path: root/erts/doc/src/erl_tracer.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2016</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>erl_tracer</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>erl_tracer</module>
  <modulesummary>Erlang tracer behavior.</modulesummary>
  <description>
    <p>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 <c>enabled</c> and <c>trace</c>
      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.</p>

    <note>
      <p>All functions in this behavior must be implemented as NIFs.
        This limitation can be removed in a future releases.
        An <seealso marker="#example">example tracer module NIF</seealso>
        implementation is provided at the end of this page.</p>
    </note>

    <warning>
      <p>Do not send messages or issue port commands to the <c>Tracee</c>
        in any of the callbacks. This is not allowed and can cause all
        sorts of strange behavior, including, but not limited to, infinite
        recursions.</p>
    </warning>
  </description>

  <datatypes>
    <datatype>
      <name name="trace_tag_call"/>
    </datatype>
    <datatype>
      <name name="trace_tag_gc"/>
    </datatype>
    <datatype>
      <name name="trace_tag_ports"/>
    </datatype>
    <datatype>
      <name name="trace_tag_procs"/>
    </datatype>
    <datatype>
      <name name="trace_tag_receive"/>
    </datatype>
    <datatype>
      <name name="trace_tag_running_ports"/>
    </datatype>
    <datatype>
      <name name="trace_tag_running_procs"/>
    </datatype>
    <datatype>
      <name name="trace_tag_send"/>
    </datatype>
    <datatype>
      <name name="trace_tag"/>
      <desc>
        <p>The different trace tags that the tracer is called with.
          Each trace tag is described in detail in
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="tracee"/>
      <desc>
        <p>The process or port that the trace belongs to.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="trace_opts"/>
      <desc>
        <p>The options for the tracee:</p>
        <taglist>
          <tag><c>timestamp</c></tag>
          <item>If set the tracer has been requested to include a
            time stamp.</item>
          <tag><c>extra</c></tag>
          <item>If set the tracepoint has included additional data about
            the trace event. What the additional data is depends on which
            <c>TraceTag</c> has been triggered. The <c>extra</c> trace data
            corresponds to the fifth element in the trace tuples described in
            <seealso marker="erlang#trace_3_trace_messages">
              erlang:trace/3</seealso>.</item>
          <tag><c>match_spec_result</c></tag>
          <item>If set the tracer has been requested to include the output
            of a match specification that was run.</item>
          <tag><c>scheduler_id</c></tag>
          <item>If set the scheduler id is to be included by the tracer.</item>
        </taglist>
      </desc>
    </datatype>
    <datatype>
      <name name="tracer_state"/>
      <desc>
        <p>The state specified when calling
          <seealso marker="erlang#trace-3">
          <c>erlang:trace(PidPortSpec,true,[{tracer,Module,TracerState}])</c></seealso>.
          The tracer state is an immutable value that is passed to
          <c>erl_tracer</c> callbacks and is to
          contain all the data that is needed to generate the trace event.</p>
      </desc>
    </datatype>
  </datatypes>

  <section>
    <title>Callback Functions</title>
    <p>The following functions are to be exported from an <c>erl_tracer</c>
      callback module:</p>

    <taglist>
      <tag><seealso marker="#Module:enabled/3">
        <c>Module:enabled/3</c></seealso></tag>
      <item>Mandatory</item>
      <tag><seealso marker="#Module:trace/5">
        <c>Module:trace/5</c></seealso></tag>
      <item>Mandatory</item>
      <tag><seealso marker="#Module:enabled_call/3">
        <c>Module:enabled_call/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_call/5">
        <c>Module:trace_call/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_garbage_collection/3">
        <c>Module:enabled_garbage_collection/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_garbage_collection/5">
        <c>Module:trace_garbage_collection/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_ports/3">
        <c>Module:enabled_ports/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_ports/5">
        <c>Module:trace_ports/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_procs/3">
        <c>Module:enabled_procs/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_procs/5">
        <c>Module:trace_procs/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_receive/3">
        <c>Module:enabled_receive/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_receive/5">
        <c>Module:trace_receive/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_running_ports/3">
        <c>Module:enabled_running_ports/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_running_ports/5">
        <c>Module:trace_running_ports/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_running_procs/3">
        <c>Module:enabled_running_procs/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_running_procs/5">
        <c>Module:trace_running_procs/5</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:enabled_send/3">
        <c>Module:enabled_send/3</c></seealso></tag>
      <item>Optional</item>
      <tag><seealso marker="#Module:trace_send/5">
        <c>Module:trace_send/5</c></seealso></tag>
      <item>Optional</item>
   </taglist>
  </section>

  <funcs>
    <func>
      <name>Module:enabled(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag">
          trace_tag()</seealso> | trace_status</v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>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 <c>trace</c> is returned, the
          necessary trace data is created and the trace callback of the tracer
          is called. If <c>discard</c> is returned, this trace call is
          discarded and no call to trace is done.</p>
        <p><c>trace_status</c> is a special type of <c>TraceTag</c>, 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 <c>remove</c> is returned when the
          <c>trace_status</c> is checked, the tracer is removed from the
          tracee.</p>
        <p>This function can be called multiple times per tracepoint, so it
          is important that it is both fast and without side effects.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_call(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_call">
          trace_tag_call()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>call | return_to</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_call/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_garbage_collection(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_gc">
          trace_tag_gc()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>garbage_collection</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_garbage_collection/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_ports(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_ports">
          trace_tag_ports()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>ports</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_ports/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_procs(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_procs">
          trace_tag_procs()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>procs</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_procs/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_receive(TraceTag, TracerState, Tracee) -> Result
      </name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_receive">
          trace_tag_receive()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>'receive'</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_receive/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_running_ports(TraceTag, TracerState, Tracee) ->
        Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_running_ports">
          trace_tag_running_ports()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>running_ports</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_running_ports/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_running_procs(TraceTag, TracerState, Tracee) ->
        Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_running_procs">
          trace_tag_running_procs()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3">
          <c>running_procs | running</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_running_procs/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:enabled_send(TraceTag, TracerState, Tracee) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_send">
          trace_tag_send()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>Result = trace | discard | remove</v>
      </type>
      <desc>
        <p>This callback is called whenever a tracepoint with trace flag
          <seealso marker="erlang#trace-3"><c>send</c></seealso>
          is triggered.</p>
        <p>If <c>enabled_send/3</c> is undefined,
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace(TraceTag, TracerState, Tracee, TraceTerm,
        Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag">
          trace_tag()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled/3"><c>Module:enabled/3</c></seealso>
          callback returned <c>trace</c>. In it any side effects needed by
          the tracer are to be done. The tracepoint payload is located in
          the <c>TraceTerm</c>. The content of the <c>TraceTerm</c>
          depends on which <c>TraceTag</c> is triggered.
          <c>TraceTerm</c> corresponds to the
          fourth element in the trace tuples described in
          <seealso marker="erlang#trace_3_trace_messages">
          <c>erlang:trace/3</c></seealso>.</p>
        <p>If the trace tuple has five elements, the fifth element
          will be sent as the <c>extra</c> value in the <c>Opts</c> maps.</p>
      </desc>
    </func>

    <func>
      <name name="trace">Module:trace(seq_trace, TracerState, Label,
        SeqTraceInfo, Opts) -> Result</name>
      <fsummary>Check if a sequence trace event is to be generated.</fsummary>
      <type>
        <v>TracerState = term()</v>
        <v>Label = term()</v>
        <v>SeqTraceInfo = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>The <c>TraceTag</c> <c>seq_trace</c> is handled slightly
          differently. There is no <c>Tracee</c> for <c>seq_trace</c>, instead
          the <c>Label</c> associated with the <c>seq_trace</c> event is
          specified.</p>
        <p>For more information on what <c>Label</c> and <c>SeqTraceInfo</c>
          can be, see <seealso marker="kernel:seq_trace">
          <c>seq_trace(3)</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_call(TraceTag, TracerState, Tracee, TraceTerm,
          Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_call">
          trace_tag_call()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_call/3">
          <c>Module:enabled_call/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_call/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_garbage_collection(TraceTag, TracerState, Tracee,
        TraceTerm, Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_gc">
          trace_tag_gc()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_garbage_collection/3">
          <c>Module:enabled_garbage_collection/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_garbage_collection/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_ports(TraceTag, TracerState, Tracee, TraceTerm,
        Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_ports">
          trace_tag()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_ports/3">
          <c>Module:enabled_ports/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_ports/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_procs(TraceTag, TracerState, Tracee, TraceTerm,
        Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_procs">
          trace_tag()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_procs/3">
          <c>Module:enabled_procs/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_procs/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_receive(TraceTag, TracerState, Tracee, TraceTerm,
        Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_receive">
          trace_tag_receive()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_receive/3">
          <c>Module:enabled_receive/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_receive/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_running_ports(TraceTag, TracerState, Tracee,
        TraceTerm, Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_running_ports">
          trace_tag_running_ports()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_running_ports/3">
          <c>Module:enabled_running_ports/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_running_ports/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_running_procs(TraceTag, TracerState, Tracee,
        TraceTerm, Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_running_procs">
          trace_tag_running_procs()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_running_procs/3">
          <c>Module:enabled_running_procs/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_running_procs/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>

    <func>
      <name>Module:trace_send(TraceTag, TracerState, Tracee, TraceTerm,
        Opts) -> Result</name>
      <fsummary>Check if a trace event is to be generated.</fsummary>
      <type>
        <v>TraceTag = <seealso marker="#type-trace_tag_send">
          trace_tag_send()</seealso></v>
        <v>TracerState = term()</v>
        <v>Tracee = <seealso marker="#type-tracee">tracee()</seealso></v>
        <v>TraceTerm = term()</v>
        <v>Opts = <seealso marker="#type-trace_opts">trace_opts()</seealso></v>
        <v>Result = ok</v>
      </type>
      <desc>
        <p>This callback is called when a tracepoint is triggered and the
          <seealso marker="#Module:enabled_send/3">
          <c>Module:enabled_send/3</c></seealso>
          callback returned <c>trace</c>.</p>
        <p>If <c>trace_send/5</c> is undefined,
          <seealso marker="#Module:trace/5"><c>Module:trace/5</c></seealso>
          is called instead.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="example"></marker>
    <title>Erl Tracer Module Example</title>
    <p>In this example, a tracer module with a NIF back end sends a
      message for each <c>send</c> 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.</p>

    <p>The following is an example session using it on Linux:</p>

    <pre>
$ 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&gt; c(erl_msg_tracer), erl_msg_tracer:load().
ok
2&gt; Tracer = spawn(fun F() -&gt; receive M -&gt; io:format("~p~n",[M]), F() end end).
&lt;0.37.0&gt;
3&gt; erlang:trace(new, true, [send,{tracer, erl_msg_tracer, Tracer}]).
0
{&lt;0.39.0&gt;,&lt;0.27.0&gt;}
4&gt; {ok, D} = file:open("/tmp/tmp.data",[write]).
{trace,#Port&lt;0.486&gt;,&lt;0.40.0&gt;}
{trace,&lt;0.40.0&gt;,&lt;0.21.0&gt;}
{trace,#Port&lt;0.487&gt;,&lt;0.4.0&gt;}
{trace,#Port&lt;0.488&gt;,&lt;0.4.0&gt;}
{trace,#Port&lt;0.489&gt;,&lt;0.4.0&gt;}
{trace,#Port&lt;0.490&gt;,&lt;0.4.0&gt;}
{ok,&lt;0.40.0&gt;}
{trace,&lt;0.41.0&gt;,&lt;0.27.0&gt;}
5&gt;</pre>

    <p><c>erl_msg_tracer.erl</c>:</p>

    <pre>
-module(erl_msg_tracer).

-export([enabled/3, trace/5, load/0]).

load() ->
    erlang:load_nif("erl_msg_tracer", []).

enabled(_, _, _) ->
    error.

trace(_, _, _, _, _) ->
    error.</pre>

    <p><c>erl_msg_tracer.c</c>:</p>

    <pre>
#include &lt;erl_nif.h&gt;

/* 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], &amp;to_pid))
        if (!enif_is_process_alive(env, &amp;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 {argv[2], argv[4]} to
 * argv[2]: Tracee
 * argv[3]: Recipient
 * argv[4]: Options, ignored
 */
static ERL_NIF_TERM trace(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    ErlNifPid to_pid;

    if (enif_get_local_pid(env, argv[1], &amp;to_pid)) {
        ERL_NIF_TERM msg = enif_make_tuple3(env, enif_make_atom(env, "trace"), argv[2], argv[4]);
        enif_send(env, &amp;to_pid, NULL, msg);
    }

    return enif_make_atom(env, "ok");
}</pre>
  </section>
</erlref>