Provided by: liblttng-ust-dev_2.13.5-1ubuntu1_amd64 
NAME
lttng-ust - LTTng user space tracing
SYNOPSIS
#include <lttng/tracepoint.h>
#define LTTNG_UST_TP_ARGS(args...)
#define LTTNG_UST_TP_ENUM_VALUES(values...)
#define LTTNG_UST_TP_FIELDS(fields...)
#define LTTNG_UST_TRACEPOINT_ENUM(prov_name, enum_name, mappings)
#define LTTNG_UST_TRACEPOINT_EVENT(prov_name, t_name, args, fields)
#define LTTNG_UST_TRACEPOINT_EVENT_CLASS(cls_prov_name, cls_name,
args, fields)
#define LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(cls_prov_name, cls_name,
inst_prov_name, t_name, args)
#define LTTNG_UST_TRACEPOINT_LOGLEVEL(prov_name, t_name, level)
#define lttng_ust_do_tracepoint(prov_name, t_name, ...)
#define lttng_ust_field_array(int_type, field_name, expr, count)
#define lttng_ust_field_array_nowrite(int_type, field_name, expr, count)
#define lttng_ust_field_array_hex(int_type, field_name, expr, count)
#define lttng_ust_field_array_nowrite_hex(int_type, field_name, expr,
count)
#define lttng_ust_field_array_network(int_type, field_name, expr, count)
#define lttng_ust_field_array_network_nowrite(int_type, field_name,
expr, count)
#define lttng_ust_field_array_network_hex(int_type, field_name, expr,
count)
#define lttng_ust_field_array_network_nowrite_hex(int_type, field_name,
expr, count)
#define lttng_ust_field_array_text(char, field_name, expr, count)
#define lttng_ust_field_array_text_nowrite(char, field_name, expr,
count)
#define lttng_ust_field_enum(prov_name, enum_name, int_type, field_name,
expr)
#define lttng_ust_field_enum_nowrite(prov_name, enum_name, int_type,
field_name, expr)
#define lttng_ust_field_enum_value(label, value)
#define lttng_ust_field_enum_range(label, start, end)
#define lttng_ust_field_float(float_type, field_name, expr)
#define lttng_ust_field_float_nowrite(float_type, field_name, expr)
#define lttng_ust_field_integer(int_type, field_name, expr)
#define lttng_ust_field_integer_hex(int_type, field_name, expr)
#define lttng_ust_field_integer_network(int_type, field_name, expr)
#define lttng_ust_field_integer_network_hex(int_type, field_name, expr)
#define lttng_ust_field_integer_nowrite(int_type, field_name, expr)
#define lttng_ust_field_sequence(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_nowrite(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_hex(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_nowrite_hex(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_network(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_network_nowrite(int_type, field_name,
expr, len_type,
len_expr)
#define lttng_ust_field_sequence_network_hex(int_type, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_sequence_network_nowrite_hex(int_type,
field_name,
expr, len_type,
len_expr)
#define lttng_ust_field_sequence_text(char, field_name, expr, len_type,
len_expr)
#define lttng_ust_field_sequence_text_nowrite(char, field_name, expr,
len_type, len_expr)
#define lttng_ust_field_string(field_name, expr)
#define lttng_ust_field_string_nowrite(field_name, expr)
#define lttng_ust_tracepoint(prov_name, t_name, ...)
#define lttng_ust_tracepoint_enabled(prov_name, t_name)
Link with, following this manual page:
• -llttng-ust -ldl
• If you define _LGPL_SOURCE before including <lttng/tracepoint.h> (directly or
indirectly): -llttng-ust-common
DESCRIPTION
The Linux Trace Toolkit: next generation <http://lttng.org/> is an open source software
package used for correlated tracing of the Linux kernel, user applications, and user
libraries.
LTTng-UST is the user space tracing component of the LTTng project. It is a port to user
space of the low-overhead tracing capabilities of the LTTng Linux kernel tracer. The
liblttng-ust library is used to trace user applications and libraries.
Note
This man page is about the liblttng-ust library. The LTTng-UST project also provides
Java and Python packages to trace applications written in those languages. How to
instrument and trace Java and Python applications is documented in the online LTTng
documentation <http://lttng.org/docs/>.
There are three ways to use liblttng-ust:
• Using the lttng_ust_tracef(3) API, which is similar to printf(3).
• Using the lttng_ust_tracelog(3) API, which is lttng_ust_tracef(3) with a log level
parameter.
• Defining your own tracepoints. See the Creating a tracepoint provider section below.
Compatibility with previous APIs
Since LTTng-UST 2.13, the LTTNG_UST_COMPAT_API_VERSION definition controls which LTTng-UST
APIs are available (compiled):
Undefined
All APIs are available.
N (0 or positive integer)
API version N, and all the following existing APIs, are available. Previous APIs are
not available (not compiled).
The following table shows the mapping from LTTng-UST versions (up to LTTng-UST 2.13.5) to
available API versions:
┌──────────────────┬────────────────────────┐
│LTTng-UST version │ Available API versions │
├──────────────────┼────────────────────────┤
│ │ │
│2.0 to 2.12 │ 0 │
├──────────────────┼────────────────────────┤
│ │ │
│2.13 │ 0 and 1 │
└──────────────────┴────────────────────────┘
This manual page only documents version 1 of the API.
If you wish to have access to version 0 of the API (for example, the tracepoint(),
ctf_integer(), and TRACEPOINT_EVENT() macros), then either don’t define
LTTNG_UST_COMPAT_API_VERSION, or define it to 0 before including any LTTng-UST header.
Creating a tracepoint provider
Creating a tracepoint provider is the first step of using liblttng-ust. The next steps
are:
• Instrumenting your application with lttng_ust_tracepoint() calls
• Building your application with LTTng-UST support, either statically or dynamically.
A tracepoint provider is a compiled object containing the event probes corresponding to
your custom tracepoint definitions. A tracepoint provider contains the code to get the
size of an event and to serialize it, amongst other things.
To create a tracepoint provider, start with the following tracepoint provider header
template:
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider
#undef LTTNG_UST_TRACEPOINT_INCLUDE
#define LTTNG_UST_TRACEPOINT_INCLUDE "./tp.h"
#if !defined(_TP_H) || \
defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H
#include <lttng/tracepoint.h>
/*
* LTTNG_UST_TRACEPOINT_EVENT(), LTTNG_UST_TRACEPOINT_EVENT_CLASS(),
* LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(),
* LTTNG_UST_TRACEPOINT_LOGLEVEL(), and `LTTNG_UST_TRACEPOINT_ENUM()`
* are used here.
*/
#endif /* _TP_H */
#include <lttng/tracepoint-event.h>
In this template, the tracepoint provider is named my_provider
(LTTNG_UST_TRACEPOINT_PROVIDER definition). The file needs to bear the name of the
LTTNG_UST_TRACEPOINT_INCLUDE definition (tp.h in this case). Between #include
<lttng/tracepoint.h> and #endif go the invocations of the LTTNG_UST_TRACEPOINT_EVENT(),
LTTNG_UST_TRACEPOINT_EVENT_CLASS(), LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(),
LTTNG_UST_TRACEPOINT_LOGLEVEL(), and LTTNG_UST_TRACEPOINT_ENUM() macros.
Note
You can avoid writing the prologue and epilogue boilerplate in the template file above
by using the lttng-gen-tp(1) tool shipped with LTTng-UST.
The tracepoint provider header file needs to be included in a source file which looks like
this:
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#include "tp.h"
Together, those two files (let’s call them tp.h and tp.c) form the tracepoint provider
sources, ready to be compiled.
You can create multiple tracepoint providers to be used in a single application, but each
one must have its own header file.
The LTTNG_UST_TRACEPOINT_EVENT() usage section below shows how to use the
LTTNG_UST_TRACEPOINT_EVENT() macro to define the actual tracepoints in the tracepoint
provider header file.
See the EXAMPLE section below for a complete example.
LTTNG_UST_TRACEPOINT_EVENT() usage
The LTTNG_UST_TRACEPOINT_EVENT() macro is used in a template provider header file (see the
Creating a tracepoint provider section above) to define LTTng-UST tracepoints.
The LTTNG_UST_TRACEPOINT_EVENT() usage template is as follows:
LTTNG_UST_TRACEPOINT_EVENT(
/* Tracepoint provider name */
my_provider,
/* Tracepoint/event name */
my_tracepoint,
/* List of tracepoint arguments (input) */
LTTNG_UST_TP_ARGS(
...
),
/* List of fields of eventual event (output) */
LTTNG_UST_TP_FIELDS(
...
)
)
The LTTNG_UST_TP_ARGS() macro contains the input arguments of the tracepoint. Those
arguments can be used in the argument expressions of the output fields defined in
LTTNG_UST_TP_FIELDS().
The format of the LTTNG_UST_TP_ARGS() parameters is: C type, then argument name; repeat as
needed, up to ten times. For example:
LTTNG_UST_TP_ARGS(
int, my_int,
const char *, my_string,
FILE *, my_file,
double, my_float,
struct my_data *, my_data
)
The LTTNG_UST_TP_FIELDS() macro contains the output fields of the tracepoint, that is, the
actual data that can be recorded in the payload of an event emitted by this tracepoint.
The LTTNG_UST_TP_FIELDS() macro contains a list of lttng_ust_field_*() macros NOT
separated by commas. The available macros are documented in the Available
lttng_ust_field_*() field type macros section below.
Available field macros
This section documents the available lttng_ust_field_*() macros that can be inserted in
the LTTNG_UST_TP_FIELDS() macro of the LTTNG_UST_TRACEPOINT_EVENT() macro.
Standard integer, displayed in base 10:
lttng_ust_field_integer(int_type, field_name, expr)
lttng_ust_field_integer_nowrite(int_type, field_name, expr)
Standard integer, displayed in base 16:
lttng_ust_field_integer_hex(int_type, field_name, expr)
Integer in network byte order (big endian), displayed in base 10:
lttng_ust_field_integer_network(int_type, field_name, expr)
Integer in network byte order, displayed in base 16:
lttng_ust_field_integer_network_hex(int_type, field_name, expr)
Floating point number:
lttng_ust_field_float(float_type, field_name, expr)
lttng_ust_field_float_nowrite(float_type, field_name, expr)
Null-terminated string:
lttng_ust_field_string(field_name, expr)
lttng_ust_field_string_nowrite(field_name, expr)
Statically-sized array of integers (_hex versions displayed in hexadecimal, _network
versions in network byte order):
lttng_ust_field_array(int_type, field_name, expr, count)
lttng_ust_field_array_nowrite(int_type, field_name, expr, count)
lttng_ust_field_array_hex(int_type, field_name, expr, count)
lttng_ust_field_array_nowrite_hex(int_type, field_name, expr, count)
lttng_ust_field_array_network(int_type, field_name, expr, count)
lttng_ust_field_array_network_nowrite(int_type, field_name, expr,
count)
lttng_ust_field_array_network_hex(int_type, field_name, expr, count)
lttng_ust_field_array_network_nowrite_hex(int_type, field_name,
expr, count)
Statically-sized array, printed as text; no need to be null-terminated:
lttng_ust_field_array_text(char, field_name, expr, count)
lttng_ust_field_array_text_nowrite(char, field_name, expr, count)
Dynamically-sized array of integers (_hex versions displayed in hexadecimal, _network
versions in network byte order):
lttng_ust_field_sequence(int_type, field_name, expr, len_type,
len_expr)
lttng_ust_field_sequence_nowrite(int_type, field_name, expr,
len_type, len_expr)
lttng_ust_field_sequence_hex(int_type, field_name, expr, len_type,
len_expr)
lttng_ust_field_sequence_nowrite_hex(int_type, field_name, expr,
len_type, len_expr)
lttng_ust_field_sequence_network(int_type, field_name, expr,
len_type, len_expr)
lttng_ust_field_sequence_network_nowrite(int_type, field_name, expr,
len_type, len_expr)
lttng_ust_field_sequence_network_hex(int_type, field_name, expr,
len_type, len_expr)
lttng_ust_field_sequence_network_nowrite_hex(int_type, field_name,
expr, len_type,
len_expr)
Dynamically-sized array, displayed as text; no need to be null-terminated:
lttng_ust_field_sequence_text(char, field_name, expr, len_type,
len_expr)
lttng_ust_field_sequence_text_nowrite(char, field_name, expr,
len_type, len_expr)
Enumeration. The enumeration field must be defined before using this macro with the
LTTNG_UST_TRACEPOINT_ENUM() macro. See the LTTNG_UST_TRACEPOINT_ENUM() usage section for
more information.
lttng_ust_field_enum(prov_name, enum_name, int_type, field_name,
expr)
lttng_ust_field_enum_nowrite(prov_name, enum_name, int_type,
field_name, expr)
The parameters are:
count
Number of elements in array/sequence. This must be known at compile time.
enum_name
Name of an enumeration field previously defined with the LTTNG_UST_TRACEPOINT_ENUM()
macro. See the LTTNG_UST_TRACEPOINT_ENUM() usage section for more information.
expr
C expression resulting in the field’s value. This expression can use one or more
arguments passed to the tracepoint. The arguments of a given tracepoint are defined in
the LTTNG_UST_TP_ARGS() macro (see the Creating a tracepoint provider section above).
field_name
Event field name (C identifier syntax, NOT a literal string).
float_type
Float C type (float or double). The size of this type determines the size of the
floating point number field.
int_type
Integer C type. The size of this type determines the size of the integer/enumeration
field.
len_expr
C expression resulting in the sequence’s length. This expression can use one or more
arguments passed to the tracepoint.
len_type
Unsigned integer C type of sequence’s length.
prov_name
Tracepoint provider name. This must be the same as the tracepoint provider name used
in a previous field definition.
The _nowrite versions omit themselves from the recorded trace, but are otherwise
identical. Their primary purpose is to make some of the event context available to the
event filters without having to commit the data to sub-buffers. See lttng-enable-event(1)
to learn more about dynamic event filtering.
See the EXAMPLE section below for a complete example.
LTTNG_UST_TRACEPOINT_ENUM() usage
An enumeration field is a list of mappings between an integers, or a range of integers,
and strings (sometimes called labels or enumerators). Enumeration fields can be used to
have a more compact trace when the possible values for a field are limited.
An enumeration field is defined with the LTTNG_UST_TRACEPOINT_ENUM() macro:
LTTNG_UST_TRACEPOINT_ENUM(
/* Tracepoint provider name */
my_provider,
/* Enumeration name (unique in the whole tracepoint provider) */
my_enum,
/* Enumeration mappings */
LTTNG_UST_TP_ENUM_VALUES(
...
)
)
LTTNG_UST_TP_ENUM_VALUES() contains a list of enumeration mappings, NOT separated by
commas. Two macros can be used in the LTTNG_UST_TP_ENUM_VALUES():
lttng_ust_field_enum_value() and lttng_ust_field_enum_range().
lttng_ust_field_enum_value() is a single value mapping:
lttng_ust_field_enum_value(label, value)
This macro maps the given label string to the value value.
lttng_ust_field_enum_range() is a range mapping:
lttng_ust_field_enum_range(label, start, end)
This macro maps the given label string to the range of integers from start to end,
inclusively. Range mappings may overlap, but the behaviour is implementation-defined: each
trace reader handles overlapping ranges as it wishes.
See the EXAMPLE section below for a complete example.
LTTNG_UST_TRACEPOINT_EVENT_CLASS() usage
A tracepoint class is a class of tracepoints sharing the same field types and names. A
tracepoint instance is one instance of such a declared tracepoint class, with its own
event name.
LTTng-UST creates one event serialization function per tracepoint class. Using
LTTNG_UST_TRACEPOINT_EVENT() creates one tracepoint class per tracepoint definition,
whereas using LTTNG_UST_TRACEPOINT_EVENT_CLASS() and LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()
creates one tracepoint class, and one or more tracepoint instances of this class. In other
words, many tracepoints can reuse the same serialization code. Reusing the same code, when
possible, can reduce cache pollution, thus improve performance.
The LTTNG_UST_TRACEPOINT_EVENT_CLASS() macro accepts the same parameters as the
LTTNG_UST_TRACEPOINT_EVENT() macro, except that instead of an event name, its second
parameter is the tracepoint class name:
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider
/* ... */
LTTNG_UST_TRACEPOINT_EVENT_CLASS(
/* Tracepoint class provider name */
my_provider,
/* Tracepoint class name */
my_tracepoint_class,
/* List of tracepoint arguments (input) */
LTTNG_UST_TP_ARGS(
...
),
/* List of fields of eventual event (output) */
LTTNG_UST_TP_FIELDS(
...
)
)
Once the tracepoint class is defined, you can create as many tracepoint instances as
needed:
#define LTTNG_UST_TRACEPOINT_PROVIDER natality
/* ... */
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
/* Name of the tracepoint class provider */
my_provider,
/* Tracepoint class name */
my_tracepoint_class,
/* Name of the local (instance) tracepoint provider */
natality,
/* Tracepoint/event name */
my_tracepoint,
/* List of tracepoint arguments (input) */
LTTNG_UST_TP_ARGS(
...
)
)
As you can see, the LTTNG_UST_TRACEPOINT_EVENT_INSTANCE() does not contain the
LTTNG_UST_TP_FIELDS() macro, because they are defined at the
LTTNG_UST_TRACEPOINT_EVENT_CLASS() level.
Note that the LTTNG_UST_TRACEPOINT_EVENT_INSTANCE() macro requires two provider names:
• The name of the tracepoint class provider (my_provider in the example above).
This is the same as the first argument of the LTTNG_UST_TRACEPOINT_EVENT_CLASS()
expansion to refer to.
• The name of the local, or instance, provider (natality in the example above).
This is the provider name which becomes the prefix part of the name of the events
which such a tracepoint creates.
The two provider names may be different if the tracepoint class and the tracepoint
instance macros are in two different translation units.
See the EXAMPLE section below for a complete example.
LTTNG_UST_TRACEPOINT_LOGLEVEL() usage
Optionally, a log level can be assigned to a defined tracepoint. Assigning different
levels of severity to tracepoints can be useful: when controlling tracing sessions, you
can choose to only enable events falling into a specific log level range using the
--loglevel and --loglevel-only options of the lttng-enable-event(1) command.
Log levels are assigned to tracepoints that are already defined using the
LTTNG_UST_TRACEPOINT_LOGLEVEL() macro. The latter must be used after having used
LTTNG_UST_TRACEPOINT_EVENT() or LTTNG_UST_TRACEPOINT_EVENT_INSTANCE() for a given
tracepoint. The LTTNG_UST_TRACEPOINT_LOGLEVEL() macro is used as follows:
LTTNG_UST_TRACEPOINT_LOGLEVEL(
/* Tracepoint provider name */
my_provider,
/* Tracepoint/event name */
my_tracepoint,
/* Log level */
LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO
)
The available log level definitions are:
LTTNG_UST_TRACEPOINT_LOGLEVEL_EMERG
System is unusable.
LTTNG_UST_TRACEPOINT_LOGLEVEL_ALERT
Action must be taken immediately.
LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT
Critical conditions.
LTTNG_UST_TRACEPOINT_LOGLEVEL_ERR
Error conditions.
LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING
Warning conditions.
LTTNG_UST_TRACEPOINT_LOGLEVEL_NOTICE
Normal, but significant, condition.
LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO
Informational message.
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_SYSTEM
Debug information with system-level scope (set of programs).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROGRAM
Debug information with program-level scope (set of processes).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROCESS
Debug information with process-level scope (set of modules).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_MODULE
Debug information with module (executable/library) scope (set of units).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT
Debug information with compilation unit scope (set of functions).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION
Debug information with function-level scope.
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_LINE
Debug information with line-level scope (default log level).
LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG
Debug-level message.
See the EXAMPLE section below for a complete example.
Instrumenting your application
Once the tracepoint provider is created (see the Creating a tracepoint provider section
above), you can instrument your application with the defined tracepoints thanks to the
lttng_ust_tracepoint() macro:
#define lttng_ust_tracepoint(prov_name, t_name, ...)
With:
prov_name
Tracepoint provider name.
t_name
Tracepoint/event name.
...
Tracepoint arguments, if any.
Make sure to include the tracepoint provider header file anywhere you use
lttng_ust_tracepoint() for this provider.
Note
Even though LTTng-UST supports lttng_ust_tracepoint() call site duplicates having the
same provider and tracepoint names, it is recommended to use a provider/tracepoint
name pair only once within the application source code to help map events back to
their call sites when analyzing the trace.
Sometimes, arguments to the tracepoint are expensive to compute (take call stack, for
example). To avoid the computation when the tracepoint is disabled, you can use the
lttng_ust_tracepoint_enabled() and lttng_ust_do_tracepoint() macros:
#define lttng_ust_tracepoint_enabled(prov_name, t_name)
#define lttng_ust_do_tracepoint(prov_name, t_name, ...)
lttng_ust_tracepoint_enabled() returns a non-zero value if the tracepoint named t_name
from the provider named prov_name is enabled at run time.
lttng_ust_do_tracepoint() is like lttng_ust_tracepoint(), except that it doesn’t check if
the tracepoint is enabled. Using lttng_ust_tracepoint() with
lttng_ust_tracepoint_enabled() is dangerous since lttng_ust_tracepoint() also contains the
lttng_ust_tracepoint_enabled() check, thus a race condition is possible in this situation:
if (lttng_ust_tracepoint_enabled(my_provider, my_tracepoint)) {
stuff = prepare_stuff();
}
lttng_ust_tracepoint(my_provider, my_tracepoint, stuff);
If the tracepoint is enabled after the condition, then stuff is not prepared: the emitted
event will either contain wrong data, or the whole application could crash (segmentation
fault, for example).
Note
Neither lttng_ust_tracepoint_enabled() nor lttng_ust_do_tracepoint() have a
STAP_PROBEV() call, so if you need it, you should emit this call yourself.
Statically linking the tracepoint provider
With the static linking method, compiled tracepoint providers are copied into the target
application.
Define LTTNG_UST_TRACEPOINT_DEFINE definition below the LTTNG_UST_TRACEPOINT_CREATE_PROBES
definition in the tracepoint provider source:
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE
#include "tp.h"
Create the tracepoint provider object file:
$ cc -c -I. tp.c
Note
Although an application instrumented with LTTng-UST tracepoints can be compiled with a
C++ compiler, tracepoint probes should be compiled with a C compiler.
At this point, you can archive this tracepoint provider object file, possibly with other
object files of your application or with other tracepoint provider object files, as a
static library:
$ ar rc tp.a tp.o
Using a static library does have the advantage of centralising the tracepoint providers
objects so they can be shared between multiple applications. This way, when the tracepoint
provider is modified, the source code changes don’t have to be patched into each
application’s source code tree. The applications need to be relinked after each change,
but need not to be otherwise recompiled (unless the tracepoint provider’s API changes).
Then, link your application with this object file (or with the static library containing
it) and with liblttng-ust and libdl (libc on a BSD system):
$ cc -o app tp.o app.o -llttng-ust -ldl
Dynamically loading the tracepoint provider
The second approach to package the tracepoint provider is to use the dynamic loader: the
library and its member functions are explicitly sought, loaded at run time.
In this scenario, the tracepoint provider is compiled as a shared object.
The process to create the tracepoint provider shared object is pretty much the same as the
static linking method, except that:
• Since the tracepoint provider is not part of the application,
LTTNG_UST_TRACEPOINT_DEFINE must be defined, for each tracepoint provider, in exactly
one source file of the application
• LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE must be defined next to
LTTNG_UST_TRACEPOINT_DEFINE
Regarding LTTNG_UST_TRACEPOINT_DEFINE and LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE, the
recommended practice is to use a separate C source file in your application to define
them, then include the tracepoint provider header files afterwards. For example, as tp-
define.c:
#define LTTNG_UST_TRACEPOINT_DEFINE
#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE
#include "tp.h"
The tracepoint provider object file used to create the shared library is built like it is
using the static linking method, but with the -fpic option:
$ cc -c -fpic -I. tp.c
It is then linked as a shared library like this:
$ cc -shared -Wl,--no-as-needed -o tp.so tp.o -llttng-ust
This tracepoint provider shared object isn’t linked with the user application: it must be
loaded manually. This is why the application is built with no mention of this tracepoint
provider, but still needs libdl:
$ cc -o app app.o tp-define.o -ldl
There are two ways to dynamically load the tracepoint provider shared object:
• Load it manually from the application using dlopen(3)
• Make the dynamic loader load it with the LD_PRELOAD environment variable (see
ld.so(8))
If the application does not dynamically load the tracepoint provider shared object using
one of the methods above, tracing is disabled for this application, and the events are not
listed in the output of lttng-list(1).
Note that it is not safe to use dlclose(3) on a tracepoint provider shared object that is
being actively used for tracing, due to a lack of reference counting from LTTng-UST to the
shared object.
For example, statically linking a tracepoint provider to a shared object which is to be
dynamically loaded by an application (a plugin, for example) is not safe: the shared
object, which contains the tracepoint provider, could be dynamically closed (dlclose(3))
at any time by the application.
To instrument a shared object, either:
• Statically link the tracepoint provider to the application, or
• Build the tracepoint provider as a shared object (following the procedure shown in
this section), and preload it when tracing is needed using the LD_PRELOAD environment
variable.
Using LTTng-UST with daemons
Some extra care is needed when using liblttng-ust with daemon applications that call
fork(2), clone(2), or BSD’s rfork(2) without a following exec(3) family system call. The
library liblttng-ust-fork.so needs to be preloaded before starting the application with
the LD_PRELOAD environment variable (see ld.so(8)).
To use liblttng-ust with a daemon application which closes file descriptors that were not
opened by it, preload the liblttng-ust-fd.so library before you start the application.
Typical use cases include daemons closing all file descriptors after fork(2), and buggy
applications doing “double-closes”.
Context information
Context information can be prepended by the LTTng-UST tracer before each event, or before
specific events.
Context fields can be added to specific channels using lttng-add-context(1).
The following context fields are supported by LTTng-UST:
General context fields
cpu_id
CPU ID.
Note
This context field is always enabled, and it cannot be added with lttng-add-
context(1). Its main purpose is to be used for dynamic event filtering. See
lttng-enable-event(1) for more information about event filtering.
ip
Instruction pointer: enables recording the exact address from which an event was
emitted. This context field can be used to reverse-lookup the source location that
caused the event to be emitted.
pthread_id
POSIX thread identifier.
Can be used on architectures where pthread_t maps nicely to an unsigned long type.
Process context fields
procname
Thread name, as set by exec(3) or prctl(2). It is recommended that programs set
their thread name with prctl(2) before hitting the first tracepoint for that
thread.
vpid
Virtual process ID: process ID as seen from the point of view of the current
process ID namespace (see pid_namespaces(7)).
vtid
Virtual thread ID: thread ID as seen from the point of view of the current process
ID namespace (see pid_namespaces(7)).
perf context fields
perf:thread:COUNTER
perf counter named COUNTER. Use lttng add-context --list to list the available
perf counters.
Only available on IA-32 and x86-64 architectures.
perf:thread:raw:rN:NAME
perf counter with raw ID N and custom name NAME. See lttng-add-context(1) for more
details.
Namespace context fields (see namespaces(7))
cgroup_ns
Inode number of the current control group namespace (see cgroup_namespaces(7)) in
the proc file system.
ipc_ns
Inode number of the current IPC namespace (see ipc_namespaces(7)) in the proc file
system.
mnt_ns
Inode number of the current mount point namespace (see mount_namespaces(7)) in the
proc file system.
net_ns
Inode number of the current network namespace (see network_namespaces(7)) in the
proc file system.
pid_ns
Inode number of the current process ID namespace (see pid_namespaces(7)) in the
proc file system.
time_ns
Inode number of the current clock namespace (see time_namespaces(7)) in the proc
file system.
user_ns
Inode number of the current user namespace (see user_namespaces(7)) in the proc
file system.
uts_ns
Inode number of the current UTS namespace (see uts_namespaces(7)) in the proc file
system.
Credential context fields (see credentials(7))
vuid
Virtual real user ID: real user ID as seen from the point of view of the current
user namespace (see user_namespaces(7)).
vgid
Virtual real group ID: real group ID as seen from the point of view of the current
user namespace (see user_namespaces(7)).
veuid
Virtual effective user ID: effective user ID as seen from the point of view of the
current user namespace (see user_namespaces(7)).
vegid
Virtual effective group ID: effective group ID as seen from the point of view of
the current user namespace (see user_namespaces(7)).
vsuid
Virtual saved set-user ID: saved set-user ID as seen from the point of view of the
current user namespace (see user_namespaces(7)).
vsgid
Virtual saved set-group ID: saved set-group ID as seen from the point of view of
the current user namespace (see user_namespaces(7)).
LTTng-UST state dump
If an application that uses liblttng-ust becomes part of a tracing session, information
about its currently loaded shared objects, their build IDs, and their debug link
information are emitted as events by the tracer.
The following LTTng-UST state dump events exist and must be enabled to record application
state dumps. Note that, during the state dump phase, LTTng-UST can also emit shared
library load/unload events (see Shared library load/unload tracking below).
lttng_ust_statedump:start
Emitted when the state dump begins.
This event has no fields.
lttng_ust_statedump:end
Emitted when the state dump ends. Once this event is emitted, it is guaranteed that,
for a given process, the state dump is complete.
This event has no fields.
lttng_ust_statedump:bin_info
Emitted when information about a currently loaded executable or shared object is
found.
Fields:
┌───────────────┬──────────────────────────────────┐
│Field name │ Description │
├───────────────┼──────────────────────────────────┤
│baddr │ Base address of loaded │
│ │ executable. │
├───────────────┼──────────────────────────────────┤
│memsz │ Size of loaded executable in │
│ │ memory. │
├───────────────┼──────────────────────────────────┤
│path │ Path to loaded executable file. │
├───────────────┼──────────────────────────────────┤
│is_pic │ Whether or not the executable is │
│ │ position-independent code. │
├───────────────┼──────────────────────────────────┤
│has_build_id │ Whether or not the executable │
│ │ has a build ID. If this field is │
│ │ 1, you can expect that an │
│ │ lttng_ust_statedump:build_id │
│ │ event record follows this one │
│ │ (not necessarily immediately │
│ │ after). │
├───────────────┼──────────────────────────────────┤
│has_debug_link │ Whether or not the executable │
│ │ has debug link information. If │
│ │ this field is 1, you can expect │
│ │ that an │
│ │ lttng_ust_statedump:debug_link │
│ │ event record follows this one │
│ │ (not necessarily immediately │
│ │ after). │
└───────────────┴──────────────────────────────────┘
lttng_ust_statedump:build_id
Emitted when a build ID is found in a currently loaded shared library. See Debugging
Information in Separate Files <https://sourceware.org/gdb/onlinedocs/gdb/Separate-
Debug-Files.html> for more information about build IDs.
Fields:
┌───────────┬─────────────────────────────────┐
│Field name │ Description │
├───────────┼─────────────────────────────────┤
│baddr │ Base address of loaded library. │
├───────────┼─────────────────────────────────┤
│build_id │ Build ID. │
└───────────┴─────────────────────────────────┘
lttng_ust_statedump:debug_link
Emitted when debug link information is found in a currently loaded shared library. See
Debugging Information in Separate Files
<https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html> for more
information about debug links.
Fields:
┌───────────┬─────────────────────────────────┐
│Field name │ Description │
├───────────┼─────────────────────────────────┤
│baddr │ Base address of loaded library. │
├───────────┼─────────────────────────────────┤
│crc │ Debug link file’s CRC. │
├───────────┼─────────────────────────────────┤
│filename │ Debug link file name. │
└───────────┴─────────────────────────────────┘
lttng_ust_statedump:procname
The process procname at process start.
Fields:
┌───────────┬───────────────────┐
│Field name │ Description │
├───────────┼───────────────────┤
│procname │ The process name. │
└───────────┴───────────────────┘
Shared library load/unload tracking
The LTTng-UST state dump and the LTTng-UST helper library to instrument the dynamic linker
(see liblttng-ust-dl(3)) can emit shared library load/unload tracking events.
The following shared library load/unload tracking events exist and must be enabled to
track the loading and unloading of shared libraries:
lttng_ust_lib:load
Emitted when a shared library (shared object) is loaded.
Fields:
┌───────────────┬──────────────────────────────────┐
│Field name │ Description │
├───────────────┼──────────────────────────────────┤
│baddr │ Base address of loaded library. │
├───────────────┼──────────────────────────────────┤
│memsz │ Size of loaded library in │
│ │ memory. │
├───────────────┼──────────────────────────────────┤
│path │ Path to loaded library file. │
├───────────────┼──────────────────────────────────┤
│has_build_id │ Whether or not the library has a │
│ │ build ID. If this field is 1, │
│ │ you can expect that an │
│ │ lttng_ust_lib:build_id event │
│ │ record follows this one (not │
│ │ necessarily immediately after). │
├───────────────┼──────────────────────────────────┤
│has_debug_link │ Whether or not the library has │
│ │ debug link information. If this │
│ │ field is 1, you can expect that │
│ │ an lttng_ust_lib:debug_link │
│ │ event record follows this one │
│ │ (not necessarily immediately │
│ │ after). │
└───────────────┴──────────────────────────────────┘
lttng_ust_lib:unload
Emitted when a shared library (shared object) is unloaded.
Fields:
┌───────────┬──────────────────────────┐
│Field name │ Description │
├───────────┼──────────────────────────┤
│baddr │ Base address of unloaded │
│ │ library. │
└───────────┴──────────────────────────┘
lttng_ust_lib:build_id
Emitted when a build ID is found in a loaded shared library (shared object). See
Debugging Information in Separate Files
<https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html> for more
information about build IDs.
Fields:
┌───────────┬─────────────────────────────────┐
│Field name │ Description │
├───────────┼─────────────────────────────────┤
│baddr │ Base address of loaded library. │
├───────────┼─────────────────────────────────┤
│build_id │ Build ID. │
└───────────┴─────────────────────────────────┘
lttng_ust_lib:debug_link
Emitted when debug link information is found in a loaded shared library (shared
object). See Debugging Information in Separate Files
<https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html> for more
information about debug links.
Fields:
┌───────────┬─────────────────────────────────┐
│Field name │ Description │
├───────────┼─────────────────────────────────┤
│baddr │ Base address of loaded library. │
├───────────┼─────────────────────────────────┤
│crc │ Debug link file’s CRC. │
├───────────┼─────────────────────────────────┤
│filename │ Debug link file name. │
└───────────┴─────────────────────────────────┘
Detect if LTTng-UST is loaded
To detect if liblttng-ust is loaded from an application:
1. Define the lttng_ust_loaded weak symbol globally:
int lttng_ust_loaded __attribute__((weak));
This weak symbol is set by the constructor of liblttng-ust.
2. Test lttng_ust_loaded where needed:
/* ... */
if (lttng_ust_loaded) {
/* LTTng-UST is loaded */
} else {
/* LTTng-UST is NOT loaded */
}
/* ... */
EXAMPLE
Note
A few examples are available in the directory of LTTng-UST’s source tree.
This example shows all the features documented in the previous sections. The static
linking method is chosen here to link the application with the tracepoint provider.
You can compile the source files and link them together statically like this:
$ cc -c -I. tp.c
$ cc -c app.c
$ cc -o app tp.o app.o -llttng-ust -ldl
Using the lttng(1) tool, create an LTTng tracing session, enable all the events of this
tracepoint provider, and start tracing:
$ lttng create my-session
$ lttng enable-event --userspace 'my_provider:*'
$ lttng start
You may also enable specific events:
$ lttng enable-event --userspace my_provider:big_event
$ lttng enable-event --userspace my_provider:event_instance2
Run the application:
$ ./app some arguments
Stop the current tracing session and inspect the recorded events:
$ lttng stop
$ lttng view
Tracepoint provider header file
tp.h:
#undef LTTNG_UST_TRACEPOINT_PROVIDER
#define LTTNG_UST_TRACEPOINT_PROVIDER my_provider
#undef LTTNG_USTTRACEPOINT_INCLUDE
#define LTTNG_USTTRACEPOINT_INCLUDE "./tp.h"
#if !defined(_TP_H) || \
defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H
#include <lttng/tracepoint.h>
#include <stdio.h>
#include "app.h"
LTTNG_UST_TRACEPOINT_EVENT(
my_provider,
simple_event,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
const char *, my_string_arg
),
LTTNG_UST_TP_FIELDS(
lttng_ust_field_string(argc, my_string_arg)
lttng_ust_field_integer(int, argv, my_integer_arg)
)
)
LTTNG_UST_TRACEPOINT_ENUM(
my_provider,
my_enum,
LTTNG_UST_TP_ENUM_VALUES(
lttng_ust_field_enum_value("ZERO", 0)
lttng_ust_field_enum_value("ONE", 1)
lttng_ust_field_enum_value("TWO", 2)
lttng_ust_field_enum_range("A RANGE", 52, 125)
lttng_ust_field_enum_value("ONE THOUSAND", 1000)
)
)
LTTNG_UST_TRACEPOINT_EVENT(
my_provider,
big_event,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
const char *, my_string_arg,
FILE *, stream,
double, flt_arg,
int *, array_arg
),
LTTNG_UST_TP_FIELDS(
lttng_ust_field_integer(int, int_field1, my_integer_arg * 2)
lttng_ust_field_integer_hex(long int, stream_pos,
ftell(stream))
lttng_ust_field_float(double, float_field, flt_arg)
lttng_ust_field_string(string_field, my_string_arg)
lttng_ust_field_array(int, array_field, array_arg, 7)
lttng_ust_field_array_text(char, array_text_field,
array_arg, 5)
lttng_ust_field_sequence(int, seq_field, array_arg, int,
my_integer_arg / 10)
lttng_ust_field_sequence_text(char, seq_text_field,
array_arg, int,
my_integer_arg / 5)
lttng_ust_field_enum(my_provider, my_enum, int,
enum_field, array_arg[1])
)
)
LTTNG_UST_TRACEPOINT_LOGLEVEL(my_provider, big_event,
LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING)
LTTNG_UST_TRACEPOINT_EVENT_CLASS(
my_provider,
my_tracepoint_class,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
),
LTTNG_UST_TP_FIELDS(
lttng_ust_field_integer(int, a, my_integer_arg)
lttng_ust_field_integer(unsigned long, b, app_struct_arg->b)
lttng_ust_field_string(c, app_struct_arg->c)
)
)
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
my_provider,
event_instance1,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
)
)
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
my_provider,
event_instance2,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
)
)
LTTNG_UST_TRACEPOINT_LOGLEVEL(my_provider, event_instance2,
LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO)
LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
my_provider,
event_instance3,
LTTNG_UST_TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
)
)
#endif /* _TP_H */
#include <lttng/tracepoint-event.h>
Tracepoint provider source file
tp.c:
#define LTTNG_UST_TRACEPOINT_CREATE_PROBES
#define LTTNG_UST_TRACEPOINT_DEFINE
#include "tp.h"
Application header file
app.h:
#ifndef _APP_H
#define _APP_H
struct app_struct {
unsigned long b;
const char *c;
double d;
};
#endif /* _APP_H */
Application source file
app.c:
#include <stdlib.h>
#include <stdio.h>
#include "tp.h"
#include "app.h"
static int array_of_ints[] = {
100, -35, 1, 23, 14, -6, 28, 1001, -3000,
};
int main(int argc, char* argv[])
{
FILE *stream;
struct app_struct app_struct;
lttng_ust_tracepoint(my_provider, simple_event, argc, argv[0]);
stream = fopen("/tmp/app.txt", "w");
if (!stream) {
fprintf(stderr,
"Error: Cannot open /tmp/app.txt for writing\n");
return EXIT_FAILURE;
}
if (fprintf(stream, "0123456789") != 10) {
fclose(stream);
fprintf(stderr, "Error: Cannot write to /tmp/app.txt\n");
return EXIT_FAILURE;
}
lttng_ust_tracepoint(my_provider, big_event, 35,
"hello tracepoint", stream, -3.14,
array_of_ints);
fclose(stream);
app_struct.b = argc;
app_struct.c = "[the string]";
lttng_ust_tracepoint(my_provider, event_instance1, 23,
&app_struct);
app_struct.b = argc * 5;
app_struct.c = "[other string]";
lttng_ust_tracepoint(my_provider, event_instance2, 17,
&app_struct);
app_struct.b = 23;
app_struct.c = "nothing";
lttng_ust_tracepoint(my_provider, event_instance3, -52,
&app_struct);
return EXIT_SUCCESS;
}
ENVIRONMENT VARIABLES
LTTNG_HOME
Alternative user’s home directory. This variable is useful when the user running the
instrumented application has a non-writable home directory.
Unix sockets used for the communication between liblttng-ust and the LTTng session and
consumer daemons (part of the LTTng-tools project) are located in a specific directory
under $LTTNG_HOME (or $HOME if $LTTNG_HOME is not set).
LTTNG_UST_ALLOW_BLOCKING
If set, allow the application to retry event tracing when there’s no space left for
the event record in the sub-buffer, therefore effectively blocking the application
until space is made available or the configured timeout is reached.
To allow an application to block during tracing, you also need to specify a blocking
timeout when you create a channel with the --blocking-timeout option of the lttng-
enable-channel(1) command.
This option can be useful in workloads generating very large trace data throughput,
where blocking the application is an acceptable trade-off to prevent discarding event
records.
Warning
Setting this environment variable may significantly affect application timings.
LTTNG_UST_ABORT_ON_CRITICAL
If set, abort the instrumented application on a critical error message.
LTTNG_UST_CLOCK_PLUGIN
Path to the shared object which acts as the clock override plugin. An example of such
a plugin can be found in the LTTng-UST documentation under
LTTNG_UST_DEBUG
If set, enable liblttng-ust's debug and error output.
LTTNG_UST_GETCPU_PLUGIN
Path to the shared object which acts as the getcpu() override plugin. An example of
such a plugin can be found in the LTTng-UST documentation under
LTTNG_UST_REGISTER_TIMEOUT
Waiting time for the registration done session daemon command before proceeding to
execute the main program (milliseconds).
The value 0 means do not wait. The value -1 means wait forever. Setting this
environment variable to 0 is recommended for applications with time constraints on the
process startup time.
Default: 3000.
LTTNG_UST_WITHOUT_BADDR_STATEDUMP
If set, prevents liblttng-ust from performing a base address state dump (see the
LTTng-UST state dump section above).
LTTNG_UST_WITHOUT_PROCNAME_STATEDUMP
If set, prevents liblttng-ust from performing a procname state dump (see the LTTng-UST
state dump section above).
BUGS
If you encounter any issue or usability problem, please report it on the LTTng bug tracker
<https://bugs.lttng.org/projects/lttng-ust>.
RESOURCES
• LTTng project website <http://lttng.org>
• LTTng documentation <http://lttng.org/docs>
• Git repositories <http://git.lttng.org>
• GitHub organization <http://github.com/lttng>
• Continuous integration <http://ci.lttng.org/>
• Mailing list <http://lists.lttng.org> for support and development: lttng-
dev@lists.lttng.org
• IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net
COPYRIGHTS
This library is part of the LTTng-UST project.
This library is distributed under the GNU Lesser General Public License, version 2.1
<http://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>. See the for more details.
THANKS
Thanks to Ericsson for funding this work, providing real-life use cases, and testing.
Special thanks to Michel Dagenais and the DORSAL laboratory
<http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal for the LTTng journey.
AUTHORS
LTTng-UST was originally written by Mathieu Desnoyers, with additional contributions from
various other people. It is currently maintained by Mathieu Desnoyers
<mailto:mathieu.desnoyers@efficios.com>.
SEE ALSO
lttng_ust_tracef(3), lttng_ust_tracelog(3), lttng-gen-tp(1), lttng-ust-dl(3), lttng-ust-
cyg-profile(3), lttng(1), lttng-enable-event(1), lttng-list(1), lttng-add-context(1),
babeltrace(1), dlopen(3), ld.so(8)