redbug

Copyright © 2004-2020 mats cronqvist

Version: 1.2.2

Authors: mats cronqvist {github.com/massemanet}.

SYNOPSIS

start in non-blocking mode

       redbug:start(Trc,[Opts]) -> {integer(NoOfProcs),integer(NoOfFuncs)} |
                                {atom(ErrorType),term(ErrorReason)}

start in blocking mode

       redbug:start(Trc,[Opts]) -> {atom(StopReason),list(TraceMessages)} |
                                {atom(ErrorType),term(ErrorReason)}

stop

       redbug:stop() -> stopped | not_started

help

       redbug:help() -> ok

DESCRIPTION

redbug is a tool to interact with the Erlang trace facility. It will instruct the Erlang VM to generate so-called 'trace messages' when certain events (such as a particular function being called) occur. It uses a safe subset of the tracing functionality, and exits if it feels overloaded, e.g. if it gets flooded by trace messages. It runs in the background, collecting trace messages, until it reaches one of its termination criteria (number of messages/file size or elapsed time). The trace messages are either printed (i.e. human readable) to a file or to the screen; or written to a trc file. Using a trc file puts less stress on the system, but there is no way to count the messages (so the 'msgs' opt is ignored), and the files can only be read by special tools (such as 'bread'). Printing and trc files cannot be combined. By default (i.e. if the 'file' opt is not given), messages are printed.

Trace types

       list('send'|'receive'|string(RTP))

where RTP (Redbug Trace Pattern) has the form:

       "MFA when GUARDS -> ACTIONS"

where MFA can be;

       "mod", "mod:fun", "mod:fun/3" or "mod:fun('_',atom,X)"

GUARD is something like;

       "X==1" or "is_atom(A)"

and ACTION is;

       "return" and/or "stack" (separated by ",")

So, an RTP looks something like this;

       "ets:lookup(T,hostname) when is_integer(T) -> stack"

Note that bindings (like we're binding 'T' above) works as expected. So this RTP;

       "maps:to_list(#{a:=T,c:=#{d:=T}})"

will not trigger on this call;

      maps:to_list(#{a=>b,c=>#{d=>e,f=>g}}).

but will trigger on this;

       maps:to_list(#{a=>b,c=>#{d=>b,f=>g}}).

Options

   Opts: list({Opt,Val}) | #{Opt => Val}
general opts:
    time         (15000)       stop trace after this many milliseconds
    msgs         (10)          stop trace after this many messages
    target       (node())      node to trace on
    cookie       ('')          target node cookie
    blocking     (false)       block start/2, return a list of messages
    procs        (all)         atom(all|new|running) | list(Proc)
                               where Proc is pid() | atom(RegName) | {pid,I2,I3}
    max_queue    (5000)        exit if redbug-internal queue gets this long
    max_msg_size (50000)       exit if seeing a message bigger than this
    debug        (false)       bigger error messages
print-related opts
    arity        (false)       print arity instead of argument list
    buffered     (no)          buffer messages till end of trace
    print_pid    (false)       print pid instead of registered name
    print_calls  (true)        print calls
    print_file   (standard_io) print to this file
    print_msec   (false)       print milliseconds on time stamps
    print_depth  (999999)      formatting depth for "~P"
    print_re     ("")          print only strings that match this regexp
    print_return (true)        print return value
    print_fun    ('')          custom print fun. gets called once for each trace
                               message. It can be a fun/1, (called as F(Msg),
                               return value is ignored), or a fun/2 (called as
                               F(Msg,Acc), return is next Acc)

trc file related opts

    file         (none)        use a trc file based on this name
    file_size    (1)           size of each trc file
    file_count   (8)           number of trc files

EXAMPLES

Basic call trace
    1> redbug:start("erlang:demonitor").
    {30,2}
    15:39:00 <{erlang,apply,2}> {erlang,demonitor,[#Ref<0.0.0.21493>]}
    15:39:00 <{erlang,apply,2}> {erlang,demonitor,[#Ref<0.0.0.21499>]}
    15:39:00 <{erlang,apply,2}> {erlang,demonitor,[#Ref<0.0.0.21500>]}
    redbug done, timeout - 3
As above, print pids
    2> redbug:start("erlang:demonitor",[{msgs,1},print_pid]).
    {30,2}
    15:42:04 <0.31.0> {erlang,demonitor,[#Ref<0.0.0.21616>]}
    redbug done, msg_count - 1
As above, print return value. The return value is a separate message.
    3> redbug:start("erlang:demonitor->return",[{msgs,2},print_pid]).
    {30,2}
    15:43:22 <0.31.0> {erlang,demonitor,[#Ref<0.0.0.21677>]}
    15:43:22 <0.31.0> erlang:demonitor/1 -> true
    redbug done, msg_count - 2
As above, also print the call stack. Note that not all functions in the call chain are on the stack, only functions we will return to (this is a consequence of tail call optimization.)
    4> redbug:start("erlang:demonitor->return,stack",[{msgs,2},print_pid]).
    {30,2}
    15:44:35 <0.31.0> {erlang,demonitor,[#Ref<0.0.0.21726>]}
      shell:eval_loop/3
      shell:eval_exprs/7
      shell:exprs/7
      erl_eval:do_apply/6
      redbug:start/2
    15:44:35 <0.31.0> erlang:demonitor/1 -> true
    redbug done, msg_count - 2
Trace on messages that the shell process receives.
    5> redbug:start('receive',[{procs,[self()]}]).
    {1,0}
    15:15:47 <{erlang,apply,2}> <<< {running,1,0}
    15:17:49 <{erlang,apply,2}> <<< timeout
    redbug done, timeout - 2
As above, but also trace on sends from the shell process. note that in this case the 'print_pid' opt would hide that there is a send to the group server.
    7> redbug:start([send,'receive'],[{procs,[self()]}]).
    {1,0}
    15:36:36 <{erlang,apply,2}> <<< {running,1,0}
    15:36:36 <{erlang,apply,2}> <{group,server,3}> <<< {io_request,<0.31.0>,
                                                        <0.24.0>,
                                                        {get_geometry,columns}}
    redbug done, timeout - 2
Call trace with a function head match. Note that the first call to ets:tab2list/1 does not trigger the tracer.
    8> redbug:start("ets:tab2list(inet_db)",[{msgs,2},print_pid]).
    {30,1}
    9> ets:tab2list(ac_tab),ok.
    ok
    10> ets:tab2list(inet_db),ok.
    ok
    15:47:15 <0.31.0> {ets,tab2list,[inet_db]}
    redbug done, timeout - 1
As above, but use the 'blocking' opt. redbug:start/2 blocks until end of trace, and returns the stop reason and a list of trace messages.
    10> spawn(fun()->receive after 2000->ets:tab2list(inet_db) end end).
    <0.540.0>
    11> redbug:start("ets:tab2list(inet_db)",[blocking]).
    {timeout,[{call,{{ets,tab2list,[inet_db]},<<>>},
                    {<0.540.0>,{erlang,apply,2}},
                    {15,50,43,776041}}]}

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