Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all bug

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");
       }