Logging

The logging API provides four main parts (basics, filtering, threading & blackbox). The idea behind this logging system is not to be prescriptive but to provide a set of tools to help the developer achieve what they want quickly and easily.

Basic logging API.

Call qb_log() to generate a log message. Then to write the message somewhere meaningful call qb_log_ctl() to configure the targets.

Simplest possible use:

 main() {
        qb_log_init("simple-log", LOG_DAEMON, LOG_INFO);
        // ...
        qb_log(LOG_WARNING, "watch out");
        // ...
        qb_log_fini();
 }

Configuring log targets.

A log target can be syslog, stderr, the blackbox, stdout, or a text file. By default only syslog is enabled.

To enable a target do the following:

        qb_log_ctl(QB_LOG_BLACKBOX, QB_LOG_CONF_ENABLED, QB_TRUE);

syslog, stderr, the blackbox, and stdout are static (they don't need to be created, just enabled or disabled). However you can open multiple logfiles (QB_LOG_TARGET_MAX - QB_LOG_TARGET_STATIC_MAX). To do this, use the following code:

        mytarget = qb_log_file_open("/var/log/mylogfile");
        qb_log_ctl(mytarget, QB_LOG_CONF_ENABLED, QB_TRUE);

Once your targets are enabled/opened you can configure them as follows: Configure the size of blackbox

        qb_log_ctl(QB_LOG_BLACKBOX, QB_LOG_CONF_SIZE, 1024*10);

Make logging to file threaded:

        qb_log_ctl(mytarget, QB_LOG_CONF_THREADED, QB_TRUE);

To workaround your syslog daemon filtering all messages > LOG_INFO

        qb_log_ctl(QB_LOG_SYSLOG, QB_LOG_CONF_PRIORITY_BUMP,
                   LOG_INFO - LOG_DEBUG);

To ensure all logs to file targets are fsync'ed (default QB_FALSE)

        qb_log_ctl(mytarget, QB_LOG_CONF_FILE_SYNC, QB_TRUE);

Filtering messages.

To have more power over what log messages go to which target you can apply filters to the targets. What happens is the desired callsites have the correct bit set. Then when the log message is generated it gets sent to the targets based on which bit is set in the callsite's "target" bitmap. Messages can be filtered based on the:

1. filename + priority
2. function name + priority
3. format string + priority

So to make all logs from evil_function() go to stderr, do the following:

        qb_log_filter_ctl(QB_LOG_STDERR, QB_LOG_FILTER_ADD,
                          QB_LOG_FILTER_FUNCTION, "evil_function", LOG_TRACE);

So to make all logs from totem* (with a priority <= LOG_INFO) go to stderr, do the following:

        qb_log_filter_ctl(QB_LOG_STDERR, QB_LOG_FILTER_ADD,
                          QB_LOG_FILTER_FILE, "totem", LOG_INFO);

So to make all logs with the substring "ringbuffer" go to stderr, do the following:

        qb_log_filter_ctl(QB_LOG_STDERR, QB_LOG_FILTER_ADD,
                          QB_LOG_FILTER_FORMAT, "ringbuffer", LOG_TRACE);

Thread safe non-blocking logging.

Logging is only thread safe when threaded logging is in use. If you plan on logging from multiple threads, you must initialize libqb's logger thread and use qg_log_filter_ctl to set the QB_LOG_CONF_THREADED flag on all the logging targets in use.

To achieve non-blocking logging, so that any calls to write() or syslog() will not hold up your program, you can use threaded logging as well.Threaded logging use:

 main() {
        qb_log_init("simple-log", LOG_DAEMON, LOG_INFO);
        qb_log_ctl(QB_LOG_SYSLOG, QB_LOG_CONF_THREADED, QB_TRUE);
        // ...
        daemonize();
        // call this after you fork()
        qb_log_thread_start();
        // ...
        qb_log(LOG_WARNING, "watch out");
        // ...
        qb_log_fini();
 }

A blackbox for in-field diagnosis.

This stores log messages in a ringbuffer so they can be written to file if the program crashes (you will need to catch SIGSEGV). These can then be easily printed out later.

Note:

the blackbox is not enabled by default.

Blackbox usage:

 static void sigsegv_handler(int sig)
 {
        (void)signal (SIGSEGV, SIG_DFL);
        qb_log_blackbox_write_to_file("simple-log.fdata");
        qb_log_fini();
        raise(SIGSEGV);
 }

 main() {

        signal(SIGSEGV, sigsegv_handler);

        qb_log_init("simple-log", LOG_DAEMON, LOG_INFO);
        qb_log_filter_ctl(QB_LOG_BLACKBOX, QB_LOG_FILTER_ADD,
                          QB_LOG_FILTER_FILE, "*", LOG_DEBUG);
        qb_log_ctl(QB_LOG_BLACKBOX, QB_LOG_CONF_SIZE, 1024*10);
        qb_log_ctl(QB_LOG_BLACKBOX, QB_LOG_CONF_ENABLED, QB_TRUE);
        // ...
        qb_log(LOG_WARNING, "watch out");
        // ...
        qb_log_fini();
 }

Tagging messages.

You can tag messages using the second argument to qb_logt() or by using qb_log_filter_ctl(). This can be used to add feature or sub-system information to the logs.

 const char* my_tags_stringify(uint32_t tags) {
        if (qb_bit_is_set(tags, QB_LOG_TAG_LIBQB_MSG_BIT) {
                return "libqb";
        } else if (tags == 3) {
                return "three";
        } else {
                return "MAIN";
        }
 }
 main() {
        // ...
        qb_log_tags_stringify_fn_set(my_tags_stringify);
        qb_log_format_set(QB_LOG_STDERR, "[%5g] %p %b");
        // ...
        qb_logt(LOG_INFO, 3, "hello");
        qb_logt(LOG_INFO, 0, "hello");
 }

The code above will produce:

 [libqb] some message
 [three] info hello
 [MAIN ] info hello

See also:

qblog.h