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       dbg - The Text Based Trace Facility


       This module implements a text based interface to the trace/3 and the trace_pattern/2 BIFs.
       It makes it possible to trace functions, processes and messages on text  based  terminals.
       It can be used instead of, or as complement to, the pman module.

       For some examples of how to use dbg from the Erlang shell, see the simple example section.

       The  utilities  are  also  suitable to use in system testing on large systems, where other
       tools have too  much  impact  on  the  system  performance.  Some  primitive  support  for
       sequential tracing is also included, see the advanced topics section.


       fun2ms(LiteralFun) -> MatchSpec


                 LiteralFun = fun() literal
                 MatchSpec = term()

              Pseudo  function  that  by  means  of a parse_transform translates the literalfun()
              typed as parameter in the function call to a match specification  as  described  in
              the  match_spec  manual  of  ERTS  users guide. (with literal I mean that the fun()
              needs to textually be written as the parameter of the function, it cannot  be  held
              in a variable which in turn is passed to the function).

              The  parse  transform is implemented in the module ms_transform and the source must
              include the file ms_transform.hrl in STDLIB  for  this  pseudo  function  to  work.
              Failing to include the hrl file in the source will result in a runtime error, not a
              compile time ditto. The include  file  is  easiest  included  by  adding  the  line
              -include_lib("stdlib/include/ms_transform.hrl"). to the source file.

              The  fun()  is  very restricted, it can take only a single parameter (the parameter
              list to match), a sole variable or a list. It needs to use the is_XXX  guard  tests
              and  one cannot use language constructs that have no representation in a match_spec
              (like if, case, receive etc). The return value from the  fun  will  be  the  return
              value of the resulting match_spec.


              1> dbg:fun2ms(fun([M,N]) when N > 3 -> return_trace() end).

              Variables from the environment can be imported, so that this works:

              2> X=3.
              3> dbg:fun2ms(fun([M,N]) when N > X -> return_trace() end).

              The  imported  variables will be replaced by match_spec const expressions, which is
              consistent with the static scoping for Erlang  fun()s.  Local  or  global  function
              calls  can  not  be  in  the  guard  or  body  of the fun however. Calls to builtin
              match_spec functions of course is allowed:

              4> dbg:fun2ms(fun([M,N]) when N > X, is_atomm(M) -> return_trace() end).
              Error: fun containing local erlang function calls ('is_atomm' called in guard)\
               cannot be translated into match_spec
              5> dbg:fun2ms(fun([M,N]) when N > X, is_atom(M) -> return_trace() end).

              As you can see by the example, the function can be called from the shell  too.  The
              fun()  needs  to  be  literally in the call when used from the shell as well. Other
              means than the parse_transform are used in the shell case, but  more  or  less  the
              same  restrictions  apply  (the exception being records, as they are not handled by
              the shell).

              If the parse_transform is not applied to a module which calls this pseudo function,
              the  call  will  fail in runtime (with a badarg). The module dbg actually exports a
              function with this name, but it should never really be called except for when using
              the  function in the shell. If the parse_transform is properly applied by including
              the ms_transform.hrl header file, compiled code will never call the  function,  but
              the function call is replaced by a literal match_spec.

              More information is provided by the ms_transform manual page in STDLIB.

       h() -> ok

              Gives a list of items for brief online help.

       h(Item) -> ok


                 Item = atom()

              Gives a brief help text for functions in the dbg module. The available items can be
              listed with dbg:h/0

       p(Item) -> {ok, MatchDesc} | {error, term()}

              Equivalent to p(Item, [m]).

       p(Item, Flags) -> {ok, MatchDesc} | {error, term()}


                 MatchDesc = [MatchNum]
                 MatchNum = {matched, node(), integer()} | {matched, node(), 0, RPCError}
                 RPCError = term()

              Traces Item in accordance to the value specified by Flags. The variation of Item is
              listed below:

                * If the Item is a pid(), the corresponding process is traced. The process may be
                  a remote process (on another Erlang node). The node must  be  in  the  list  of
                  traced nodes (seen/1 and tracer/0/2/3).

                * If  the  Item  is  the  atom  all,  all  processes in the system as well as all
                  processes created hereafter are to be traced. This also affects all nodes added
                  with the n/1 or tracer/0/2/3 function.

                * If  the Item is the atom new, no currently existing processes are affected, but
                  every process created after the call is.This also affects all nodes added  with
                  the n/1 or tracer/0/2/3 function.

                * If  the  Item  is the atom existing, all existing processes are traced, but new
                  processes will not be affected.This also affects all nodes added with  the  n/1
                  or tracer/0/2/3 function.

                * If  the  Item  is an atom other than all, new or existing, the process with the
                  corresponding registered name is traced.The process may be a remote process (on
                  another  Erlang  node).  The  node  must  be added with the n/1 or tracer/0/2/3

                * If the Item is an integer, the process <0.Item.0> is traced.

                * If the Item is a tuple {X, Y, Z}, the process <X.Y.Z> is traced.

                * If the Item is a string "<X.Y.Z>" as returned from pid_to_list/1,  the  process
                  <X.Y.Z> is traced.

              Flags can be a single atom, or a list of flags. The available flags are:

                s (send):
                  Traces the messages the process sends.

                r (receive):
                  Traces the messages the process receives.

                m (messages):
                  Traces the messages the process receives and sends.

                c (call):
                  Traces  global  function  calls for the process according to the trace patterns
                  set in the system (see tp/2).

                p (procs):
                  Traces process related events to the process.

                sos (set on spawn):
                  Lets all processes created by the traced process inherit the trace flags of the
                  traced process.

                sol (set on link):
                  Lets  another  process,  P2,  inherit  the  trace  flags  of the traced process
                  whenever the traced process links to P2.

                sofs (set on first spawn):
                  This is the same as sos, but only for the first process spawned by  the  traced

                sofl (set on first link):
                  This  is  the  same as sol, but only for the first call to link/1 by the traced

                  Sets all flags except silent.

                  Clears all flags.

              The list can also include any of the flags allowed in erlang:trace/3

              The function returns either an error tuple or a tuple {ok, List}. The List consists
              of  specifications  of how many processes that matched (in the case of a pure pid()
              exactly 1). The specification of matched processes is {matched, Node,  N}.  If  the
              remote  processor  call,rpc,  to  a  remote  node  fails,  the rpc error message is
              delivered as a fourth argument and the number of matched processes are 0. Note that
              the  result  {ok,  List} may contain a list where rpc calls to one, several or even
              all nodes failed.

       c(Mod, Fun, Args)

              Equivalent to c(Mod, Fun, Args, all).

       c(Mod, Fun, Args, Flags)

              Evaluates the expression apply(Mod, Fun, Args) with the trace flags in  Flags  set.
              This is a convenient way to trace processes from the Erlang shell.

       i() -> ok

              Displays information about all traced processes.


              Same as tp({Module, '_', '_'}, MatchSpec)


              Same as tp({Module, Function, '_'}, MatchSpec)

       tp(Module, Function, Arity, MatchSpec)

              Same as tp({Module, Function, Arity}, MatchSpec)

       tp({Module, Function, Arity}, MatchSpec) -> {ok, MatchDesc} | {error, term()}


                 Module = atom() | '_'
                 Function = atom() | '_'
                 Arity = integer() |'_'
                 MatchSpec = integer() | Built-inAlias | [] | match_spec()
                 Built-inAlias = x | c | cx
                 MatchDesc = [MatchInfo]
                 MatchInfo = {saved, integer()} | MatchNum
                 MatchNum = {matched, node(), integer()} | {matched, node(), 0, RPCError}

              This  function enables call trace for one or more functions. All exported functions
              matching the  {Module,  Function,  Arity}  argument  will  be  concerned,  but  the
              match_spec()  may  further  narrow  down the set of function calls generating trace

              For a description of the match_spec() syntax, please turn to the User's guide  part
              of  the  online  documentation  for  the  runtime  system (erts). The chapter Match
              Specification in Erlang explains the general match specification "language".

              The Module, Function and/or Arity parts of the tuple may be specified as  the  atom
              '_'  which  is  a  "wild-card" matching all modules/functions/arities. Note, if the
              Module is specified as '_', the Function and Arity parts have to  be  specified  as
              '_' too. The same holds for the Functions relation to the Arity.

              All  nodes  added  with  n/1  or tracer/0/2/3 will be affected by this call, and if
              Module is not '_' the module will be loaded on all nodes.

              The function returns either an error tuple or a tuple {ok, List}. The List consists
              of  specifications  of  how  many  functions  that  matched, in the same way as the
              processes are presented in the return value of p/2.

              There may be a tuple {saved, N} in the return value, if the MatchSpec is other than
              [].  The  integer  N may then be used in subsequent calls to this function and will
              stand as an "alias" for the given expression. There are also a couple  of  built-in
              aliases for common expressions, see ltp/0 below for details.

              If  an  error  is  returned,  it  can  be due to errors in compilation of the match
              specification. Such errors are presented as a  list  of  tuples  {error,  string()}
              where the string is a textual explanation of the compilation error. An example:

              (x@y)4> dbg:tp({dbg,ltp,0},[{[],[],[{message, two, arguments}, {noexist}]}]).
               [{error,"Special form 'message' called with wrong number of
                        arguments in {message,two,arguments}."},
                {error,"Function noexist/1 does_not_exist."}]}


              Same as tpl({Module, '_', '_'}, MatchSpec)


              Same as tpl({Module, Function, '_'}, MatchSpec)

       tpl(Module, Function, Arity, MatchSpec)

              Same as tpl({Module, Function, Arity}, MatchSpec)

       tpl({Module, Function, Arity}, MatchSpec) -> {ok, MatchDesc} | {error, term()}

              This  function  works  as  tp/2,  but  enables  tracing  for local calls (and local
              functions) as well as for global calls (and functions).


              Same as ctp({'_', '_', '_'})


              Same as ctp({Module, '_', '_'})

       ctp(Module, Function)

              Same as ctp({Module, Function, '_'})

       ctp(Module, Function, Arity)

              Same as ctp({Module, Function, Arity})

       ctp({Module, Function, Arity}) -> {ok, MatchDesc} | {error, term()}


                 Module = atom() | '_'
                 Function = atom() | '_'
                 Arity = integer() | '_'
                 MatchDesc = [MatchNum]
                 MatchNum = {matched, node(), integer()} | {matched, node(), 0, RPCError}

              This function disables call tracing on the specified functions.  The  semantics  of
              the  parameter  is the same as for the corresponding function specification in tp/2
              or tpl/2. Both local and global call trace is disabled.

              The return value reflects how many functions that matched, and  is  constructed  as
              described  in  tp/2.  No  tuple  {saved,  N}  is however ever returned (for obvious


              Same as ctpl({'_', '_', '_'})


              Same as ctpl({Module, '_', '_'})

       ctpl(Module, Function)

              Same as ctpl({Module, Function, '_'})

       ctpl(Module, Function, Arity)

              Same as ctpl({Module, Function, Arity})

       ctpl({Module, Function, Arity}) -> {ok, MatchDesc} | {error, term()}

              This function works as ctp/1, but only disables tracing set up with tpl/2 (not with


              Same as ctpg({'_', '_', '_'})


              Same as ctpg({Module, '_', '_'})

       ctpg(Module, Function)

              Same as ctpg({Module, Function, '_'})

       ctpg(Module, Function, Arity)

              Same as ctpg({Module, Function, Arity})

       ctpg({Module, Function, Arity}) -> {ok, MatchDesc} | {error, term()}

              This  function works as ctp/1, but only disables tracing set up with tp/2 (not with

       ltp() -> ok

              Use this function to recall all match specifications previously used in the session
              (i.  e.  previously  saved during calls to tp/2, and built-in match specifications.
              This is very useful, as a complicated match_spec can be  quite  awkward  to  write.
              Note that the match specifications are lost if stop/0 is called.

              Match  specifications  used  can be saved in a file (if a read-write file system is
              present) for use in later debugging sessions, see wtp/1 and rtp/1

              There  are  three  built-in  trace  patterns:  exception_trace,  caller_trace   and
              caller_exception_trace  (or x, c and cx respectively). Exception trace sets a trace
              which will show function names, parameters, return  values  and  exceptions  thrown
              from  functions.  Caller  traces display function names, parameters and information
              about which function called it. An example using a built-in alias:

              (x@y)4> dbg:tp(lists,sort,cx).
              (x@y)4> lists:sort([2,1]).
              (<0.32.0>) call lists:sort([2,1]) ({erl_eval,do_apply,5})
              (<0.32.0>) returned from lists:sort/1 -> [1,2]

       dtp() -> ok

              Use this function to "forget" all match specifications saved during calls to  tp/2.
              This  is  useful  when  one wants to restore other match specifications from a file
              with rtp/1. Use dtp/1 to delete specific saved match specifications.

       dtp(N) -> ok


                 N = integer()

              Use this function to "forget" a specific match specification saved during calls  to

       wtp(Name) -> ok | {error, IOError}


                 Name = string()
                 IOError = term()

              This  function  will save all match specifications saved during the session (during
              calls to tp/2) and built-in match specifications in  a  text  file  with  the  name
              designated by Name. The format of the file is textual, why it can be edited with an
              ordinary text editor, and then restored with rtp/1.

              Each match spec in the file ends with  a  full  stop  (.)  and  new  (syntactically
              correct) match specifications can be added to the file manually.

              The function returns ok or an error tuple where the second element contains the I/O
              error that made the writing impossible.

       rtp(Name) -> ok | {error, Error}


                 Name = string()
                 Error = term()

              This function reads match specifications from a file (possibly)  generated  by  the
              wtp/1  function. It checks the syntax of all match specifications and verifies that
              they are correct. The error handling principle is "all or nothing", i. e.  if  some
              of  the match specifications are wrong, none of the specifications are added to the
              list of saved match specifications for the running system.

              The  match  specifications  in  the  file  are  merged  with  the   current   match
              specifications,  so that no duplicates are generated. Use ltp/0 to see what numbers
              were assigned to the specifications from the file.

              The function will return an error, either due to I/O problems (like a non  existing
              or  non readable file) or due to file format problems. The errors from a bad format
              file are in a more or less textual format, which will give a hint to what's causing
              the problem.

       n(Nodename) -> {ok, Nodename} | {error, Reason}


                 Nodename = atom()
                 Reason = term()

              The  dbg server keeps a list of nodes where tracing should be performed. Whenever a
              tp/2 call or a p/2 call is made,  it  is  executed  for  all  nodes  in  this  list
              including  the  local node (except for p/2 with a specific pid() as first argument,
              in which case the command is executed only on the node where the designated process

              This  function  adds a remote node (Nodename) to the list of nodes where tracing is
              performed. It starts a tracer process on the remote node, which will send all trace
              messages  to the tracer process on the local node (via the Erlang distribution). If
              no tracer process is running on the local node, the error reason no_local_tracer is
              returned.  The tracer process on the local node must be started with the tracer/0/2

              If Nodename is the local node, the error reason cant_add_local_node is returned.

              If a trace port (seetrace_port/2) is running on the local node,  remote  nodes  can
              not     be     traced     with    a    tracer    process.    The    error    reason
              cant_trace_remote_pid_to_local_port is  returned.  A  trace  port  can  however  be
              started on the remote node with the tracer/3 function.

              The function will also return an error if the node Nodename is not reachable.

       cn(Nodename) -> ok


                 Nodename = atom()

              Clears  a node from the list of traced nodes. Subsequent calls to tp/2 and p/2 will
              not consider that node, but tracing already activated on the node will continue  to
              be in effect.

              Returns ok, cannot fail.

       ln() -> ok

              Shows the list of traced nodes on the console.

       tracer() -> {ok, pid()} | {error, already_started}

              This  function  starts a server on the local node that will be the recipient of all
              trace messages. All subsequent calls to p/2 will result in  messages  sent  to  the
              newly started trace server.

              A  trace  server  started  in  this way will simply display the trace messages in a
              formatted way in the Erlang shell  (i.  e.  use  io:format).  See  tracer/2  for  a
              description of how the trace message handler can be customized.

              To start a similar tracer on a remote node, use n/1.

       tracer(Type, Data) -> {ok, pid()} | {error, Error}


                 Type = port | process
                 Data = PortGenerator | HandlerSpec
                 HandlerSpec = {HandlerFun, InitialData}
                 HandlerFun = fun() (two arguments)
                 InitialData = term()
                 PortGenerator = fun() (no arguments)
                 Error = term()

              This  function starts a tracer server with additional parameters on the local node.
              The first parameter, the Type, indicates if trace messages should be handled  by  a
              receiving  process  (process)  or  by a tracer port (port). For a description about
              tracer ports see trace_port/2.

              If Type is a process, a message handler function can  be  specified  (HandlerSpec).
              The  handler  function,  which should be a fun taking two arguments, will be called
              for each trace message, with the first argument containing the message as it is and
              the  second  argument  containing  the return value from the last invocation of the
              fun. The initial value of the second parameter is specified in the InitialData part
              of  the  HandlerSpec. The HandlerFun may choose any appropriate action to take when
              invoked, and can save a state for the next invocation by returning it.

              If Type is a port, then the second  parameter  should  be  a  fun  which  takes  no
              arguments  and  returns  a  newly  opened  trace  port  when  called. Such a fun is
              preferably generated by calling trace_port/2.

              If an error is returned, it can either be due to a tracer  server  already  running
              ({error,already_started}) or due to the HandlerFun throwing an exception.

              To start a similar tracer on a remote node, use tracer/3.

       tracer(Nodename, Type, Data) -> {ok, Nodename} | {error, Reason}


                 Nodename = atom()

              This  function  is  equivalent to tracer/2, but acts on the given node. A tracer is
              started on the node (Nodename) and the node is added to the list of traced nodes.

              This function is not equivalent to n/1. While n/1 starts  a  process  tracer  which
              redirects  all  trace  information  to a process tracer on the local node (i.e. the
              trace control node), tracer/3 starts a tracer of any type which is  independent  of
              the tracer on the trace control node.

              For details, seetracer/2.

       trace_port(Type, Parameters) -> fun()


                 Type = ip | file
                 Parameters = Filename | WrapFilesSpec | IPPortSpec
                 Filename = string() | [string()] | atom()
                 WrapFilesSpec  = {Filename, wrap, Suffix} | {Filename, wrap, Suffix, WrapSize} |
                 {Filename, wrap, Suffix, WrapSize, WrapCnt}
                 Suffix = string()
                 WrapSize = integer() >= 0 | {time, WrapTime}
                 WrapTime = integer() >= 1
                 WrapCnt = integer() >= 1
                 IpPortSpec = PortNumber | {PortNumber, QueSize}
                 PortNumber = integer()
                 QueSize = integer()

              This function creates a trace port generating fun. The fun takes no  arguments  and
              returns  a newly opened trace port. The return value from this function is suitable
              as a second parameter to tracer/2, i.e. dbg:tracer(port, dbg:trace_port(ip, 4711)).

              A trace port is an Erlang port to a dynamically linked in driver that handles trace
              messages  directly,  without the overhead of sending them as messages in the Erlang
              virtual machine.

              Two trace drivers are currently implemented, the file and the ip trace drivers. The
              file  driver  sends all trace messages into one or several binary files, from where
              they later can be fetched and processed with the trace_client/2  function.  The  ip
              driver  opens  a  TCP/IP  port  where  it  listens  for  connections. When a client
              (preferably started by calling trace_client/2 on another Erlang node) connects, all
              trace  messages  are  sent over the IP network for further processing by the remote

              Using a trace port significantly lowers the overhead imposed by using tracing.

              The file trace  driver  expects  a  filename  or  a  wrap  files  specification  as
              parameter.  A  file  is  written  with  a  high  degree of buffering, why all trace
              messages are not guaranteed to be saved in the file in case of a system crash. That
              is the price to pay for low tracing overhead.

              A  wrap  files specification is used to limit the disk space consumed by the trace.
              The trace is written to a limited number of files each with  a  limited  size.  The
              actual filenames are Filename ++ SeqCnt ++ Suffix, where SeqCnt counts as a decimal
              string from 0 to WrapCnt and then around again from 0. When a trace term written to
              the  current file makes it longer than WrapSize, that file is closed, if the number
              of files in this wrap trace is as many as WrapCnt the oldest file is deleted then a
              new file is opened to become the current. Thus, when a wrap trace has been stopped,
              there are at most WrapCnt trace files saved with a size of at least  WrapSize  (but
              not  much  bigger),  except for the last file that might even be empty. The default
              values are WrapSize = 128*1024 and WrapCnt = 8.

              The SeqCnt values in the filenames are all in the range 0 through  WrapCnt  with  a
              gap in the circular sequence. The gap is needed to find the end of the trace.

              If  the  WrapSize is specified as {time, WrapTime}, the current file is closed when
              it has been open more than WrapTime milliseconds, regardless of it being  empty  or

              The ip trace driver has a queue of QueSize messages waiting to be delivered. If the
              driver cannot deliver messages as fast as they are produced by the runtime  system,
              a special message is sent, which indicates how many messages that are dropped. That
              message will arrive at the handler function  specified  in  trace_client/3  as  the
              tuple  {drop,  N} where N is the number of consecutive messages dropped. In case of
              heavy tracing, drop's are likely to occur, and they surely occur if  no  client  is
              reading the trace messages.


              Equivalent to flush_trace_port(node()).

       flush_trace_port(Nodename) -> ok | {error, Reason}

              Equivalent to trace_port_control(Nodename,flush).


              Equivalent to trace_port_control(node(),Operation).

       trace_port_control(Nodename,Operation) -> ok | {ok, Result} | {error, Reason}


                 Nodename = atom()

              This  function is used to do a control operation on the active trace port driver on
              the given node (Nodename). Which operations are allowed as  well  as  their  return
              values depend on which trace driver is used.

              Returns  either  ok  or  {ok,  Result}  if the operation was successful, or {error,
              Reason} if the current tracer is a process or if it is a port  not  supporting  the

              The allowed values for Operation are:

                  This  function  is  used  to  flush  the  internal buffers held by a trace port
                  driver. Currently only the file trace driver supports this  operation.  Returns

                  Returns  {ok,  IpPort}  where  IpPortis  the  IP port number used by the driver
                  listen socket. Only the ip trace driver supports this operation.

       trace_client(Type, Parameters) -> pid()


                 Type = ip | file | follow_file
                 Parameters = Filename | WrapFilesSpec | IPClientPortSpec
                 Filename = string() | [string()] | atom()
                 WrapFilesSpec = see trace_port/2
                 Suffix = string()
                 IpClientPortSpec = PortNumber | {Hostname, PortNumber}
                 PortNumber = integer()
                 Hostname = string()

              This function starts a trace client that reads the output created by a  trace  port
              driver  and  handles  it  in mostly the same way as a tracer process created by the
              tracer/0 function.

              If Type is file, the client reads all trace  messages  stored  in  the  file  named
              Filename  or specified by WrapFilesSpec (must be the same as used when creating the
              trace, see trace_port/2) and let's the default handler function format the messages
              on  the console. This is one way to interpret the data stored in a file by the file
              trace port driver.

              If Type is follow_file, the client behaves as in the file case, but keeps trying to
              read  (and  process)  more data from the file until stopped by stop_trace_client/1.
              WrapFilesSpec is not allowed as second argument for this Type.

              If Type is ip, the client connects to  the  TCP/IP  port  PortNumber  on  the  host
              Hostname, from where it reads trace messages until the TCP/IP connection is closed.
              If no Hostname is specified, the local host is assumed.

              As an example, one can let trace messages be  sent  over  the  network  to  another
              Erlang node (preferably not distributed), where the formatting occurs:

              On  the  node  stack  there's  an  Erlang  node  ant@stack,  in the shell, type the

              ant@stack> dbg:tracer(port, dbg:trace_port(ip,4711)).
              ant@stack> dbg:p(self(), send).

              All trace messages are now sent to the trace port driver, which in turn listens for
              connections  on  the  TCP/IP  port  4711. If we want to see the messages on another
              node, preferably on another host, we do like this:

              -> dbg:trace_client(ip, {"stack", 4711}).

              If we now send a message from the shell on the node ant@stack, where all sends from
              the shell are traced:

              ant@stack> self() ! hello.

              The following will appear at the console on the node that started the trace client:

              (<0.23.0>) <0.23.0> ! hello
              (<0.23.0>) <0.22.0> ! {shell_rep,<0.23.0>,{value,hello,[],[]}}

              The last line is generated due to internal message passing in the Erlang shell. The
              process id's will vary.

       trace_client(Type, Parameters, HandlerSpec) -> pid()


                 Type = ip | file | follow_file
                 Parameters = Filename | WrapFilesSpec | IPClientPortSpec
                 Filename = string() | [string()] | atom()
                 WrapFilesSpec = see trace_port/2
                 Suffix = string()
                 IpClientPortSpec = PortNumber | {Hostname, PortNumber}
                 PortNumber = integer()
                 Hostname = string()
                 HandlerSpec = {HandlerFun, InitialData}
                 HandlerFun = fun() (two arguments)
                 InitialData = term()

              This function works exactly as trace_client/2, but allows you  to  write  your  own
              handler  function.  The  handler  function  works  mostly  as  the one described in
              tracer/2, but will also have to be prepared to handle trace messages  of  the  form
              {drop,  N},  where  N  is the number of dropped messages. This pseudo trace message
              will only occur if the ip trace driver is used.

              For trace type file, the pseudo trace message end_of_trace will appear at  the  end
              of the trace. The return value from the handler function is in this case ignored.

       stop_trace_client(Pid) -> ok


                 Pid = pid()

              This function shuts down a previously started trace client. The Pid argument is the
              process id returned from the trace_client/2 or trace_client/3 call.


              Equivalent to get_tracer(node()).

       get_tracer(Nodename) -> {ok, Tracer}


                 Nodename = atom()
                 Tracer = port() | pid()

              Returns the process or port to which all trace messages are sent.

       stop() -> stopped

              Stops the dbg server and clears all trace flags for all  processes  and  all  trace
              patterns  for all functions. Also shuts down all trace clients and closes all trace

              Note that no trace patterns are affected by this function.

       stop_clear() -> stopped

              Same as stop/0, but also clears all trace patterns on local  and  global  functions


       The  simplest  way  of  tracing  from  the Erlang shell is to use dbg:c/3 or dbg:c/4, e.g.
       tracing the function dbg:get_tracer/0:

       (tiger@durin)84> dbg:c(dbg,get_tracer,[]).
       (<0.154.0>) <0.152.0> ! {<0.154.0>,{get_tracer,tiger@durin}}
       (<0.154.0>) out {dbg,req,1}
       (<0.154.0>) << {dbg,{ok,<0.153.0>}}
       (<0.154.0>) in {dbg,req,1}
       (<0.154.0>) << timeout

       Another way of tracing from the shell is to explicitly start a tracer  and  then  set  the
       trace  flags  of  your  choice on the processes you want to trace, e.g. trace messages and
       process events:

       (tiger@durin)66> Pid = spawn(fun() -> receive {From,Msg} -> From ! Msg end end).
       (tiger@durin)67> dbg:tracer().
       (tiger@durin)68> dbg:p(Pid,[m,procs]).
       (tiger@durin)69> Pid ! {self(),hello}.
       (<0.126.0>) << {<0.116.0>,hello}
       (<0.126.0>) << timeout
       (<0.126.0>) <0.116.0> ! hello
       (<0.126.0>) exit normal
       (tiger@durin)70> flush().
       Shell got hello

       If you set the call trace flag, you also have to set a trace pattern for the functions you
       want to trace:

       (tiger@durin)77> dbg:tracer().
       (tiger@durin)78> dbg:p(all,call).
       (tiger@durin)79> dbg:tp(dbg,get_tracer,0,[]).
       (tiger@durin)80> dbg:get_tracer().
       (<0.116.0>) call dbg:get_tracer()
       (tiger@durin)81> dbg:tp(dbg,get_tracer,0,[{'_',[],[{return_trace}]}]).
       (tiger@durin)82> dbg:get_tracer().
       (<0.116.0>) call dbg:get_tracer()
       (<0.116.0>) returned from dbg:get_tracer/0 -> {ok,<0.143.0>}


       The  dbg module is primarily targeted towards tracing through the erlang:trace/3 function.
       It is sometimes desired to trace messages in a more delicate way, which can be  done  with
       the help of the seq_trace module.

       seq_trace  implements  sequential  tracing (known in the AXE10 world, and sometimes called
       "forlopp tracing"). dbg can interpret messages  generated  from  seq_trace  and  the  same
       tracer  function for both types of tracing can be used. The seq_trace messages can even be
       sent to a trace port for further analysis.

       As a match specification can turn on  sequential  tracing,  the  combination  of  dbg  and
       seq_trace  can  be  quite  powerful.  This  brief example shows a session where sequential
       tracing is used:

       1> dbg:tracer().
       2> {ok, Tracer} = dbg:get_tracer().
       3> seq_trace:set_system_tracer(Tracer).
       4> dbg:tp(dbg, get_tracer, 0, [{[],[],[{set_seq_token, send, true}]}]).
       5> dbg:p(all,call).
       6> dbg:get_tracer(), seq_trace:set_token([]).
       (<0.25.0>) call dbg:get_tracer()
       SeqTrace [0]: (<0.25.0>) <0.30.0> ! {<0.25.0>,get_tracer} [Serial: {2,4}]
       SeqTrace [0]: (<0.30.0>) <0.25.0> ! {dbg,{ok,<0.31.0>}} [Serial: {4,5}]

       This session sets the system_tracer to the same process as the ordinary tracer process (i.
       e.  <0.31.0>)  and  sets the trace pattern for the function dbg:get_tracer to one that has
       the action of setting a sequential token. When the function is called by a traced  process
       (all  processes are traced in this case), the process gets "contaminated" by the token and
       seq_trace  messages  are  sent  both  for  the  server  request  and  the  response.   The
       seq_trace:set_token([])  after  the  call  clears the seq_trace token, why no messages are
       sent when the answer propagates via the shell  to  the  console  port.  The  output  would
       otherwise have been more noisy.


       When  tracing  function  calls on a group leader process (an IO process), there is risk of
       causing a deadlock. This will happen if a group leader process generates a  trace  message
       and  the tracer process, by calling the trace handler function, sends an IO request to the
       same group leader. The problem can only occur if the trace handler prints to tty using  an
       io  function  such as format/2. Note that when dbg:p(all,call) is called, IO processes are
       also traced. Here's an example:

       %% Using a default line editing shell
       1> dbg:tracer(process, {fun(Msg,_) -> io:format("~p~n", [Msg]), 0 end, 0}).
       2> dbg:p(all, [call]).
       3> dbg:tp(mymod,[{'_',[],[]}]).
       4> mymod: % TAB pressed here
       %% -- Deadlock --

       Here's another example:

       %% Using a shell without line editing (oldshell)
       1> dbg:tracer(process).
       2> dbg:p(all, [call]).
       3> dbg:tp(lists,[{'_',[],[]}]).
       % -- Deadlock --

       The reason we get a deadlock in the first example is because when TAB is pressed to expand
       the   function   name,   the   group   leader   (which   handles  character  input)  calls
       mymod:module_info(). This generates a trace message which,  in  turn,  causes  the  tracer
       process to send an IO request to the group leader (by calling io:format/2). We end up in a

       In the second example we use the default trace handler function. This  handler  prints  to
       tty  by  sending IO requests to the user process. When Erlang is started in oldshell mode,
       the shell process will have user as its group leader and so will  the  tracer  process  in
       this  example.  Since user calls functions in lists we end up in a deadlock as soon as the
       first IO request is sent.

       Here are a few suggestions for how to avoid deadlock:

         * Don't trace the group leader of the tracer process. If tracing has  been  switched  on
           for  all  processes,  call  dbg:p(TracerGLPid,clear)  to stop tracing the group leader
           (TracerGLPid). process_info(TracerPid,group_leader) tells you which  process  this  is
           (TracerPid is returned from dbg:get_tracer/0).

         * Don't trace the user process if using the default trace handler function.

         * In  your  own  trace handler function, call erlang:display/1 instead of an io function
           or, if user is not used as group leader, print to user instead of  the  default  group
           leader. Example: io:format(user,Str,Args).