LTTNG-UST(3)

NAME
lttng-ust - LTTng user space tracing

SYNOPSIS

#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.

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 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 */
        }
        /* ... */

EXAMPLE

    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;
    }

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_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
    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 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
    If set, prevents liblttng-ust from performing a base address 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 COPYING <https://github.com/lttng/lttng-ust/blob/master/COPYING> file 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

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)