Provided by: libqb-dev_2.0.6-1_amd64 
NAME
qblog.h - The logging API provides four main parts (basics, filtering, threading &
blackbox).
SYNOPSIS
#include <qb/qblog.h>
DESCRIPTION
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:
qb_log_ctl(B_LOG_SYSLOG, QB_LOG_CONF_ENABLED, QB_FALSE);
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 (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):
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 (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,
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 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);
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.
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
NOTE
the blackbox is not enabled by default.
SEE ALSO
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
Copyright (c) 2017 Red Hat, Inc.