Babeltrace contributor’s guide

Table of Contents

Babeltrace library
- Object reference counting and lifetime
Logging
Valgrind
Testing

This is a partial contributor’s guide for the Babeltrace project. If you have any questions that are not answered by this guide, please post them on Babeltrace’s mailing list.

Babeltrace library

Object reference counting and lifetime

This section covers the rationale behind the design of Babeltrace’s object lifetime management. This applies to the Babeltrace library, as well as to the CTF writer library (although the public reference counting functions are not named the same way).

Starting from Babeltrace 2.0, all publicly exposed objects inherit a common base: bt_object. This base provides a number of facilities to all objects, chief amongst which are lifetime management functions.

The lifetime of some public objects is managed by reference counting. In this case, the API offers the bt_*_get_ref() and bt_*_put_ref() functions which respectively increment and decrement an object’s reference count.

As far as lifetime management in concerned, Babeltrace makes a clear distinction between regular objects, which have a single parent, and root objects, which don’t.

The problem

Let us consider a problematic case to illustrate the need for this distinction.

A user of the Babeltrace library creates a trace class, which has a stream class (the class of a stream) and that stream class, in turn, has an event class (the class of an event).

Nothing prevents this user from releasing his reference on any one of these objects in any order. However, all objects in the trace—stream class—event class hierarchy can be retrieved from any other.

For instance, the user could discard his reference on both the event class and the stream class, only keeping a reference on the trace class. From this trace class reference, stream classes can be enumerated, providing the user with a new reference to the stream class he discarded earlier. Event classes can also be enumerated from stream classes, providing the user with references to the individual event classes.

Conversely, the user could also hold a reference to an event class and retrieve its parent stream class. The trace class, in turn, can then be retrieved from the stream class.

This example illustrates what could be interpreted as a circular reference dependency existing between these objects. Of course, if the objects in such a scenario were to hold references to each other (in both directions), we would be in presence of a circular ownership resulting in a leak of both objects as their reference counts would never reach zero.

Nonetheless, the API must offer the guarantee that holding a node to any node of the graph keeps all other reachable nodes alive.

The solution

The scheme employed in Babeltrace to break this cycle consists in the "children" holding reverse component references to their parents. That is, in the context of the trace IR, that event classes hold a reference to their parent stream class and stream classes hold a reference to their parent trace class.

On the other hand, parents hold claiming aggregation references to their children. A claiming aggregation reference means that the object being referenced should not be deleted as long as the reference still exists. In this respect, it can be said that parents truly hold the ownership of their children, since they control their lifetime. Conversely, the reference counting mechanism is leveraged by children to notify parents that no other child indirectly exposes the parent.

When a parented object’s reference count reaches zero, it invokes bt_*_put_ref() on its parent and does not free itself. However, from that point, the object depends on its parent to signal the moment when it can be safely reclaimed.

The invocation of bt_*_put_ref() by the last children holding a reference to its parent might trigger a cascade of bt_*_put_ref() from child to parent. Eventually, a root object is reached. At that point, if this orphaned object’s reference count reaches zero, the object invokes the destructor method defined by everyone of its children as part of their base struct bt_object. The key point here is that the cascade of destructor will necessarily originate from the root and propagate in preorder to the children. These children will propagate the destruction to their own children before reclaiming their own memory. This ensures that a node’s pointer to its parent is always valid since the parent has the responsibility of tearing-down their children before cleaning themselves up.

Assuming a reference to an object is acquired by calling bt_*_get_ref() while its reference count is zero, the object acquires, in turn, a reference on its parent using bt_*_get_ref(). At that point, the child can be thought of as having converted its weak reference to its parent into a regular reference. That is why this reference is referred to as a claiming aggregation reference.

Caveats

This scheme imposes a number of strict rules defining the relation between objects:

Objects may only have one parent.
Objects, beside the root, are only retrievable from their direct parent or children.

Example

The initial situation is rather simple: User A is holding a reference to a trace class, TC1. As per the rules previously enounced, stream classes SC1 and SC2 don’t hold a reference to TC1 since their own reference counts are zero. The same holds true for EC1, EC2 and EC3 with respect to SC1 and SC2.

In this second step, we can see that User A has acquired a reference on SC2 through the trace class, TC1.

The stream class’s reference count transitions from zero to one, triggering the acquisition of a strong reference on TC1 from SC2.

Hence, at this point, the trace class’s ownership is shared by User A and SC2.

Next, User A acquires a reference on the EC3 event class through its parent stream class, SC2. Again, the transition of an object’s reference count from 0 to 1 triggers the acquisition of a reference on its parent.

Note that SC2’s reference count was incremented to 2. The trace class’s reference count remains unchanged.

User A decides to drop its reference on SC2. SC2's reference count returns back to 1, everything else remaining unchanged.

User A can then decide to drop its reference on the trace class. This results in a reversal of the initial situation: User A now owns an event, EC3, which is keeping everything else alive and reachable.

If another object, User B, enters the picture and acquires a reference on the SC1 stream class, we see that SC1's reference count transitioned from 0 to 1, triggering the acquisition of a reference on TC1.

User B hands off a reference to EC1, acquired through SC1, to another object, User C. The acquisition of a reference on EC1, which transitions from 0 to 1, triggers the acquisition of a reference on its parent, SC1.

At some point, User A releases its reference on EC3. Since EC3's reference count transitions to zero, it releases its reference on SC2. SC2's reference count, in turn, reaches zero and it releases its reference to TC1.

TC1's reference count is now 1 and no further action is taken.

User B releases its reference on SC1. User C becomes the sole owner of the whole hierarchy through his ownership of EC1.

Finally, User C releases his ownership of EC1, triggering the release of the whole hierarchy. Let’s walk through the reclamation of the whole graph.

Mirroring what happened when User A released its last reference on EC3, the release of EC1 by User C causes its reference count to fall to zero.

This transition to zero causes EC1 to release its reference on SC1. SC1's reference count reaching zero causes it to release its reference on TC1.

Since the reference count of TC1, a root object, has reached zero, it invokes the destructor method on its children. This method is recursive and causes the stream classes to call the destructor method on their event classes.

The event classes are reached and, having no children of their own, are reclaimed.

The stream classes having destroyed their children, are then reclaimed by the trace class.

Finally, the stream classes having been reclaimed, TC1 is reclaimed.

Logging

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 message iterator, and any deeply nested validation of a CTF IR object (within the ctf plugin), for example. With the appropriate logging statements manually placed in the source code, we can find the cause of a bug faster.

While care must be taken when placing DEBUG to FATAL logging statements, you should liberally instrument your Babeltrace module with TRACE logging statements to help future you and other developers understand what’s happening at run time.

Logging 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 LIBBABELTRACE2_INIT_LOG_LEVEL environment variable.

This API is based on 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, like color support and using the project’s BT_DEBUG_MODE definition instead of the standard NDEBUG.

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

Headers

The logging API headers are:

<babeltrace2/logging.h>: Public header which a library user can use to set and get libbabeltrace2’s current log level.
"logging/log.h": Internal, generic logging API which you can use in any Babeltrace module. This is the translation of zf_log.h.

This header offers the generic logging statement macros.
"lib/logging.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 "logging/log.h".

This header offers the library-specific logging statement macros.
"logging/comp-logging.h": Specific internal header to use within a component class.

This header offers the component-specific logging statement macros.

Log levels

The internal logging API offers the following log levels, in ascending order of severity:

Log level name	Log level short name	Internal API enumerator	Public API enumerator
TRACE	`T`	`BT_LOG_TRACE`	`BT_LOGGING_LEVEL_TRACE`
DEBUG	`D`	`BT_LOG_DEBUG`	`BT_LOGGING_LEVEL_DEBUG`
INFO	`I`	`BT_LOG_INFO`	`BT_LOGGING_LEVEL_INFO`
WARNING	`W`	`BT_LOG_WARNING`	`BT_LOGGING_LEVEL_WARNING`
ERROR	`E`	`BT_LOG_ERROR`	`BT_LOGGING_LEVEL_ERROR`
FATAL	`F`	`BT_LOG_FATAL`	`BT_LOGGING_LEVEL_FATAL`
NONE	`N`	`BT_LOG_NONE`	`BT_LOGGING_LEVEL_NONE`

The short name is accepted by the log level environment variables and by the CLI’s --log-level options.

See how to decide which one to use below.

There are two important log level expressions:

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 of the logging statements which can be executed. This applies to all the modules (CLI, library, plugins, bindings, 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.

You can set this level at configuration time with the BABELTRACE_MINIMAL_LOG_LEVEL environment variable, for example:

$ BABELTRACE_MINIMAL_LOG_LEVEL=INFO ./configure

The default build-time log level is DEBUG. For optimal performance, set it to INFO, which effectively disables all fast path logging in all the Babeltrace modules. You can’t set it to WARNING, ERROR, FATAL, or NONE because the impact on performance is minuscule starting from the INFO log level anyway and we want any Babeltrace build to always be able to print INFO-level logs.

The library’s public API provides bt_logging_get_minimal_level() to get the configured minimal 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 still 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.

zf_log has 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 expression before including "logging/log.h".

In the library, "lib/logging.h" defines its own BT_LOG_OUTPUT_LEVEL to the library’s log level symbol before it includes "logging/log.h" itself.

In libbabeltrace2, the user can set the current run-time log level with the bt_logging_set_global_level() function, for example:

bt_logging_set_global_level(BT_LOGGING_LEVEL_INFO);

The library’s initial run-time log level is defined by the LIBBABELTRACE2_INIT_LOG_LEVEL environment variable, or set to NONE if this environment variable is undefined.

Other modules have their own way of setting their run-time log level.

For example, the CLI uses the BABELTRACE_CLI_LOG_LEVEL environment variable, as well as its global --log-level option:

$ babeltrace2 --log-level=I ...

The components use their own log level (as returned by bt_component_get_logging_level()). With the CLI, you can set a specific component’s log level with its own, position-dependent --log-level option:

$ babeltrace2 /path/to/trace -c sink.ctf.fs --log-level=D

Code which is common to the whole project, for example src/common and src/compat, use function parameters to get its run-time log level, for example:

BT_HIDDEN
char *bt_common_get_home_plugin_path(int log_level);

Typically, when a logging-enabled module calls such a function, it passes its own log level expression directly (BT_LOG_OUTPUT_LEVEL):

path = bt_common_get_home_plugin_path(BT_LOG_OUTPUT_LEVEL);

Otherwise, just pass BT_LOG_NONE:

path = bt_common_get_home_plugin_path(BT_LOG_NONE);

Generic logging statement macros

The Babeltrace logging statement macros work just like printf() (except the BT_LOG*_STR() ones) and contain their log level (short name) in their name.

Each of the following macros evaluate the build-time log level definition and run-time log level expression (as defined by BT_LOG_OUTPUT_LEVEL) to log conditionally.

See Instrument a C source file (generic) and Instrument a C header file (generic) to learn how to be able to use the following macros.

BT_LOGT("format string", ...): Generic trace logging statement.
BT_LOGD("format string", ...): Generic debug logging statement.
BT_LOGI("format string", ...): Generic info logging statement.
BT_LOGW("format string", ...): Generic warning logging statement.
BT_LOGE("format string", ...): Generic error logging statement.
BT_LOGF("format string", ...): Generic fatal logging statement.
BT_LOGT_STR("preformatted string"): Generic preformatted string trace logging statement.
BT_LOGD_STR("preformatted string"): Generic preformatted string debug logging statement.
BT_LOGI_STR("preformatted string"): Generic preformatted string info logging statement.
BT_LOGW_STR("preformatted string"): Generic preformatted string warning logging statement.
BT_LOGE_STR("preformatted string"): Generic preformatted string error logging statement.
BT_LOGF_STR("preformatted string"): Generic preformatted string fatal logging statement.
BT_LOGT_MEM(data_ptr, data_size, "format string", ...): Generic memory trace logging statement.
BT_LOGD_MEM(data_ptr, data_size, "format string", ...): Generic memory debug logging statement.
BT_LOGI_MEM(data_ptr, data_size, "format string", ...): Generic memory info logging statement.
BT_LOGW_MEM(data_ptr, data_size, "format string", ...): Generic memory warning logging statement.
BT_LOGE_MEM(data_ptr, data_size, "format string", ...): Generic memory error logging statement.
BT_LOGF_MEM(data_ptr, data_size, "format string", ...): Generic memory fatal logging statement.
BT_LOGT_ERRNO("initial message", "format string", ...): Generic errno string trace logging statement.
BT_LOGD_ERRNO("initial message", "format string", ...): Generic errno string debug logging statement.
BT_LOGI_ERRNO("initial message", "format string", ...): Generic errno string info logging statement.
BT_LOGW_ERRNO("initial message", "format string", ...): Generic errno string warning logging statement.
BT_LOGE_ERRNO("initial message", "format string", ...): Generic errno string error logging statement.
BT_LOGF_ERRNO("initial message", "format string", ...): Generic errno string fatal logging statement.

Library-specific logging statement macros

The Babeltrace library contains an internal logging API based on the generic logging framework. You can use it to log known Babeltrace objects without having to manually log each member.

See Instrument a library C source file and Instrument a library C header file to learn how to be able to use the following macros.

The library logging statement macros are named BT_LIB_LOG*() instead of BT_LOG*():

BT_LIB_LOGT("format string", ...): Library trace logging statement.
BT_LIB_LOGD("format string", ...): Library debug logging statement.
BT_LIB_LOGI("format string", ...): Library info logging statement.
BT_LIB_LOGW("format string", ...): Library warning logging statement.
BT_LIB_LOGE("format string", ...): Library error logging statement.
BT_LIB_LOGF("format string", ...): Library fatal logging statement.
BT_LIB_LOGW_APPEND_CAUSE("format string", ...): Library warning logging statement, and unconditional error cause appending.
BT_LIB_LOGE_APPEND_CAUSE("format string", ...): Library error logging statement, and unconditional error cause appending.
BT_LIB_LOGF_APPEND_CAUSE("format string", ...): Library fatal logging statement, and unconditional error cause appending.

The macros above accept the typical printf() conversion specifiers with the following limitations:

The * width specifier is not accepted.
The * precision specifier is not accepted.
The j and t length modifiers are not accepted.
The n format specifier is not accepted.
The format specifiers defined in <inttypes.h> are not accepted, except for PRId64, PRIu64, PRIx64, PRIX64, PRIo64, and PRIi64.

The Babeltrace library custom conversion specifier is accepted. Its syntax is either %!u to format a UUID (bt_uuid type), or:

Introductory %! sequence.
Optional: [ followed by a custom prefix for the printed fields of this specifier, followed by ]. The standard form is to end this prefix with - so that, for example, with the prefix tc-, the complete field name becomes tc-addr.
Optional: + to print extended object members. This depends on the provided format specifier.
Format specifier (see below).

The available format specifiers are:

Specifier	Object	Expected C type
`F`	Trace IR field class	`const struct bt_field_class *`
`f`	Trace IR field	`const struct bt_field *`
`P`	Trace IR field path	`const struct bt_field_path *`
`E`	Trace IR event class	`const struct bt_event_class *`
`e`	Trace IR event	`const struct bt_event *`
`S`	Trace IR stream class.	`const struct bt_stream_class *`
`s`	Trace IR stream	`const struct bt_stream *`
`a`	Trace IR packet	`const struct bt_packet *`
`T`	Trace IR trace class	`const struct bt_trace_class *`
`t`	Trace IR trace	`const struct bt_trace *`
`K`	Trace IR clock class	`const struct bt_clock_class *`
`k`	Trace IR clock snapshot	`const struct bt_clock_snapshot *`
`v`	Value object	`const struct bt_value *`
`R`	Integer range set	`const struct bt_integer_range_set *`
`n`	Message	`const struct bt_message *`
`I`	Message iterator class	`struct bt_message_iterator_class *`
`i`	Message iterator	`struct bt_message_iterator *`
`C`	Component class	`struct bt_component_class *`
`c`	Component	`const struct bt_component *`
`p`	Port	`const struct bt_port *`
`x`	Connection	`const struct bt_connection *`
`g`	Graph	`const struct bt_graph *`
`z`	Interrupter	`struct bt_interrupter *`
`l`	Plugin	`const struct bt_plugin *`
`r`	Error cause	`const struct bt_error_cause *`
`o`	Object pool	`const struct bt_object_pool *`
`O`	Object	`const struct bt_object *`

Conversion specifier examples:

%!f
%![my-event-]+e
%!t
%!+F

The , string (comma and space) is printed between individual fields, but not after the last one. Therefore, you must put this separator in the format string between two conversion specifiers, for example:

BT_LIB_LOGW("Message: count=%u, %!E, %!+K", count, event_class, clock_class);

Example with a custom prefix:

BT_LIB_LOGI("Some message: %![ec-a-]e, %![ec-b-]+e", ec_a, ec_b);

It is safe to pass NULL as any Babeltrace object parameter: the macros only print its null address.

Warning

Build-time printf() format checks are disabled for the BT_LIB_LOG*() macros because there are custom conversion specifiers, so make sure to test your logging statements.

Component-specific logging statement macros

There are available logging macros for components. They prepend a prefix including the component’s name to the logging message.

See Instrument a component class C source file and Instrument a component class C header file to learn how to be able to use the following macros.

The component logging statement macros are named BT_COMP_LOG*() instead of BT_LOG*():

BT_COMP_LOGT("format string", ...): Component trace logging statement.
BT_COMP_LOGD("format string", ...): Component debug logging statement.
BT_COMP_LOGI("format string", ...): Component info logging statement.
BT_COMP_LOGW("format string", ...): Component warning logging statement.
BT_COMP_LOGE("format string", ...): Component error logging statement.
BT_COMP_LOGF("format string", ...): Component fatal logging statement.
BT_COMP_LOGT_STR("preformatted string"): Component preformatted string trace logging statement.
BT_COMP_LOGD_STR("preformatted string"): Component preformatted string debug logging statement.
BT_COMP_LOGI_STR("preformatted string"): Component preformatted string info logging statement.
BT_COMP_LOGW_STR("preformatted string"): Component preformatted string warning logging statement.
BT_COMP_LOGE_STR("preformatted string"): Component preformatted string error logging statement.
BT_COMP_LOGF_STR("preformatted string"): Component preformatted string fatal logging statement.
BT_COMP_LOGT_ERRNO("initial message", "format string", ...): Component errno string trace logging statement.
BT_COMP_LOGD_ERRNO("initial message", "format string", ...): Component errno string debug logging statement.
BT_COMP_LOGI_ERRNO("initial message", "format string", ...): Component errno string info logging statement.
BT_COMP_LOGW_ERRNO("initial message", "format string", ...): Component errno string warning logging statement.
BT_COMP_LOGE_ERRNO("initial message", "format string", ...): Component errno string error logging statement.
BT_COMP_LOGF_ERRNO("initial message", "format string", ...): Component errno string fatal logging statement.
BT_COMP_LOGT_MEM(data_ptr, data_size, "format string", ...): Component memory trace logging statement.
BT_COMP_LOGD_MEM(data_ptr, data_size, "format string", ...): Component memory debug logging statement.
BT_COMP_LOGI_MEM(data_ptr, data_size, "format string", ...): Component memory info logging statement.
BT_COMP_LOGW_MEM(data_ptr, data_size, "format string", ...): Component memory warning logging statement.
BT_COMP_LOGE_MEM(data_ptr, data_size, "format string", ...): Component memory error logging statement.
BT_COMP_LOGF_MEM(data_ptr, data_size, "format string", ...): Component memory fatal logging statement.

Conditional logging

BT_LOG_IF(cond, statement)

Execute statement only if cond is true.

Example:

BT_LOG_IF(i < count / 2, BT_LOGD("Log this: i=%d", i));

To check the build-time log level:

#if BT_LOG_ENABLED_DEBUG
...
#endif

This tests if the DEBUG level was enabled at build time. This means that the current, run-time 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_TRACE
BT_LOG_ENABLED_DEBUG
BT_LOG_ENABLED_INFO
BT_LOG_ENABLED_WARNING
BT_LOG_ENABLED_ERROR
BT_LOG_ENABLED_FATAL

To check the current, run-time log level:

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_TRACE
BT_LOG_ON_DEBUG
BT_LOG_ON_INFO
BT_LOG_ON_WARNING
BT_LOG_ON_ERROR
BT_LOG_ON_FATAL

Those macros check the 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:

int number = get_number_of_event_classes_with_property_x(...);
BT_LOGD("Bla bla: number=%d", number);

Do this instead:

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:

BT_LOGD("Bla bla: number=%d", get_number_of_event_classes_with_property_x(...));

Guides

Instrument a C source file (generic)

To instrument a C source file (.c):

At the top of the file, before the first #include line (if any), define your file’s logging tag name:
#define BT_LOG_TAG "SUBSYS/MY-MODULE/MY-FILE"

Below the line above, define the source file’s log level expression, BT_LOG_OUTPUT_LEVEL. This expression is evaluated for each logging statement to know the current run-time log level.

Examples:

/* Global log level variable */
#define BT_LOG_OUTPUT_LEVEL module_global_log_level

/* Local log level variable; must exist where you use BT_LOG*() */
#define BT_LOG_OUTPUT_LEVEL log_level

/* Object's log level; `obj` must exist where you use BT_LOG*() */
#define BT_LOG_OUTPUT_LEVEL (obj->log_level)

Include "logging/log.h":
```
#include "logging/log.h"
```
In the file, instrument your code with the generic logging statement macros.

Instrument a C header file (generic)

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

Do not include "logging/log.h"!
Do one of:
1. In the file, instrument your code with the generic logging statement macros, making each of them conditional to the existence of the macro you’re using:
  static inline int some_function(int x) { /* ... */ #ifdef BT_LOGT BT_LOGT(...); #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, and if so, what is their logging tag and run-time log level expression.
2. Require that logging be enabled:
  /* Protection: this file uses BT_LOG*() macros directly */ #ifndef BT_LOG_SUPPORTED # error Please include "logging/log.h" before including this file. #endif
  Then, in the file, instrument your code with the generic logging statement macros.

Instrument a library C source file

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

At the top of the file, before the first #include line (if any), define your file’s logging tag name (this tag must start with LIB/):
#define BT_LOG_TAG "LIB/THE-FILE"
Include "lib/logging.h":
```
#include "lib/logging.h"
```
In the file, instrument your code with the library logging statement macros or with the generic logging statement macros.

Instrument a library C header file

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

Do not include "lib/logging.h"!

Require that library logging be enabled:

/* Protection: this file uses BT_LIB_LOG*() macros directly */
#ifndef BT_LIB_LOG_SUPPORTED
# error Please include "lib/logging.h" before including this file.
#endif

In the file, instrument your code with the library logging statement macros or with the generic logging statement macros.

Instrument a component class C source file

To instrument a component class C source file (.c):

At the top of the file, before the first #include line (if any), define your file’s logging tag name (this tag must start with PLUGIN/ followed by the component class identifier):
#define BT_LOG_TAG "PLUGIN/SRC.MY-PLUGIN.MY-SRC"
Below the line above, define the source file’s log level expression, BT_LOG_OUTPUT_LEVEL. This expression is evaluated for each logging statement to know the current run-time log level.

For a component class file, it is usually a member of a local component private structure variable:
```
#define BT_LOG_OUTPUT_LEVEL (my_comp->log_level)
```
Below the line above, define BT_COMP_LOG_SELF_COMP to an expression which, evaluated in the context of the logging statements, evaluates to the self component address (bt_self_component *) of the component.

This is usually a member of a local component private structure variable:
```
#define BT_COMP_LOG_SELF_COMP (my_comp->self_comp)
```
Include "logging/comp-logging.h":
```
#include "logging/comp-logging.h"
```

In the component initialization method, make sure to set the component private structure’s log level member to the initial component’s log level:

struct my_comp {
    bt_logging_level log_level;
    /* ... */
};

BT_HIDDEN
bt_self_component_status my_comp_init(
        bt_self_component_source *self_comp_src,
        bt_value *params, void *init_method_data)
{
    struct my_comp *my_comp = g_new0(struct my_comp, 1);
    bt_self_component *self_comp =
        bt_self_component_source_as_self_component(self_comp_src);
    const bt_component *comp = bt_self_component_as_component(self_comp);

    BT_ASSERT(my_comp);
    my_comp->log_level = bt_component_get_logging_level(comp);

    /* ... */
}

In the file, instrument your code with the component logging statement macros.

Instrument a component class C header file

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

Do not include "logging/comp-logging.h"!

Require that component logging be enabled:

/* Protection: this file uses BT_COMP_LOG*() macros directly */
#ifndef BT_COMP_LOG_SUPPORTED
# error Please include "logging/comp-logging.h" before including this file.
#endif

In the file, instrument your code with the component logging statement macros.

Choose a logging tag

Each logging-enabled C source file must define BT_LOG_TAG to a logging tag. A logging tag is a namespace to identify the logging messages of this specific source file.

In general, a logging tag name must be only uppercase letters, digits, and the -, ., and / characters.

Use / to show the subsystem to source file hierarchy.

For the Babeltrace library, start with LIB/.

For the CTF writer library, start with CTF-WRITER/.

For component classes, use:

PLUGIN/CCTYPE.PNAME.CCNAME[/FILE]

With:

CCTYPE: Component class’s type (SRC, FLT, or SINK).
PNAME: Plugin’s name.
CCNAME: Component class’s name.
FILE: Additional information to specify the source file name or module.

For plugins (files common to many component classes), use:

PLUGIN/PNAME[/FILE]

With:

PNAME: Plugin’s name.
FILE: Additional information to specify the source file name or module.

Choose a log level

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

Log level	Description	Use cases	Expected 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. Failed assertion (within `BT_ASSERT()`). Unsatisfied library precondition (within `BT_ASSERT_PRE()` or `BT_ASSERT_PRE_DEV()`). Unsatisfied library postcondition (within `BT_ASSERT_POST()` or `BT_ASSERT_POST_DEV()`).	Almost none: always enabled.
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. Wrong component initialization parameters. Corrupted, unrecoverable trace data. 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 any error in terminal elements: CLI and plugins.	Almost none: always enabled.
WARNING	An error which still allows the execution to continue, but you judge that it should be reported to the user. WARNING-level logging statements are for any error or weird action that is directly or indirectly caused by the user, often through some bad input data. 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).	Missing data within something that is expected to have it, but there’s an alternative. Invalid file, but recoverable/fixable.	Almost none: always enabled.
INFO	Any useful information which a non-developer user would possibly understand. Anything logged with this level must not happen repetitively on the fast path, that is, nothing related to each message, for example. This level is used for sporadic and one-shot events.	CLI or component configuration report. Successful plugin, component, or message iterator initialization. In the library: anything related to plugins, graphs, component classes, components, message iterators, connections, and ports which is not on the fast path. Successful connection to or disconnection from another system. An optional subsystem cannot be loaded. An optional field/datum cannot be found.	Very little: always enabled.
DEBUG	Something that only Babeltrace developers would be interested into, which can occur on the fast path, but not more often than once per message. The DEBUG level is the default build-time log level as, since it’s not too verbose, the performance is similar to an INFO build.	Object construction and destruction. Object recycling (except fields). Object copying (except fields and values). Object freezing (whatever the type, as freezing only occurs in developer mode). Object interruption. Calling user methods and logging the result. Setting object properties (except fields and values).	Noticeable, but not as much as the TRACE level: could be executed in production if you’re going to need a thorough log for support tickets without having to rebuild the project.
TRACE	Low-level debugging context information (anything that does not fit the other log levels). More appropriate for tracing in general.	Reference count change. Fast path, low level state machine’s state change. Get or set an object’s property. Object comparison’s intermediate results.	Huge: not executed in production.

Important

Make sure not to use a WARNING (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 scenario: the user could legitimately use this function to check if the name/key exists in the object. In this case, use the TRACE level (or do not log at all).

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 logging messages can be hard to parse, analyze, and filter, however, so prefer multiple logging 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; use kebab case if possible. If value is a non-alphanumeric string, put it between double quotes ("%s" specifier). Always use the PRId64 and PRIu64 specifiers to log an int64_t or an uint64_t value. Use %d to log a boolean value.

Example:

"Cannot read stream data for indexing: path=\"%s\", name=\"%s\", "
"stream-id=%" PRIu64 ", stream-fd=%d, "
"index=%" PRIu64 ", status=%s, is-mapped=%d"

By following a standard format for the statement fields, it is easier to use tools like Logstash or even Splunk to split fields and analyze logs.

Prefer the following suffixes in field names:

Field name suffix	Description	Format specifier
`-addr`	Memory address	`%p`
`-fd`	File descriptor	`%d`
`-fp`	File stream (`FILE *`)	`%p`
`-id`	Object’s ID	`%" PRIu64 "`
`-index`	Index	`%" 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 log level, the logging tag, the source’s function name, file name and line number, and the message.

When Babeltrace supports terminal color codes (depends on the BABELTRACE_TERM_COLOR environment variable’s value and what the standard output and error streams are plugged into), INFO-level lines are blue, WARNING-level lines are yellow, and ERROR-level and FATAL-level lines are red.

Log line 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 DEBUG-level logging messages that the FIELD-CLASS module generates:

$ babeltrace2 --log-level=D /path/to/trace |& ag 'D FIELD-CLASS'

Valgrind

To use Valgrind on an application (for example, the CLI or a test) which loads libbabeltrace2, use:

$ G_SLICE=always-malloc G_DEBUG=gc-friendly PYTHONMALLOC=malloc \
  LIBBABELTRACE2_NO_DLCLOSE=1 valgrind --leak-check=full app

G_SLICE=always-malloc and G_DEBUG=gc-friendly is for GLib and PYTHONMALLOC=malloc is for the Python interpreter, if it is used by the Python plugin provider (Valgrind will probably show a lot of errors which originate from the Python interpreter anyway).

LIBBABELTRACE2_NO_DLCLOSE=1 makes libbabeltrace2 not close the shared libraries (plugins) which it loads. You need this to see the appropriate backtrace when Valgrind shows errors.

Testing

Environment

tests/utils/utils.sh sets the environment variables for any Babeltrace test script.

utils.sh only needs to know the path to the tests directory within the source and the build directories. By default, utils.sh assumes the build is in tree, that is, you ran ./configure from the source’s root directory, and sets the BT_TESTS_SRCDIR and BT_TESTS_BUILDDIR environment variables accordingly. You can override those variables, for example if you build out of tree.

All test scripts eventually do something like this to source utils.sh, according to where they are located relative to the tests directory:

if [ "x${BT_TESTS_SRCDIR:-}" != "x" ]; then
    UTILSSH="$BT_TESTS_SRCDIR/utils/utils.sh"
else
    UTILSSH="$(dirname "$0")/../utils/utils.sh"
fi

Python

You can use the tests/utils/run_python_bt2 script to run any command within an environment making the build’s bt2 Python package available.

run_python_bt2 uses utils.sh which needs to know the build directory, so make sure you set the BT_TESTS_BUILDDIR environment variable correctly if you build out of tree, for example:

$ export BT_TESTS_BUILDDIR=/path/to/build/babeltrace/tests

You can run any command which needs the bt2 Python package through run_python_bt2, for example:

$ ./tests/utils/run_python_bt2 ipython3

Report format

All test scripts output the test results following the Test Anything Protocol (TAP) format.

The TAP format has two mechanisms to print additional information about a test:

Print a line starting with # to the standard output.

This is usually done with the diag() C function or the diag shell function.
Print to the standard error.

Python bindings

The bt2 Python package tests are located in tests/bindings/python/bt2.

Python test runner

tests/utils/python/testrunner.py is Babeltrace’s Python test runner which loads Python files containing unit tests, finds all the test cases, and runs the tests, producing a TAP report.

You can see the test runner command’s help with:

$ python3 ./tests/utils/python/testrunner.py --help

By default, the test runner reports failing tests (TAP’s not ok line), but continues to run other tests. You can use the --failfast option to make the test runner fail as soon as a test fails.

Guides

To run all the bt2 Python package tests:

Run:

$ ./tests/utils/run_python_bt2 ./tests/bindings/python/bt2/test_python_bt2

or:

$ ./tests/utils/run_python_bt2 python3 ./tests/utils/python/testrunner.py \
  ./tests/bindings/python/bt2/ -p '*.py'

To run all the tests in a test module (for example, test_value.py):

Run:

$ ./tests/utils/run_python_bt2 python3 ./tests/utils/python/testrunner.py \
  ./tests/bindings/python/bt2 -t test_value

To run a specific test case (for example, RealValueTestCase within test_value.py):

Run:

$ ./tests/utils/run_python_bt2 python3 ./tests/utils/python/testrunner.py \
  ./tests/bindings/python/bt2/ -t test_value.RealValueTestCase

To run a specific test (for example, RealValueTestCase.test_assign_pos_int within test_value.py):

Run:

$ ./tests/utils/run_python_bt2 python3 ./tests/utils/python/testrunner.py \
  ./tests/bindings/python/bt2/ -t test_value.RealValueTestCase.test_assign_pos_int

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

CONTRIBUTING.adoc

CONTRIBUTING.adoc

Babeltrace contributor’s guide

Babeltrace library

Object reference counting and lifetime

The problem

The solution

Caveats

Example

Logging

Logging API

Headers

Log levels

Generic logging statement macros

Library-specific logging statement macros

Component-specific logging statement macros

Conditional logging

Guides

Instrument a C source file (generic)

Instrument a C header file (generic)

Instrument a library C source file

Instrument a library C header file

Instrument a component class C source file

Instrument a component class C header file

Choose a logging tag

Choose a log level

Write an appropriate message

Output

Valgrind

Testing

Environment

Python

Report format

Python bindings

Python test runner

Guides

Files

CONTRIBUTING.adoc

Latest commit

History

CONTRIBUTING.adoc

File metadata and controls

Babeltrace contributor’s guide

Babeltrace library

Object reference counting and lifetime

The problem

The solution

Caveats

Example

Logging

Logging API

Headers

Log levels

Generic logging statement macros

Library-specific logging statement macros

Component-specific logging statement macros

Conditional logging

Guides

Instrument a C source file (generic)

Instrument a C header file (generic)

Instrument a library C source file

Instrument a library C header file

Instrument a component class C source file

Instrument a component class C header file

Choose a logging tag

Choose a log level

Write an appropriate message

Output

Valgrind

Testing

Environment

Python

Report format

Python bindings

Python test runner

Guides