HPX uses a sophisticated logging framework allowing to follow in detail what operations have been performed inside the HPX library in what sequence. This information proves to be very useful for diagnosing problems or just for improving the understanding what is happening in HPX as a consequence of invoking HPX API functionality.

Default Logging

Enabling default logging is a simple process. The detailed description in the remainder of this section explains different ways to customize the defaults. Default logging can be enabled by using one of the following:

Customizing Logging

Generally, logging can be customized either using environment variable settings or using by an ini configuration file. Logging is generated in several categories, each of which can be customized independently. All customizable configuration parameters have reasonable defaults, allowing to use logging without any additional configuration effort. The following table lists the available categories.

By default, all logging output is redirected to the console instance of an application, where it is collected and written to a file, one file for each logging category.

Each logging category can be customized at two levels, the parameters for each are stored in the ini configuration sections hpx.logging.CATEGORY and hpx.logging.console.CATEGORY (where 'CATEGORY' is the category shortcut as listed in the table above). The former influences logging at the source locality and the latter modifies the logging behaviour for each of the categories at the console instance of an application.

Levels

All HPX logging output have seven different logging levels. These levels can be set explicitly or through environmental variables in the main HPX ini file as shown below. The logging levels and their associated integral values are shown in the table below, ordered from most verbose to least verbose. By default, all HPX logs are set to 0, e.g. all logging output is disabled by default.

	Tip
	The easiest way to enable logging output is to set the environment variable corresponding to the logging category to an integral value as described in the table above. For instance, setting HPX_LOGLEVEL=5 will enable full logging output for the general category. Please note, that the syntax and means of setting environment variables varies between operating systems.

Configuration

Logs will be saved to destinations as configured by the user. By default, logging output is saved on the console instance of an application to hpx.<CATEGORY>.<PID>.log (where <CATEGORY> and <PID> are placeholders for the category shortcut and the OS process id). The output for the general logging category is saved to hpx.<PID>.log. The default settings for the general logging category are shown here (the syntax is described in the section The HPX INI File Format):

[hpx.logging]
level = ${HPX_LOGLEVEL:0}
destination = ${HPX_LOGDESTINATION:console}
format = ${HPX_LOGFORMAT:(T%locality%/%hpxthread%.%hpxphase%/%hpxcomponent%) P%parentloc%/%hpxparent%.%hpxparentphase% %time%($hh:$mm.$ss.$mili) [%idx%]|\\n}

The logging level is taken from the environment variable HPX_LOGLEVEL and defaults to zero, e.g. no logging. The default logging destination is read from the environment variable HPX_LOGDESTINATION. On any of the localities it defaults to console which redirects all generated logging output to the console instance of an application. The following table lists the possible destinations for any logging output. It is possible to specify more than one destination separated by whitespace.

The logging format is read from the environment variable HPX_LOGFORMAT and it defaults to a complex format description. This format consists of several placeholder fields (for instance %locality%) which will be replaced by concrete values when the logging output is generated. All other information is transferred verbatim to the output. The table below describes the available field placeholders. The separator character | separates the logging message prefix formatted as shown and the actual log message which will replace the separator.

	Note
	Not all of the field placeholder may be expanded for all generated logging output. If no value is available for a particular field it is replaced with a sequence of '-' characters.

Here is an example line from a logging output generated by one of the HPX examples (please note that this is generated on a single line, without line break):

(T00000000/0000000002d46f90.01/00000000009ebc10) P--------/0000000002d46f80.02 17:49.37.320 [000000000000004d]
    <info>  [RT] successfully created component {0000000100ff0001, 0000000000030002} of type: component_barrier[7(3)]

The default settings for the general logging category on the console is shown here:

[hpx.logging.console]
level = ${HPX_LOGLEVEL:$[hpx.logging.level]}
destination = ${HPX_CONSOLE_LOGDESTINATION:file(hpx.$[system.pid].log)}
format = ${HPX_CONSOLE_LOGFORMAT:|}

These settings define how the logging is customized once the logging output is received by the console instance of an application. The logging level is read from the environment variable HPX_LOGLEVEL (as set for the console instance of the application). The level defaults to the same values as the corresponding settings in the general logging configuration shown before. The destination on the console instance is set to be a file which name is generated based from its OS process id. Setting the environment variable HPX_CONSOLE_LOGDESTINATION allows customization of the naming scheme for the output file. The logging format is set to leave the original logging output unchanged, as received from one of the localities the application runs on.