NAME¶

lttng-ust — Linux Trace Toolkit Next Generation User-Space Tracer

SYNOPSIS¶

Link liblttng-ust.so with applications, following this manpage.

DESCRIPTION¶

LTTng-UST, the Linux Trace Toolkit Next Generation Userspace Tracer, is port of the low-overhead tracing capabilities of the LTTng kernel tracer to user-space. The library "liblttng-ust" enables tracing of applications and libraries.

USAGE¶

The simple way to generate the lttng-ust tracepoint probes is to use the lttng-gen-tp(1) tool. See the lttng-gen-tp(1) manpage for explanation. Here is the way to do it manually, without the lttng-gen-tp(1) helper script, through an example:

CREATION OF TRACEPOINT PROVIDER¶

To create a tracepoint provider, within a build tree similar to
examples/easy-ust installed with lttng-ust documentation, a
sample_component_provider.h for the general layout. This manpage will
focus on the various types that can be recorded into a trace event:

TRACEPOINT_EVENT(
	/*
	 * provider name, not a variable but a string starting with a
	 * letter and containing either letters, numbers or underscores. 
	 * Needs to be the same as TRACEPOINT_PROVIDER. Needs to
	 * follow the namespacing guide-lines in lttng/tracepoint.h:
	 * 
	 * Must be included before include tracepoint provider 
	 * ex.: project_event
	 * ex.: project_component_event
	 *
	 * Optional company name goes here
	 *  ex.: com_efficios_project_component_event
	 *
	 * In this example, "sample" is the project, and "component" is the
	 * component.
	 */
	sample_component,

	/*
	 * tracepoint name, same format as sample provider. Does not
	 * need to be declared before. in this case the name is
	 * "message" 
	 */
	message,

	/*
	 * TP_ARGS macro contains the arguments passed for the tracepoint 
	 * it is in the following format
	 *	      TP_ARGS(type1, name1, type2, name2, ... type10,
				 name10)
	 * where there can be from zero to ten elements. 
	 * typeN is the datatype, such as int, struct or double **. 
	 * name is the variable name (in "int myInt" the name would be
	 * myint) 
	 *	      TP_ARGS() is valid to mean no arguments
	 *	      TP_ARGS(void) is valid too
	 */
	TP_ARGS(int, anint, int, netint, long *, values,
		 char *, text, size_t, textlen,
		 double, doublearg, float, floatarg),

	/*
	 * TP_FIELDS describes how to write the fields of the trace event. 
	 * You can put expressions in the "argument expression" area,
	 * typically using the input arguments from TP_ARGS.
	 */
	TP_FIELDS(
		/*
		 * ctf_integer: standard integer field.
		 * args: (type, field name, argument expression)
		 */
		ctf_integer(int, intfield, anint)
		ctf_integer(long, longfield, anint)

		/*
		 * ctf_integer_hex: integer field printed as hexadecimal.
		 * args: (type, field name, argument expression)
		 */
		ctf_integer_hex(int, intfield2, anint)

		/*
		 * ctf_integer_network: integer field in network byte
		 * order. (_hex: printed as hexadecimal too)
		 * args: (type, field name, argument expression)
		 */
		ctf_integer_network(int, netintfield, netint)
		ctf_integer_network_hex(int, netintfieldhex, netint)

		/*
		 * ctf_array: a statically-sized array.
		 * args: (type, field name, argument expression, value)
		 */ 
		ctf_array(long, arrfield1, values, 3)

		/*
		 * ctf_array_text: a statically-sized array, printed as
		 * a string. No need to be terminated by a null
		 * character.
		 */ 
		ctf_array_text(char, arrfield2, text, 10)

		/*
		 * ctf_sequence: a dynamically-sized array.
		 * args: (type, field name, argument expression,
		 *	type of length expression, length expression)
		 */ 
		ctf_sequence(char, seqfield1, text,
			     size_t, textlen)

		/*
		 * ctf_sequence_text: a dynamically-sized array, printed
		 * as string. No need to be null-terminated.
		 */
		ctf_sequence_text(char, seqfield2, text,
			     size_t, textlen)

		/*
		 * ctf_string: null-terminated string.
		 * args: (field name, argument expression)
		 */
		ctf_string(stringfield, text)

		/*
		 * ctf_float: floating-point number.
		 * args: (type, field name, argument expression)
		 */
		ctf_float(float, floatfield, floatarg)
		ctf_float(double, doublefield, doublearg)
	)
)

ASSIGNING LOGLEVEL TO EVENTS¶

Optionally, a loglevel can be assigned to a TRACEPOINT_EVENT using the
following construct:

	TRACEPOINT_LOGLEVEL(< [com_company_]project[_component] >,
		< event >, < loglevel_name >)

 The first field is the provider name, the second field is the name of
the tracepoint, and the third field is the loglevel name.  A
TRACEPOINT_EVENT should be declared prior to the the TRACEPOINT_LOGLEVEL
for a given tracepoint name. The TRACEPOINT_PROVIDER must be already
declared before declaring a TRACEPOINT_LOGLEVEL.

The loglevels go from 0 to 14. Higher numbers imply the most verbosity
(higher event throughput expected.
  
Loglevels 0 through 6, and loglevel 14, match syslog(3) loglevels
semantic. Loglevels 7 through 13 offer more fine-grained selection of
debug information.
  
   TRACE_EMERG           0
   system is unusable
  
   TRACE_ALERT           1
   action must be taken immediately
  
   TRACE_CRIT            2
   critical conditions
  
   TRACE_ERR             3
   error conditions
  
   TRACE_WARNING         4
   warning conditions
  
   TRACE_NOTICE          5
   normal, but significant, condition
  
   TRACE_INFO            6
   informational message
  
   TRACE_DEBUG_SYSTEM    7
   debug information with system-level scope (set of programs)
  
   TRACE_DEBUG_PROGRAM   8
   debug information with program-level scope (set of processes)
  
   TRACE_DEBUG_PROCESS   9
   debug information with process-level scope (set of modules)
  
   TRACE_DEBUG_MODULE    10
   debug information with module (executable/library) scope (set of
   units)
  
   TRACE_DEBUG_UNIT      11
   debug information with compilation unit scope (set of functions)
  
   TRACE_DEBUG_FUNCTION  12
   debug information with function-level scope
  
   TRACE_DEBUG_LINE      13
   debug information with line-level scope (TRACEPOINT_EVENT default)
  
   TRACE_DEBUG           14
   debug-level message (trace_printf default)

See lttng(1) for information on how to use LTTng-UST loglevels.

ADDING TRACEPOINTS TO YOUR CODE¶

Include the provider header in each C files you plan to instrument,
following the building/linking directives in the next section.

For instance, add within a function:

		tracepoint(ust_tests_hello, tptest, i, netint, values,
			text, strlen(text), dbl, flt);

As a call to the tracepoint. It will only be activated when requested by
lttng(1) through lttng-sessiond(8).

BUILDING/LINKING THE TRACEPOINT PROVIDER¶

There are 2 ways to compile the Tracepoint Provider with the
application: either statically or dynamically. Please follow
carefully:

  1.1) Compile the Tracepoint provider with the application, either
       directly or through a static library (.a):
    - Into exactly one object of your application: define
      "TRACEPOINT_DEFINE" and include the tracepoint provider.
    - Use "-I." for the compilation unit containing the tracepoint
      provider include (e.g. tp.c).
    - Link application with "-ldl".
    - If building the provider directly into the application,
      link the application with "-llttng-ust".
    - If building a static library for the provider, link the static
      library with "-lllttng-ust".
    - Include the tracepoint provider header into all C files using
      the provider.
    - Example:
        tests/hello/  hello.c tp.c ust_tests_hello.h Makefile.example

  2) Compile the Tracepoint Provider separately from the application,
     using dynamic linking:
    - Into exactly one object of your application: define
      "TRACEPOINT_DEFINE" _and_ also define
      "TRACEPOINT_PROBE_DYNAMIC_LINKAGE", then include the tracepoint
      provider header.
    - Include the tracepoint provider header into all instrumented C
      files that use the provider.
    - Compile the tracepoint provider with "-I.".
    - Link the tracepoint provider with "-llttng-ust".
    - Link application with "-ldl".
    - Set a LD_PRELOAD environment to preload the tracepoint provider
      shared object before starting the application when tracing is
      needed.
    - Example:
      - tests/demo/   demo.c  tp*.c ust_tests_demo*.h demo-trace

  - Note about dlopen() usage: due to locking side-effects due to the
    way libc lazily resolves Thread-Local Storage (TLS) symbols when a
    library is dlopen'd, linking the tracepoint probe or liblttng-ust
    with dlopen() is discouraged. They should be linked with the
    application using "-llibname" or loaded with LD_PRELOAD.
  - Enable instrumentation and control tracing with the "lttng" command
    from lttng-tools. See lttng-tools doc/quickstart.txt.
  - Note for C++ support: although an application instrumented with
    tracepoints can be compiled with g++, tracepoint probes should be
    compiled with gcc (only tested with gcc so far).

ENVIRONMENT VARIABLES¶

LTTNG_UST_DEBUG

Activate liblttng-ust debug output.

LTTNG_UST_REGISTER_TIMEOUT

The environment variable "LTTNG_UST_REGISTER_TIMEOUT" can be used to specify how long the applications should wait for sessiond "registration done" command before proceeding to execute the main program. The default is 3000ms (3 seconds). The timeout value is specified in milliseconds. The value 0 means "don't 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.

SEE ALSO¶

lttng-gen-tp(1), lttng(1), babeltrace(1), lttng-sessiond(8)

BUGS¶

No knows bugs at this point. If you encounter any issues or usability problem, please report it on our mailing list <lttng-dev@lists.lttng.org> to help improve this project.

CREDITS¶

liblttng-ust is distributed under the GNU Lesser General Public License version 2.1. The headers are distributed under the MIT license. See http://lttng.org for more information on the LTTng project. Mailing list for support and development: <lttng-dev@lists.lttng.org>. You can find us on IRC server irc.oftc.net (OFTC) in #lttng.

THANKS¶

Thanks to Ericsson for funding this work, providing real-life use-cases, and testing. Special thanks to Michel Dagenais and the DORSAL laboratory at Polytechnique de Montreal for the LTTng journey.

AUTHORS¶

liblttng-ust was originally written by Mathieu Desnoyers, with additional contributions from various other people. It is currently maintained by Mathieu Desnoyers <mathieu.desnoyers@efficios.com>.