space. — Andrew Henle
– Andrew Henle, Commented Mar 23, 2017 at 19:26

Ian Abbott · Accepted Answer · 2017-03-23 18:22:19Z

The function prototype of syslog is:

void syslog(int priority, const char *format, ...);

In particular, the second parameter is similar to a printf-style format specifier except that is also supports the specifier %m which will be replaced with an error message generated by strerror(errno).

You could log a simple string along with a priority string by calling this function:

void my_simple_syslog(int priority, const char *message) { static const char * const prio_strings[] = { [LOG_EMERG] = "EMERG", [LOG_ALERT] = "ALERT", [LOG_CRIT] = "CRIT", [LOG_ERR] = "ERR", [LOG_WARNING] = "WARNING", [LOG_NOTICE] = "NOTICE", [LOG_INFO] = "INFO", [LOG_DEBUG] = "DEBUG", }; if (priority < 0 || priority >= sizeof(prio_strings) / sizeof(prio_strings[0]) || !prio_strings[priority]) { /* priority is an unknown value */ syslog(priority, "%s [PRIORITY:%d]", message, priority); } else { syslog(priority, "%s [%s]", message, prio_strings[priority]); } }

This example call:

my_simple_syslog(LOG_ERR, "this is log massage");

produces a log message similar to:

Mar 23 17:56:37 mypc slog[3597]: this is log massage [ERR]

The downside with using this canned approach is that you cannot add extra parameters like you could with calling syslog directly.

To add extra parameters you can do something like my_simple_syslog(int priority, const char *fmt, ...) where fmt is a standard printf-style format string, then use something like vsnprintf() to build a single message string, to which you can add derived data such as [ERR] or pass to syslog() as a parameter for a %s format specifier.

Community · Accepted Answer · 2017-05-23 12:09:39Z

Note that the syslog message already contains the priority but it is stripped by the syslog daemon (e.g. rsyslog) when it is written into the file. The syslog message that is passed by the syslog() call to the syslog daemon actually looks like this:

<16>Mar 23 17:56:37 mypc slog[3597]: this is log massage

It is possible to reconfigure the syslog daemon so that the priority is also printed. Rsyslog uses templates and with NXLog you could simply do this:

Exec $raw_event = $raw_evnt + " " + $SyslogSeverity;

See also https://stackoverflow.com/a/9216977/995934

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