Using qi::log

Introduction

Header: <qi/log.hpp>

Overview

Log are hierarchical with ”.” as separator. Each level of hierarchy is called a category.

For example, for an instant messaging service, categories may be:

im.audio
im.audio.input
im.audio.output
im.audio.internal
im.video
im.video.internal
im.http

This allows easy log filtering in external log tools.

There are 7 levels of log:

silent:Hide logs. fatal:Used before the program exit. error:A classical error. warning:Useful to warn user. info:Useful to user information. verbose:Not mandatory but useful to user information. debug:Useful to developer. Not compile on release.

Writing logs

There is a log function for each level except silent. Those functions are basic streams.

std::string msg = "Message to print with number: ";
qiLogFatal("ca.te.go.ry")   << msg << 1;
qiLogError("ca.te.go.ry")   << msg << 2;
qiLogWarning("ca.te.go.ry") << msg << 3;
qiLogInfo("ca.te.go.ry")    << msg << 4;
qiLogVerbose("ca.te.go.ry") << msg << 5;
qiLogDebug("ca.te.go.ry")   << msg << 6;

Note

Do not add enlines manually, qi::log automatically adds them at the end of each message.

To avoid typing category each time (and so make mistakes), there is a scope function to set it globally: qiLogCategory(const char*);

Note

It is always allowed to set a specific category for a message even if a global category is set.

qiLogCategory("ca.te.go.ry")

qiLogFatal()   << "foo";
qiLogError()   << "foo";
qiLogWarning() << "foo";
qiLogInfo()    << "foo";
qiLogVerbose() << "foo";
qiLogDebug()   << "foo";

Warning

If log category is set globally in an include file, all files including this file will have this category as global category.

(A)synchronous logging

Log are asynchronous, we don’t want to waste time in log, furthermore log can do networking. The handling of the log output is in a separated thread.

There is a way to disable asynchronous. Start naoqi with –synchronous-log command line option. Be careful, it will slow down naoqi if you have lots of handlers.

Logs may be asynchronous, but they are actually being displayed in their request order.

Add/Remove Log Handlers

The default handler log to console. The color is enable on tty. The handler can be added or deleted. You just need to give a delegate to a log function with the following prototype:

void logfct(const qi::LogLevel verb,
            const qi::Clock::time_point date,
            const qi::SystemClock::time_point systemDate,
            const char *category,
            const char *msg,
            const char *file = "",
            const char *fct = "",
            const int line = 0);

Then you can add the handler with addHandler(name, fctLog)

:cpp:`addHandler("nameofloghandler", logfct);`

and remove it with removeHandler(name).

removeHandler("nameofloghandler");

Filtering logs output

It is possible to change the output of log in 3 different ways:

• category,
• level,
• context.

Level

level via QI_LOG_LEVEL environment variable.

If QI_LOG_LEVEL is set to "fatal" only fatal logs are displayed.

if QI_LOG_LEVEL is set to "info", fatal, error, warning and info logs are displayed.

Note

By default, level is set to info.

It is possible to use corresponding number instead of name of levels.

0. Silent
1. Fatal
2. Error
3. Warning
4. Info
5. Verbose
6. Debug

Context

context are the meta information collected during the creation of log.

These information are:

• Level,
• Date,
• ThreadId,
• Category,
• File,
• Function.

To set context use QI_LOG_CONTEXT, which is a bit-field.

1:Level as complete string 2:Level as a single letter 4:Date of emission 8:ThreadId 16:Category 32:File name and line number 64:Function name 128:End of line

Useful values of contexts:

26:Short level + threadId + category 30:Short level + threadId + date + category 126:Short level + threadId + date + category + file + function 254:Short level + threadId + date + category + file + function + eol

Category filtering

category can be filtered via QI_LOG_FILTERS environment variable.

- is used to remove a category, + to add it, : is the separator.

QI_LOG_FILTERS="-im.audio:+im.video"

QI_LOG_FILTERS support globbing.

QI_LOG_FILTERS="-im*:+im.video*"

It is possible via QI_LOG_FILTERS to set different level of verbosity for each category via =.

# set level of verbosity to warning for im* except for im.video to debug.
QI_LOG_FILTERS="+im*=3:+im.video=6"

Warning

Due to this feature, use QI_LOG_LEVEL with QI_LOG_FILTERS may be hazardous.