QBLOG.H(3) libqb Programmer's Manual QBLOG.H(3)

qblog.h - The logging API provides four main parts (basics, filtering, threading & blackbox).

#include <qb/qblog.h>

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. While this is usual for daemons, it is rarely appropriate for ordinary programs, which should disable it when other targets (see below) are to be used:


To enable a target do the following:


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 (falling within inclusive range QB_LOG_TARGET_DYNAMIC_START up to QB_LOG_TARGET_DYNAMIC_END). 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);

Sometimes, syslog daemons are (pre)configured to filter messages not exceeding a particular priority. When this happens to be the logging target, the designated priority of the message is passed along unchanged, possibly resulting in message loss. For messages up to LOG_DEBUG importance, this can be worked around by proportionally bumping the priorities to be passed to syslog (here, the step is such that LOG_DEBUG gets promoted to LOG_INFO):



To ensure all logs to file targets are fsync'ed (new messages expressly transferred to the storage device as they keep coming, otherwise defaults to 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:

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,


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,


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,


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 qb_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);
// ...
// call this after you fork()
// ...
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.

Blackbox usage:

static void sigsegv_handler(int sig)

(void)signal (SIGSEGV, SIG_DFL);
raise(SIGSEGV); } main() {
signal(SIGSEGV, sigsegv_handler);
qb_log_init("simple-log", LOG_DAEMON, LOG_INFO);
qb_log_ctl(QB_LOG_BLACKBOX, QB_LOG_CONF_SIZE, 1024*10);
// ...
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_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

the blackbox is not enabled by default.

qb_log_filter_fn_set(3), qb_log_from_external_source_va2(3), qb_log_thread_start(3), qb_log_target_user_data_get(3), qb_log_tags_stringify_fn_set(3), qb_log_file_reopen(3), qb_log_fini(3), qb_log_callsites_dump(3), qb_log_ctl2(3), qb_log_target_user_data_set(3), qb_log_target_format(3), qb_log_thread_priority_set(3), qb_log_facility2int(3), qb_log_callsites_register(3), qb_log_filter_ctl2(3), qb_log_file_close(3), qb_log_format_set(3), qb_log_real_(3), qb_log_from_external_source_va(3), qb_log_callsite_get(3), qb_log_blackbox_write_to_file(3), qb_log_real_va_(3), qb_log_from_external_source(3), qb_log_blackbox_print_from_file(3), qb_log_facility2str(3), qb_log_callsite_get2(3), qb_log_ctl(3), qb_log_filter_ctl(3), qb_log_custom_close(3), qb_log_init(3), qb_log_file_open(3), qb_log_custom_open(3)

Copyright (c) 2017 Red Hat, Inc.

2021-03-03 LIBQB