= Babeltrace logging guide
Philippe Proulx
31 May 2017
:toc:
:toclevels: 5

This guide explains to the Babeltrace developer how to insert logging
statements in Babeltrace's CLI, library, and plugins.


== Motive

Logging is a great instrument for a developer to be able to collect
information about a running software.

Babeltrace is a complex software with many layers. When a Babeltrace
graph fails to run, what caused the failure? It could be caused by any
component, any notification iterator, and any deeply nested validation
of a CTR IR object, for example. With the appropriate logging statements
manually placed in the source code, we can find the cause of a bug
faster.

While <<level,care must be taken>> when placing _INFO_ to _FATAL_
logging statements, you should deliberately instrument your Babeltrace
module with _DEBUG_ and _VERBOSE_ logging statements to help future
you and other developers understand what's happening at run-time.


== API

The Babeltrace logging API is internal: it is not exposed to the users
of the library, only to their developers. The only thing that a library
user can control is the current log level of the library itself with
`bt_logging_set_global_level()` and the initial library's log level with
the `BABELTRACE_LOGGING_GLOBAL_LEVEL` environment variable.

This API is based on https://github.com/wonder-mice/zf_log[zf_log], a
lightweight, yet featureful, MIT-licensed core logging library for C and
$$C++$$. The zf_log source files were modified to have the `BT_` and
`bt_` prefixes, and other small changes.

The logging functions are implemented in the logging convenience
library (`logging` directory).


=== Headers

The logging API headers are:

`<babeltrace/logging.h>`::
    Public header which a library user can use to control and read
    libbabeltrace's current log level.

`<babeltrace/logging-internal.h>`::
    Internal, generic logging API which you can use in any Babeltrace
    subproject. This is the translation of `zf_log.h`.

`<babeltrace/lib-logging-internal.h>`::
    Specific internal header to use within the library. This header
    defines `BT_LOG_OUTPUT_LEVEL` to a custom, library-wide hidden
    symbol which is the library's current log level before including
    `<babeltrace/logging-internal.h>`.

Do not include `<babeltrace/logging-internal.h>` or
`<babeltrace/lib-logging-internal.h>` in a header which contains logging
statements: this header could be included in source files which define a
different <<tag,tag>>, for example.



=== Log levels ===

The API offers the following log levels:

* _VERBOSE_
* _DEBUG_
* _INFO_
* _WARN_
* _ERROR_
* _FATAL_

See <<level,how to decide which one to use>> below.

There are two important log level variables:

[[build-time-log-level]]Build-time, minimal log level::
    The minimal log level, or build-time log level, is set at build
    time and determines the minimal log level which can be executed.
    This applies to all the subprojects and modules (CLI, library,
    plugins, etc.).
+
All the logging statements with a level below this level are **not built
at all**. All the logging statements with a level equal to or greater
than this level _can_ be executed, depending on the run-time log level
(see below).
+
You can set this level at configuration time with the
`BABELTRACE_MINIMAL_LOG_LEVEL` environment variable, for example:
+
--
----
$ BABELTRACE_MINIMAL_LOG_LEVEL=WARN ./configure
----
--
+
The default build-time log level is `DEBUG`. For optimal performance,
set it to `NONE`, which effectively disables all logging in all the
Babeltrace subprojects.
+
The library's public API provides `bt_logging_get_minimal_level()` to
get the configured minimal log level.

[[run-time-log-level]]Run-time, dynamic log level::
    The dynamic log level is set at run-time and determines the current,
    active log level. All the logging statements with a level below this
    level are not executed, but they evaluate the condition. All the
    logging statements with a level equal to or greater than this level
    are executed, provided that their level is also enabled at build
    time (see above).
+
In `zf_log`, there is a concept of a global run-time log level which
uses the `_bt_log_global_output_lvl` symbol. In practice, we never use
this symbol, and always make sure that `BT_LOG_OUTPUT_LEVEL` is defined
to a module-wise or subproject-wise hidden symbol before including
`<babeltrace/logging-internal.h>`. In the library,
`<babeltrace/lib-logging-internal.h>` does this job: just include
this header which defines `BT_LOG_OUTPUT_LEVEL` to the appropriate
symbol before it includes `<babeltrace/logging-internal.h>`. In plugins,
for example, there is one log level per component class, which makes
log filtering easier during execution.
+
In libbabeltrace, the user can set the current run-time log level with
the `bt_logging_set_global_level()` function, for example:
+
--
[source,c]
----
bt_logging_set_global_level(BT_LOGGING_LEVEL_INFO);
----
--
+
The library's initial run-time log level is defined by the
`BABELTRACE_LOGGING_GLOBAL_LEVEL` environment variable (`VERBOSE`, `DEBUG`,
`INFO`, `WARN`, `ERROR`, `FATAL`, or `NONE`), or set to _NONE_ if this
environment variable is undefined.
+
Other subprojects have their own way of setting their run-time log
level. For example, the CLI uses the `BABELTRACE_CLI_LOG_LEVEL`
environment variable, and the `text.pretty` sink component class
initializes its log level thanks to the
`BABELTRACE_PLUGIN_TEXT_PRETTY_SINK_LOG_LEVEL` environment variable
(also _NONE_ by default).
+
Make sure that there is a documented way to initialize or modify the
log level of your subproject or module, and that it's set to _NONE_
by default.


[[logging-statements]]
=== Logging statement macros

The Babeltrace logging statement macros work just like `printf()` and
contain their log level in their name:

`BT_LOGV("format string", ...)`::
    Standard verbose logging statement.

`BT_LOGD("format string", ...)`::
    Standard debug logging statement.

`BT_LOGI("format string", ...)`::
    Standard info logging statement.

`BT_LOGW("format string", ...)`::
    Standard warning logging statement.

`BT_LOGE("format string", ...)`::
    Standard error logging statement.

`BT_LOGF("format string", ...)`::
    Standard fatal logging statement.

`BT_LOGV_MEM(data_ptr, data_size, "format string", ...)`::
    Memory verbose logging statement.

`BT_LOGD_MEM(data_ptr, data_size, "format string", ...)`::
    Memory debug logging statement.

`BT_LOGI_MEM(data_ptr, data_size, "format string", ...)`::
    Memory info logging statement.

`BT_LOGW_MEM(data_ptr, data_size, "format string", ...)`::
    Memory warning logging statement.

`BT_LOGE_MEM(data_ptr, data_size, "format string", ...)`::
    Memory error logging statement.

`BT_LOGF_MEM(data_ptr, data_size, "format string", ...)`::
    Memory fatal logging statement.

`BT_LOGV_STR("preformatted string")`::
    Preformatted string verbose logging statement.

`BT_LOGD_STR("preformatted string")`::
    Preformatted string debug logging statement.

`BT_LOGI_STR("preformatted string")`::
    Preformatted string info logging statement.

`BT_LOGW_STR("preformatted string")`::
    Preformatted string warning logging statement.

`BT_LOGE_STR("preformatted string")`::
    Preformatted string error logging statement.

`BT_LOGF_STR("preformatted string")`::
    Preformatted string fatal logging statement.


=== Conditional logging

`BT_LOG_IF(cond, statement)`::
    Execute `statement` only if `cond` is true.
+
Example:
+
--
[source,c]
----
BT_LOG_IF(i < count / 2, BT_LOGD("Log this: i=%d", i));
----
--

To check the <<build-time-log-level,build-time log level>>:

[source,c]
----
#if BT_LOG_ENABLED_DEBUG
...
#endif
----

This tests if the _DEBUG_ level was enabled at build-time. This
means that the current, dynamic log level _could_ be _DEBUG_, but it
could also be higher. The rule of thumb is to use only logging
statements at the same level in a `BT_LOG_ENABLED_*` conditional block.

The available definitions for build-time conditions are:

* `BT_LOG_ENABLED_VERBOSE`
* `BT_LOG_ENABLED_DEBUG`
* `BT_LOG_ENABLED_INFO`
* `BT_LOG_ENABLED_WARN`
* `BT_LOG_ENABLED_ERROR`
* `BT_LOG_ENABLED_FATAL`

To check the current, <<run-time-log-level,run-time log level>>:

[source,c]
----
if (BT_LOG_ON_DEBUG) {
    ...
}
----

This tests if the _DEBUG_ log level is dynamically turned on
(implies that it's also enabled at build-time). This check could have a
noticeable impact on performance.

The available definitions for run-time conditions are:

* `BT_LOG_ON_VERBOSE`
* `BT_LOG_ON_DEBUG`
* `BT_LOG_ON_INFO`
* `BT_LOG_ON_WARN`
* `BT_LOG_ON_ERROR`
* `BT_LOG_ON_FATAL`

Those macros check the subproject-specific or module-specific log level
symbol (defined by `BT_LOG_OUTPUT_LEVEL`).

Never, ever write code which would be executed only to compute the
fields of a logging statement outside a conditional logging scope,
for example:

[source,c]
----
int number = get_number_of_event_classes_with_property_x(...);
BT_LOGD("Bla bla: number=%d", number);
----

Do this instead:

[source,c]
----
if (BT_LOG_ON_DEBUG) {
    int number = get_number_of_event_classes_with_property_x(...);
    BT_LOGD("Bla bla: number=%d", number);
}
----

Or even this:

[source,c]
----
BT_LOGD("Bla bla: number=%d", get_number_of_event_classes_with_property_x(...));
----


[[how-to]]
=== Instrument a module

Follow those steps to make your module loggable:

. In your module's root directory, create a `logging.c` file with
  this content:
+
--
[source,c]
----
/*
 * Copyright (c) 2017 EfficiOS Inc. <http://efficios.com/>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#define BT_LOG_OUTPUT_LEVEL my_module_log_level
#include <babeltrace/logging-internal.h>

BT_LOG_INIT_LOG_LEVEL(my_module_log_level, "BABELTRACE_MY_MODULE_LOG_LEVEL");
----
--
+
Replace `my_module_log_level` with the name of the symbol which holds
the log level for your module (should be unique amongst all the log level
symbols of the project). Replace `BABELTRACE_MY_MODULE_LOG_LEVEL` with
the name of the environment variable from which to initialize your
module's log level at construction time. `BT_LOG_INIT_LOG_LEVEL()`
defines both the hidden log level symbol and a constructor which reads
the environment variable and sets the log level symbol accordingly.

. In your module's root directory, create a `logging.h` file with
  this content:
+
--
[source,c]
----
#ifndef MY_MODULE_LOGGING_H
#define MY_MODULE_LOGGING_H

/*
 * Copyright (c) 2017 EfficiOS Inc. <http://efficios.com/>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#define BT_LOG_OUTPUT_LEVEL my_module_log_level
#include <babeltrace/logging-internal.h>

BT_LOG_LEVEL_EXTERN_SYMBOL(my_module_log_level);

#endif /* MY_MODULE_LOGGING_H */
----
--
+
Again, replace `my_module_log_level` with the name of your module's
log level symbol.

. Include the `logging.c` and `logging.h` in the `_SOURCES` variable
  of your module's object in your module's root `Makefile.am`.

To instrument a C source file (`.c`):

. At the top of the file, before the first `#include` line (if any),
  define your file's tag name and include the local `logging.h`:
+
--
[source,c]
----
#define BT_LOG_TAG "MY-MODULE-MY-FILE"
#include "logging.h"
----
--
+
A logging tag name is a namespace for a specific source file. Its name
is part of every log message generated by the statements of the file. A
logging tag name _must_ be only uppercase letters, digits, and the
hyphen (`-`) character. See <<tags,Babeltrace standard tags>> for
a list of standard tags.

. Use the <<logging-statements,logging statement macros>> in the file's
  functions to instrument it.

To instrument a C header file (`.h`), if you have `static inline`
functions in it:

. Do not include any logging header.
. Use the <<logging-statements,logging statement macros>> in the file's
  functions to instrument it, making each of them conditional to the
  existence of the macro you're using:
+
--
[source,c]
----
static inline
void some_function(void)
{
    /* ... */

#ifdef BT_LOGV
    BT_LOGV(...);
#endif

    /* ... */

#ifdef BT_LOGW_STR
    BT_LOGW_STR(...);
#endif

    /* ... */
}
----
--
+
The C source files which include this header file determine if logging
is enabled or not for them (if the source file is instrumented itself),
and the tag of the included header's logging statement is the same as
the C source file.


[[tags]]
=== Babeltrace standard tags

==== CTF IR (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Attributes |`ATTRS`
|Clock class and values |`CLOCK-CLASS`
|Event class |`EVENT-CLASS`
|Event |`EVENT`
|Field path |`FIELD-PATH`
|Field types |`FIELD-TYPES`
|Fields |`FIELDS`
|Packet |`PACKET`
|Resolver |`RESOLVE`
|Stream class |`STREAM-CLASS`
|Stream |`STREAM`
|Trace |`TRACE`
|Validation |`VALIDATION`
|Visitor |`VISITOR`
|===


==== CTF writer (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Clock |`CTF-WRITER-CLOCK`
|CTF writer |`CTF-WRITER`
|Serialization |`CTF-WRITER-SER`
|===


==== Graph (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Clock class priority map |`CC-PRIO-MAP`
|Component (common) |`COMP`
|Component class |`COMP-CLASS`
|Connection |`CONNECTION`
|Filter component |`COMP-FILTER`
|Graph |`GRAPH`
|Notification iterator |`NOTIF-ITER`
|Port |`PORT`
|Sink component |`COMP-SINK`
|Source component |`COMP-SOURCE`
|===

==== Notifications (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Event notification |`NOTIF-EVENT`
|Inacitivity notification |`NOTIF-INACTIVITY`
|Notification |`NOTIF`
|Packet notification |`NOTIF-PACKET`
|Stream notification |`NOTIF-STREAM`
|===


==== Plugin (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Plugin |`PLUGIN`
|Python plugin provider |`PLUGIN-PY`
|Shared object plugin provider |`PLUGIN-SO`
|===


==== Values (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Values |`VALUES`
|===


==== Reference counting (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Reference counting |`REF`
|===


==== Common (library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Common |`COMMON`
|===


==== CLI

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|CLI (main) |`CLI`
|CLI configuration (common) |`CLI-CFG`
|CLI configuration from CLI arguments |`CLI-CFG-ARGS`
|CLI connection configuration from CLI arguments |`CLI-CFG-ARGS-CONNECT`
|===


==== libctfcopytrace (plugin convenience library)

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Clock fields |`LIBCTFCOPYTRACE-CLOCK-FIELDS`
|libctfcopytrace |`LIBCTFCOPYTRACE`
|===


==== `ctf` plugin

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Plugin (main) |`PLUGIN-CTF`
|Common: BTR |`PLUGIN-CTF-BTR`
|Common: CTF IR generation metadata visitor |`PLUGIN-CTF-METADATA-IR-VISITOR`
|Common: Metadata decoder |`PLUGIN-CTF-METADATA-DECODER`
|Common: Metadata lexer |`PLUGIN-CTF-METADATA-LEXER`
|Common: Metadata parser |`PLUGIN-CTF-METADATA-PARSER`
|Common: Notification iterator |`PLUGIN-CTF-NOTIF-ITER`
|`fs` sink (main) |`PLUGIN-CTF-FS-SINK`
|`fs` sink: write |`PLUGIN-CTF-FS-SINK-WRITE`
|`fs` source (main) |`PLUGIN-CTF-FS-SRC`
|`fs` source: data stream |`PLUGIN-CTF-FS-SRC-DS`
|`fs` source: file |`PLUGIN-CTF-FS-SRC-FILE`
|`fs` source: metadata |`PLUGIN-CTF-FS-SRC-METADATA`
|`lttng-live` source (main) |`PLUGIN-CTF-LTTNG-LIVE-SRC`
|`lttng-live` source: data stream |`PLUGIN-CTF-LTTNG-LIVE-SRC-DS`
|`lttng-live` source: metadata |`PLUGIN-CTF-LTTNG-LIVE-SRC-METADATA`
|`lttng-live` source: viewer connection |`PLUGIN-CTF-LTTNG-LIVE-SRC-VIEWER`
|===


==== `lttng-utils` plugin

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Plugin (main) |`PLUGIN-LTTNG-UTILS`
|`debug-info` filter (main) |`PLUGIN-LTTNG-UTILS-DBG-INFO-FILT`
|`debug-info` filter: binary info |`PLUGIN-LTTNG-UTILS-DBG-INFO-FILT-BIN-INFO`
|`debug-info` filter: copy |`PLUGIN-LTTNG-UTILS-DBG-INFO-FILT-COPY`
|`debug-info` filter: CRC32 |`PLUGIN-LTTNG-UTILS-DBG-INFO-FILT-CRC32`
|`debug-info` filter: DWARF |`PLUGIN-LTTNG-UTILS-DBG-INFO-FILT-DWARF`
|===


==== `text` plugin

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Plugin (main) |`PLUGIN-TEXT`
|`pretty` filter (main) |`PLUGIN-TEXT-PRETTY-SINK`
|`pretty` filter: print |`PLUGIN-TEXT-PRETTY-SINK-PRINT`
|===


==== `utils` plugin

[options="header,autowidth"]
|===
|Subsystem/object |Tag name

|Plugin (main) |`PLUGIN-UTILS`
|`dummy` sink (main) |`PLUGIN-UTILS-DUMMY-SINK`
|`muxer` filter (main) |`PLUGIN-UTILS-MUXER-FILT`
|`trimmer` filter (main) |`PLUGIN-UTILS-TRIMMER-FILT`
|`trimmer` filter: copy |`PLUGIN-UTILS-TRIMMER-FILT-COPY`
|`trimmer` filter: iterator |`PLUGIN-UTILS-TRIMMER-FILT-ITER`
|===


[[level]]
== Choose a log level

Choosing the appropriate level for your logging statement is very
important.

[options="header,autowidth",cols="default,default,asciidoc,default"]
|===
|Log level |Description |Use cases |Impact on performance

|_FATAL_
|The program, library, or plugin cannot continue to work in this
condition: it must be terminated immediately.

A _FATAL_-level logging statement should always be followed by
`abort()`.
|
* Unexpected return values from system calls.
* Logic error in internal code, for example an unexpected value in a
  `switch` statement.
|Almost none: should be executed in production.

|_ERROR_
|An important error which is somewhat not fatal, that is, the program,
library, or plugin can continue to work after this, but you judge that
it should be reported to the user.

Usually, the program cannot recover from such an error, but it can at
least exit cleanly.
|
* Memory allocation errors.
* Failed to perform an operation which should work considering the
  implementation and the satisfied preconditions. For example, the
  failure to create an empty object (no parameters): most probably
  failed internally because of an allocation error.
|Almost none: should be executed in production.

|_WARN_
|A logic error which still allows the execution to continue.

_WARN_-level logging statements are for any error or weird action that
is directly or indirectly caused by the user. For example, not having
enough memory is considered beyond the user's control, so we always
log memory errors with an _ERROR_ level (not _FATAL_ because we usually
don't abort in this condition).
|
* Not honoring a public function's preconditions (NULL parameters,
  index out of bounds, etc.).
* Adding an invalid event class to a stream class which is already
  part of a trace.
+
For example, the caller tries to set a property of a frozen stream
class.
|Almost none: can be executed in production.

|_INFO_
|Any useful information which a non-developer user would understand.
|
* Successful loading of a plugin (with name, version, etc.).
* Successful connection to or disconnection from another system.
* An _optional_ subsystem cannot be loaded.
|Very little: can be executed in production if
_INFO_ level information is desired.

|_DEBUG_
|Something that only Babeltrace developers would be interested into.
|
* High-level function entry/exit.
* Object creation, destruction, copying, and freezing.
* The result of some computation/validation.
|Noticeable, but not as much as the _VERBOSE_ level: not executed in
production.

|_VERBOSE_
|Low-level debugging context information. More appropriate for tracing
in general.
|
* Reference count change.
* Status of each iteration of a loop.
* State machine's state change.
* Data structure lookup/modification.
* List of ELF sections found in a plugin.
* Get or set an object's property.
* Object comparison's intermediate results.
|Huge: not executed in production.
|===

Make sure not to use a _WARN_ (or higher) log level when the condition
leading to the logging statement can occur under normal circumstances.
For example, a public function to get some object or property from an
object by name or key that fails to find the value is not a warning: the
user could legitimately use this function to check if the name/key
exists in the object. In this case, use the _VERBOSE_ level (or do not
log at all). If a numeric index is out of bounds, however, this
qualifies for a _WARN_ level: such API functions have documented
preconditions that the index must be in bounds (the user can always
check with a count or size function).


[[message]]
== Write an appropriate message

Follow those rules when you write a logging statement's message:

* Use an english sentence which starts with a capital letter. Start the
  sentence with the appropriate verb tense depending on the context. For
  example:
+
--
** Beginning of operation (present continuous): _Creating ..._,
   _Copying ..._, _Serializing ..._, _Freezing ..._, _Destroying ..._
** End of operation (simple past): _Created ..._, _Successfully created ..._,
   _Failed to create ..._, _Set ..._ (simple past of _to set_ which is
   also _set_)
--
+
For warning and error messages, you can start the message with _Cannot_
or _Failed to_ followed by a verb if it's appropriate.

* Do not include the log level in the message itself. For example,
  do not start the message with _Error while_ or _Warning:_.

* Do not put newlines, tabs, or other special characters in the
  message, unless you want to log a string with such characters. Note
  that multiline log messages can be hard to parse, analyze, and filter,
  however, so prefer multiple `BT_LOG*()` statements over a single
  statement with newlines.

* **If there are fields that your logging statement must record**,
  follow the message with `:` followed by a space, then with the list of
  fields (more about this below). If there are no fields, end the
  sentence with a period.

The statement's fields _must_ be a comma-separated list of
+__name__=__value__+ tokens. Keep +__name__+ as simple as possible
(lowercase if possible). If +__value__+ is a non-alphanumeric string,
put it between double quotes. Always use the `PRId64` and `PRIu64`
specifiers when logging `int64_t` and `uint64_t` values.

Example:

    "Cannot add event class to stream class: stream-class-addr=%p, "
    "stream-class-name=\"%s\", stream-class-id=%" PRId64 ", "
    "event-class-addr=%p, event-class-name=\"%s\", event-class-id=%" PRId64

By following a standard format for the statement fields, it is easier
to use tools like https://www.elastic.co/products/logstash[Logstash]
to split fields and analyze logs.

Prefer the following suffixes in field names:

[options="header,autowidth"]
|===
|Field name suffix |Description |Format specifier

|`-addr` |Memory address |`%p`
|`-fd` |File descriptor |`%d`
|`-fp` |File stream (`FILE *`) |`%p`
|`-id` |Object's ID |`%" PRId64 "` or `%" PRIu64 "`
|`-name` |Object's name |`\"%s\"`
|===


== Output

The log is printed to the standard error stream. A log line contains the
time, the process and thread IDs, the <<level,log level>>, the tag name,
the source's function name, file name and line number, and the
<<message,message>>.

Example:

    05-11 00:58:03.691 23402 23402 D VALUES bt_value_destroy@values.c:498 Destroying value: addr=0xb9c3eb0

You can easily filter the log with `grep` or `ag`. For example, to
keep only the _WARN_-level log messages that the `VALUES` module
generates:

    $ export BABELTRACE_LOGGING_GLOBAL_LEVEL=VERBOSE
    $ ./test_ctf_writer_complete 2>&1 | ag 'W VALUES'