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LTTNG-UST(3) LTTng Manual LTTNG-UST(3)
lttng-ust - LTTng user space tracing
#include <lttng/tracepoint.h>
#define TRACEPOINT_ENUM(prov_name, enum_name, mappings)
#define TRACEPOINT_EVENT(prov_name, t_name, args, fields)
#define TRACEPOINT_EVENT_CLASS(prov_name, class_name, args, fields)
#define TRACEPOINT_EVENT_INSTANCE(prov_name, class_name, t_name, args)
#define TRACEPOINT_LOGLEVEL(prov_name, t_name, level)
#define ctf_array(int_type, field_name, expr, count)
#define ctf_array_nowrite(int_type, field_name, expr, count)
#define ctf_array_hex(int_type, field_name, expr, count)
#define ctf_array_nowrite_hex(int_type, field_name, expr, count)
#define ctf_array_network(int_type, field_name, expr, count)
#define ctf_array_network_nowrite(int_type, field_name, expr, count)
#define ctf_array_network_hex(int_type, field_name, expr, count)
#define ctf_array_network_nowrite_hex(int_type, field_name, expr, count)
#define ctf_array_text(char, field_name, expr, count)
#define ctf_array_text_nowrite(char, field_name, expr, count)
#define ctf_enum(prov_name, enum_name, int_type, field_name, expr)
#define ctf_enum_nowrite(prov_name, enum_name, int_type, field_name,
expr)
#define ctf_enum_value(label, value)
#define ctf_enum_range(label, start, end)
#define ctf_float(float_type, field_name, expr)
#define ctf_float_nowrite(float_type, field_name, expr)
#define ctf_integer(int_type, field_name, expr)
#define ctf_integer_hex(int_type, field_name, expr)
#define ctf_integer_network(int_type, field_name, expr)
#define ctf_integer_network_hex(int_type, field_name, expr)
#define ctf_integer_nowrite(int_type, field_name, expr)
#define ctf_sequence(int_type, field_name, expr, len_type, len_expr)
#define ctf_sequence_nowrite(int_type, field_name, expr, len_type,
len_expr)
#define ctf_sequence_hex(int_type, field_name, expr, len_type,
len_expr)
#define ctf_sequence_nowrite_hex(int_type, field_name, expr, len_type,
len_expr)
#define ctf_sequence_network(int_type, field_name, expr, len_type,
len_expr)
#define ctf_sequence_network_nowrite(int_type, field_name, expr,
len_type, len_expr)
#define ctf_sequence_network_hex(int_type, field_name, expr, len_type,
len_expr)
#define ctf_sequence_network_nowrite_hex(int_type, field_name, expr,
len_type, len_expr)
#define ctf_sequence_text(char, field_name, expr, len_type, len_expr)
#define ctf_sequence_text_nowrite(char, field_name, expr, len_type,
len_expr)
#define ctf_string(field_name, expr)
#define ctf_string_nowrite(field_name, expr)
#define do_tracepoint(prov_name, t_name, ...)
#define tracepoint(prov_name, t_name, ...)
#define tracepoint_enabled(prov_name, t_name)
Link with -llttng-ust -ldl, following this man page.
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 tracef(3) API, which is similar to printf(3).
· Using the tracelog(3) API, which is tracef(3) with a log level
parameter.
· Defining your own tracepoints. See the Creating a tracepoint
provider section below.
Creating a tracepoint provider
Creating a tracepoint provider is the first step of using liblttng-
ust. The next steps are:
· Instrumenting your application with 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 TRACEPOINT_PROVIDER
#define TRACEPOINT_PROVIDER my_provider
#undef TRACEPOINT_INCLUDE
#define TRACEPOINT_INCLUDE "./tp.h"
#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H
#include <lttng/tracepoint.h>
/*
* TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
* TRACEPOINT_EVENT_INSTANCE(), TRACEPOINT_LOGLEVEL(),
* and `TRACEPOINT_ENUM()` are used here.
*/
#endif /* _TP_H */
#include <lttng/tracepoint-event.h>
In this template, the tracepoint provider is named my_provider
(TRACEPOINT_PROVIDER definition). The file needs to bear the name of
the TRACEPOINT_INCLUDE definition (tp.h in this case). Between
#include <lttng/tracepoint.h> and #endif go the invocations of the
TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
TRACEPOINT_EVENT_INSTANCE(), TRACEPOINT_LOGLEVEL(), and
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 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 TRACEPOINT_EVENT() usage section below shows how to use the
TRACEPOINT_EVENT() macro to define the actual tracepoints in the
tracepoint provider header file.
See the EXAMPLE section below for a complete example.
TRACEPOINT_EVENT() usage
The 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 TRACEPOINT_EVENT() usage template is as follows:
TRACEPOINT_EVENT(
/* Tracepoint provider name */
my_provider,
/* Tracepoint/event name */
my_tracepoint,
/* List of tracepoint arguments (input) */
TP_ARGS(
...
),
/* List of fields of eventual event (output) */
TP_FIELDS(
...
)
)
The 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 TP_FIELDS().
The format of the TP_ARGS() parameters is: C type, then argument
name; repeat as needed, up to ten times. For example:
TP_ARGS(
int, my_int,
const char *, my_string,
FILE *, my_file,
double, my_float,
struct my_data *, my_data
)
The 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 TP_FIELDS() macro contains a list of ctf_*() macros NOT separated
by commas. The available macros are documented in the Available
ctf_*() field type macros section below.
Available ctf_*() field type macros
This section documents the available ctf_*() macros that can be
inserted in the TP_FIELDS() macro of the TRACEPOINT_EVENT() macro.
Standard integer, displayed in base 10:
ctf_integer(int_type, field_name, expr)
ctf_integer_nowrite(int_type, field_name, expr)
Standard integer, displayed in base 16:
ctf_integer_hex(int_type, field_name, expr)
Integer in network byte order (big endian), displayed in base 10:
ctf_integer_network(int_type, field_name, expr)
Integer in network byte order, displayed in base 16:
ctf_integer_network_hex(int_type, field_name, expr)
Floating point number:
ctf_float(float_type, field_name, expr)
ctf_float_nowrite(float_type, field_name, expr)
Null-terminated string:
ctf_string(field_name, expr)
ctf_string_nowrite(field_name, expr)
Statically-sized array of integers (_hex versions displayed in
hexadecimal, _network versions in network byte order):
ctf_array(int_type, field_name, expr, count)
ctf_array_nowrite(int_type, field_name, expr, count)
ctf_array_hex(int_type, field_name, expr, count)
ctf_array_nowrite_hex(int_type, field_name, expr, count)
ctf_array_network(int_type, field_name, expr, count)
ctf_array_network_nowrite(int_type, field_name, expr, count)
ctf_array_network_hex(int_type, field_name, expr, count)
ctf_array_network_nowrite_hex(int_type, field_name, expr, count)
Statically-sized array, printed as text; no need to be
null-terminated:
ctf_array_text(char, field_name, expr, count)
ctf_array_text_nowrite(char, field_name, expr, count)
Dynamically-sized array of integers (_hex versions displayed in
hexadecimal, _network versions in network byte order):
ctf_sequence(int_type, field_name, expr, len_type, len_expr)
ctf_sequence_nowrite(int_type, field_name, expr, len_type, len_expr)
ctf_sequence_hex(int_type, field_name, expr, len_type, len_expr)
ctf_sequence_nowrite_hex(int_type, field_name, expr, len_type,
len_expr)
ctf_sequence_network(int_type, field_name, expr, len_type, len_expr)
ctf_sequence_network_nowrite(int_type, field_name, expr, len_type,
len_expr)
ctf_sequence_network_hex(int_type, field_name, expr, len_type,
len_expr)
ctf_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:
ctf_sequence_text(char, field_name, expr, len_type, len_expr)
ctf_sequence_text_nowrite(char, field_name, expr, len_type, len_expr)
Enumeration. The enumeration field must be defined before using this
macro with the TRACEPOINT_ENUM() macro. See the TRACEPOINT_ENUM()
usage section for more information.
ctf_enum(prov_name, enum_name, int_type, field_name, expr)
ctf_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
TRACEPOINT_ENUM() macro. See the 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 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.
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 TRACEPOINT_ENUM() macro:
TRACEPOINT_ENUM(
/* Tracepoint provider name */
my_provider,
/* Enumeration name (unique in the whole tracepoint provider) */
my_enum,
/* Enumeration mappings */
TP_ENUM_VALUES(
...
)
)
TP_ENUM_VALUES() contains a list of enumeration mappings, NOT
separated by commas. Two macros can be used in the TP_ENUM_VALUES():
ctf_enum_value() and ctf_enum_range().
ctf_enum_value() is a single value mapping:
ctf_enum_value(label, value)
This macro maps the given label string to the value value.
ctf_enum_range() is a range mapping:
ctf_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.
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 TRACEPOINT_EVENT() creates one tracepoint class per
tracepoint definition, whereas using TRACEPOINT_EVENT_CLASS() and
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 TRACEPOINT_EVENT_CLASS() macro accepts the same parameters as the
TRACEPOINT_EVENT() macro, except that instead of an event name, its
second parameter is the tracepoint class name:
TRACEPOINT_EVENT_CLASS(
/* Tracepoint provider name */
my_provider,
/* Tracepoint class name */
my_tracepoint_class,
/* List of tracepoint arguments (input) */
TP_ARGS(
...
),
/* List of fields of eventual event (output) */
TP_FIELDS(
...
)
)
Once the tracepoint class is defined, you can create as many
tracepoint instances as needed:
TRACEPOINT_EVENT_INSTANCE(
/* Tracepoint provider name */
my_provider,
/* Tracepoint class name */
my_tracepoint_class,
/* Tracepoint/event name */
my_tracepoint,
/* List of tracepoint arguments (input) */
TP_ARGS(
...
)
)
As you can see, the TRACEPOINT_EVENT_INSTANCE() does not contain the
TP_FIELDS() macro, because they are defined at the
TRACEPOINT_EVENT_CLASS() level.
See the EXAMPLE section below for a complete example.
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 TRACEPOINT_LOGLEVEL() macro. The latter must be used after having
used TRACEPOINT_EVENT() or TRACEPOINT_EVENT_INSTANCE() for a given
tracepoint. The TRACEPOINT_LOGLEVEL() macro is used as follows:
TRACEPOINT_LOGLEVEL(
/* Tracepoint provider name */
my_provider,
/* Tracepoint/event name */
my_tracepoint,
/* Log level */
TRACE_INFO
)
The available log level definitions are:
TRACE_EMERG
System is unusable.
TRACE_ALERT
Action must be taken immediately.
TRACE_CRIT
Critical conditions.
TRACE_ERR
Error conditions.
TRACE_WARNING
Warning conditions.
TRACE_NOTICE
Normal, but significant, condition.
TRACE_INFO
Informational message.
TRACE_DEBUG_SYSTEM
Debug information with system-level scope (set of programs).
TRACE_DEBUG_PROGRAM
Debug information with program-level scope (set of processes).
TRACE_DEBUG_PROCESS
Debug information with process-level scope (set of modules).
TRACE_DEBUG_MODULE
Debug information with module (executable/library) scope (set of
units).
TRACE_DEBUG_UNIT
Debug information with compilation unit scope (set of functions).
TRACE_DEBUG_FUNCTION
Debug information with function-level scope.
TRACE_DEBUG_LINE
Debug information with line-level scope (default log level).
TRACE_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 tracepoint()
macro:
#define 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 tracepoint() for this provider.
Note
Even though LTTng-UST supports 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 tracepoint_enabled() and
do_tracepoint() macros:
#define tracepoint_enabled(prov_name, t_name)
#define do_tracepoint(prov_name, t_name, ...)
tracepoint_enabled() returns a non-zero value if the tracepoint named
t_name from the provider named prov_name is enabled at run time.
do_tracepoint() is like tracepoint(), except that it doesn’t check if
the tracepoint is enabled. Using tracepoint() with
tracepoint_enabled() is dangerous since tracepoint() also contains
the tracepoint_enabled() check, thus a race condition is possible in
this situation:
if (tracepoint_enabled(my_provider, my_tracepoint)) {
stuff = prepare_stuff();
}
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 tracepoint_enabled() nor 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 TRACEPOINT_DEFINE definition below the
TRACEPOINT_CREATE_PROBES definition in the tracepoint provider
source:
#define TRACEPOINT_CREATE_PROBES
#define 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,
TRACEPOINT_DEFINE must be defined, for each tracepoint provider,
in exactly one source file of the application
· TRACEPOINT_PROBE_DYNAMIC_LINKAGE must be defined next to
TRACEPOINT_DEFINE
Regarding TRACEPOINT_DEFINE and 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 TRACEPOINT_DEFINE
#define 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:
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.
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.
pthread_id
POSIX thread identifier. Can be used on architectures where
pthread_t maps nicely to an unsigned long type.
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 process namespace.
vtid
Virtual thread ID: thread ID as seen from the point of view of
the process namespace.
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. │
└───────────┴────────────────────────┘
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 */
}
/* ... */
Note
A few examples are available in the doc/examples
<https://github.com/lttng/lttng-ust/tree/master/doc/examples>
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 TRACEPOINT_PROVIDER
#define TRACEPOINT_PROVIDER my_provider
#undef TRACEPOINT_INCLUDE
#define TRACEPOINT_INCLUDE "./tp.h"
#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
#define _TP_H
#include <lttng/tracepoint.h>
#include <stdio.h>
#include "app.h"
TRACEPOINT_EVENT(
my_provider,
simple_event,
TP_ARGS(
int, my_integer_arg,
const char *, my_string_arg
),
TP_FIELDS(
ctf_string(argc, my_string_arg)
ctf_integer(int, argv, my_integer_arg)
)
)
TRACEPOINT_ENUM(
my_provider,
my_enum,
TP_ENUM_VALUES(
ctf_enum_value("ZERO", 0)
ctf_enum_value("ONE", 1)
ctf_enum_value("TWO", 2)
ctf_enum_range("A RANGE", 52, 125)
ctf_enum_value("ONE THOUSAND", 1000)
)
)
TRACEPOINT_EVENT(
my_provider,
big_event,
TP_ARGS(
int, my_integer_arg,
const char *, my_string_arg,
FILE *, stream,
double, flt_arg,
int *, array_arg
),
TP_FIELDS(
ctf_integer(int, int_field1, my_integer_arg * 2)
ctf_integer_hex(long int, stream_pos, ftell(stream))
ctf_float(double, float_field, flt_arg)
ctf_string(string_field, my_string_arg)
ctf_array(int, array_field, array_arg, 7)
ctf_array_text(char, array_text_field, array_arg, 5)
ctf_sequence(int, seq_field, array_arg, int,
my_integer_arg / 10)
ctf_sequence_text(char, seq_text_field, array_arg,
int, my_integer_arg / 5)
ctf_enum(my_provider, my_enum, int,
enum_field, array_arg[1])
)
)
TRACEPOINT_LOGLEVEL(my_provider, big_event, TRACE_WARNING)
TRACEPOINT_EVENT_CLASS(
my_provider,
my_tracepoint_class,
TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
),
TP_FIELDS(
ctf_integer(int, a, my_integer_arg)
ctf_integer(unsigned long, b, app_struct_arg->b)
ctf_string(c, app_struct_arg->c)
)
)
TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
event_instance1,
TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
)
)
TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
event_instance2,
TP_ARGS(
int, my_integer_arg,
struct app_struct *, app_struct_arg
)
)
TRACEPOINT_LOGLEVEL(my_provider, event_instance2, TRACE_INFO)
TRACEPOINT_EVENT_INSTANCE(
my_provider,
my_tracepoint_class,
event_instance3,
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 TRACEPOINT_CREATE_PROBES
#define 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;
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;
}
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]";
tracepoint(my_provider, event_instance1, 23, &app_struct);
app_struct.b = argc * 5;
app_struct.c = "[other string]";
tracepoint(my_provider, event_instance2, 17, &app_struct);
app_struct.b = 23;
app_struct.c = "nothing";
tracepoint(my_provider, event_instance3, -52, &app_struct);
return EXIT_SUCCESS;
}
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 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 timeout is reached. This only has effect on channels
with blocking-timeout set.
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_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 examples/clock-override
<https://github.com/lttng/lttng-
ust/tree/master/doc/examples/clock-override>.
LTTNG_UST_DEBUG
Activates liblttng-ust's debug and error output if set to 1.
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 examples/getcpu-override
<https://github.com/lttng/lttng-
ust/tree/master/doc/examples/getcpu-override>.
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
Prevents liblttng-ust from performing a base address state dump
(see the LTTng-UST state dump section above) if set to 1.
If you encounter any issue or usability problem, please report it on
the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-ust>.
· 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
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 COPYING
<https://github.com/lttng/lttng-ust/blob/master/COPYING> file for
more details.
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.
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>.
tracef(3), 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)
This page is part of the LTTng-UST ( LTTng Userspace Tracer)
project. Information about the project can be found at
⟨http://lttng.org/⟩. It is not known how to report bugs for this man
page; if you know, please send a mail to man-pages@man7.org. This
page was obtained from the project's upstream Git repository
⟨git://git.lttng.org/lttng-ust.git⟩ on 2017-07-05. If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
LTTng 2.10.0-rc1 07/05/2017 LTTNG-UST(3)
Pages that refer to this page: lttng(1), lttng-crash(1), lttng-gen-tp(1), lttng-health-check(3), lttng-ust-cyg-profile(3), lttng-ust-dl(3), tracef(3), tracelog(3), lttng-relayd(8), lttng-sessiond(8)