Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all 
NAME
erl_tracer - Erlang tracer behavior.
DESCRIPTION
This behavior module implements the back end of the Erlang tracing system. The functions in this module
are called whenever a trace probe is triggered. Both the enabled and trace functions are called in the
context of the entity that triggered the trace probe. This means that the overhead by having the tracing
enabled is greatly effected by how much time is spent in these functions. So, do as little work as
possible in these functions.
Note:
All functions in this behavior must be implemented as NIFs. This limitation can be removed in a future
releases. An example tracer module NIF implementation is provided at the end of this page.
Warning:
Do not send messages or issue port commands to the Tracee in any of the callbacks. This is not allowed
and can cause all sorts of strange behavior, including, but not limited to, infinite recursions.
DATA TYPES
trace_tag_call() =
call | return_to | return_from | exception_from
trace_tag_gc() =
gc_minor_start | gc_minor_end | gc_major_start | gc_major_end
trace_tag_ports() =
open |
closed |
link |
unlink |
getting_linked |
getting_unlinked
trace_tag_procs() =
spawn |
spawned |
exit |
link |
unlink |
getting_linked |
getting_unlinked |
register |
unregister
trace_tag_receive() = 'receive'
trace_tag_running_ports() =
in | out | in_exiting | out_exiting | out_exited
trace_tag_running_procs() =
in | out | in_exiting | out_exiting | out_exited
trace_tag_send() = send | send_to_non_existing_process
trace_tag() =
trace_tag_send() |
trace_tag_receive() |
trace_tag_call() |
trace_tag_procs() |
trace_tag_ports() |
trace_tag_running_procs() |
trace_tag_running_ports() |
trace_tag_gc()
The different trace tags that the tracer is called with. Each trace tag is described in detail in
Module:trace/5.
tracee() = port() | pid() | undefined
The process or port that the trace belongs to.
trace_opts() =
#{extra => term(),
match_spec_result => term(),
scheduler_id => integer() >= 0,
timestamp =>
timestamp | cpu_timestamp | monotonic | strict_monotonic}
The options for the tracee:
timestamp:
If set the tracer has been requested to include a time stamp.
extra:
If set the tracepoint has included additional data about the trace event. What the additional
data is depends on which TraceTag has been triggered. The extra trace data corresponds to the
fifth element in the trace tuples described in erlang:trace/3.
match_spec_result:
If set the tracer has been requested to include the output of a match specification that was
run.
scheduler_id:
If set the scheduler id is to be included by the tracer.
tracer_state() = term()
The state specified when calling erlang:trace(PidPortSpec,true,[{tracer,Module,TracerState}]). The
tracer state is an immutable value that is passed to erl_tracer callbacks and is to contain all
the data that is needed to generate the trace event.
CALLBACK FUNCTIONS
The following functions are to be exported from an erl_tracer callback module:
Module:enabled/3:
Mandatory
Module:trace/5:
Mandatory
Module:enabled_call/3:
Optional
Module:trace_call/5:
Optional
Module:enabled_garbage_collection/3:
Optional
Module:trace_garbage_collection/5:
Optional
Module:enabled_ports/3:
Optional
Module:trace_ports/5:
Optional
Module:enabled_procs/3:
Optional
Module:trace_procs/5:
Optional
Module:enabled_receive/3:
Optional
Module:trace_receive/5:
Optional
Module:enabled_running_ports/3:
Optional
Module:trace_running_ports/5:
Optional
Module:enabled_running_procs/3:
Optional
Module:trace_running_procs/5:
Optional
Module:enabled_send/3:
Optional
Module:trace_send/5:
Optional
EXPORTS
Module:enabled(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag() | trace_status
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint is triggered. It allows the tracer to decide whether
a trace is to be generated or not. This check is made as early as possible to limit the amount of
overhead associated with tracing. If trace is returned, the necessary trace data is created and
the trace callback of the tracer is called. If discard is returned, this trace call is discarded
and no call to trace is done.
trace_status is a special type of TraceTag, which is used to check if the tracer is still to be
active. It is called in multiple scenarios, but most significantly it is used when tracing is
started using this tracer. If remove is returned when the trace_status is checked, the tracer is
removed from the tracee.
This function can be called multiple times per tracepoint, so it is important that it is both fast
and without side effects.
Module:enabled_call(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_call()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag call | return_to is triggered.
If enabled_call/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_garbage_collection(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_gc()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag garbage_collection is triggered.
If enabled_garbage_collection/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_ports(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_ports()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag ports is triggered.
If enabled_ports/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_procs(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_procs()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag procs is triggered.
If enabled_procs/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_receive(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_receive()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag 'receive' is triggered.
If enabled_receive/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_running_ports(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_running_ports()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag running_ports is triggered.
If enabled_running_ports/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_running_procs(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_running_procs()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag running_procs | running is
triggered.
If enabled_running_procs/3 is undefined, Module:enabled/3 is called instead.
Module:enabled_send(TraceTag, TracerState, Tracee) -> Result
Types:
TraceTag = trace_tag_send()
TracerState = term()
Tracee = tracee()
Result = trace | discard | remove
This callback is called whenever a tracepoint with trace flag send is triggered.
If enabled_send/3 is undefined, Module:enabled/3 is called instead.
Module:trace(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled/3 callback returned
trace. In it any side effects needed by the tracer are to be done. The tracepoint payload is
located in the TraceTerm. The content of the TraceTerm depends on which TraceTag is triggered.
TraceTerm corresponds to the fourth element in the trace tuples described in erlang:trace/3.
If the trace tuple has five elements, the fifth element will be sent as the extra value in the
Opts maps.
Module:trace(seq_trace, TracerState, Label, SeqTraceInfo, Opts) -> Result
Types:
TracerState = term()
Label = term()
SeqTraceInfo = term()
Opts = trace_opts()
Result = ok
The TraceTag seq_trace is handled slightly differently. There is no Tracee for seq_trace, instead
the Label associated with the seq_trace event is specified.
For more information on what Label and SeqTraceInfo can be, see seq_trace(3erl).
Module:trace_call(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_call()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_call/3 callback
returned trace.
If trace_call/5 is undefined, Module:trace/5 is called instead.
Module:trace_garbage_collection(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_gc()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_garbage_collection/3
callback returned trace.
If trace_garbage_collection/5 is undefined, Module:trace/5 is called instead.
Module:trace_ports(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_ports/3 callback
returned trace.
If trace_ports/5 is undefined, Module:trace/5 is called instead.
Module:trace_procs(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_procs/3 callback
returned trace.
If trace_procs/5 is undefined, Module:trace/5 is called instead.
Module:trace_receive(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_receive()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_receive/3 callback
returned trace.
If trace_receive/5 is undefined, Module:trace/5 is called instead.
Module:trace_running_ports(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_running_ports()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_running_ports/3
callback returned trace.
If trace_running_ports/5 is undefined, Module:trace/5 is called instead.
Module:trace_running_procs(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_running_procs()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_running_procs/3
callback returned trace.
If trace_running_procs/5 is undefined, Module:trace/5 is called instead.
Module:trace_send(TraceTag, TracerState, Tracee, TraceTerm, Opts) -> Result
Types:
TraceTag = trace_tag_send()
TracerState = term()
Tracee = tracee()
TraceTerm = term()
Opts = trace_opts()
Result = ok
This callback is called when a tracepoint is triggered and the Module:enabled_send/3 callback
returned trace.
If trace_send/5 is undefined, Module:trace/5 is called instead.
ERL TRACER MODULE EXAMPLE
In this example, a tracer module with a NIF back end sends a message for each send trace tag containing
only the sender and receiver. Using this tracer module, a much more lightweight message tracer is used,
which only records who sent messages to who.
The following is an example session using it on Linux:
$ gcc -I erts-8.0/include/ -fPIC -shared -o erl_msg_tracer.so erl_msg_tracer.c
$ erl
Erlang/OTP 19 [DEVELOPMENT] [erts-8.0] [source-ed2b56b] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V8.0 (abort with ^G)
1> c(erl_msg_tracer), erl_msg_tracer:load().
ok
2> Tracer = spawn(fun F() -> receive M -> io:format("~p~n",[M]), F() end end).
<0.37.0>
3> erlang:trace(new, true, [send,{tracer, erl_msg_tracer, Tracer}]).
0
{trace,<0.39.0>,<0.27.0>}
4> {ok, D} = file:open("/tmp/tmp.data",[write]).
{trace,#Port<0.486>,<0.40.0>}
{trace,<0.40.0>,<0.21.0>}
{trace,#Port<0.487>,<0.4.0>}
{trace,#Port<0.488>,<0.4.0>}
{trace,#Port<0.489>,<0.4.0>}
{trace,#Port<0.490>,<0.4.0>}
{ok,<0.40.0>}
{trace,<0.41.0>,<0.27.0>}
5>
erl_msg_tracer.erl:
-module(erl_msg_tracer).
-export([enabled/3, trace/5, load/0]).
load() ->
erlang:load_nif("erl_msg_tracer", []).
enabled(_, _, _) ->
error.
trace(_, _, _, _, _) ->
error.
erl_msg_tracer.c:
#include <erl_nif.h>
/* NIF interface declarations */
static int load(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info);
static int upgrade(ErlNifEnv* env, void** priv_data, void** old_priv_data, ERL_NIF_TERM load_info);
static void unload(ErlNifEnv* env, void* priv_data);
/* The NIFs: */
static ERL_NIF_TERM enabled(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[]);
static ERL_NIF_TERM trace(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[]);
static ErlNifFunc nif_funcs[] = {
{"enabled", 3, enabled},
{"trace", 5, trace}
};
ERL_NIF_INIT(erl_msg_tracer, nif_funcs, load, NULL, upgrade, unload)
static int load(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)
{
*priv_data = NULL;
return 0;
}
static void unload(ErlNifEnv* env, void* priv_data)
{
}
static int upgrade(ErlNifEnv* env, void** priv_data, void** old_priv_data,
ERL_NIF_TERM load_info)
{
if (*old_priv_data != NULL || *priv_data != NULL) {
return -1; /* Don't know how to do that */
}
if (load(env, priv_data, load_info)) {
return -1;
}
return 0;
}
/*
* argv[0]: TraceTag
* argv[1]: TracerState
* argv[2]: Tracee
*/
static ERL_NIF_TERM enabled(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
ErlNifPid to_pid;
if (enif_get_local_pid(env, argv[1], &to_pid))
if (!enif_is_process_alive(env, &to_pid))
if (enif_is_identical(enif_make_atom(env, "trace_status"), argv[0]))
/* tracer is dead so we should remove this tracepoint */
return enif_make_atom(env, "remove");
else
return enif_make_atom(env, "discard");
/* Only generate trace for when tracer != tracee */
if (enif_is_identical(argv[1], argv[2]))
return enif_make_atom(env, "discard");
/* Only trigger trace messages on 'send' */
if (enif_is_identical(enif_make_atom(env, "send"), argv[0]))
return enif_make_atom(env, "trace");
/* Have to answer trace_status */
if (enif_is_identical(enif_make_atom(env, "trace_status"), argv[0]))
return enif_make_atom(env, "trace");
return enif_make_atom(env, "discard");
}
/*
* argv[0]: TraceTag, should only be 'send'
* argv[1]: TracerState, process to send {Tracee, Recipient} to
* argv[2]: Tracee
* argv[3]: Message
* argv[4]: Options, map containing Recipient
*/
static ERL_NIF_TERM trace(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
ErlNifPid to_pid;
ERL_NIF_TERM recipient, msg;
if (enif_get_local_pid(env, argv[1], &to_pid)) {
if (enif_get_map_value(env, argv[4], enif_make_atom(env, "extra"), &recipient)) {
msg = enif_make_tuple3(env, enif_make_atom(env, "trace"), argv[2], recipient);
enif_send(env, &to_pid, NULL, msg);
}
}
return enif_make_atom(env, "ok");
}