<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2002</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.
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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.
The Initial Developer of the Original Code is Ericsson AB.
</legalnotice>
<title>ttb</title>
<prepared>Siri Hansen, Bartlomiej Puzon</prepared>
<responsible></responsible>
<docno>1</docno>
<approved></approved>
<checked></checked>
<date>2010-08-13</date>
<rev>PA1</rev>
<file>ttb.xml</file>
</header>
<module>ttb</module>
<modulesummary>A base for building trace tools for distributed systems.</modulesummary>
<description>
<p>The Trace Tool Builder, <c>ttb</c>, is a base for building trace
tools for distributed systems.
</p>
<p>When using <c>ttb</c>, do not use module <c>dbg</c> in application
Runtime_Tools in parallel.</p>
</description>
<funcs>
<func>
<name since="OTP R15B">start_trace(Nodes, Patterns, FlagSpec, Opts) -> Result</name>
<fsummary>Start a trace port on each specified node.</fsummary>
<type>
<v>Result = see p/2</v>
<v>Nodes = see tracer/2</v>
<v>Patterns = [tuple()]</v>
<v>FlagSpec = {Procs, Flags}</v>
<v>Proc = see p/2</v>
<v>Flags = see p/2</v>
<v>Opts = see tracer/2</v>
</type>
<desc>
<p>This function is a shortcut allowing to start a trace with one command. Each
tuple in <c>Patterns</c> is converted to a list, which in turn is passed to
<c>ttb:tpl/2,3,4</c>.</p>
<p>The call:</p>
<pre>
> <input>ttb:start_trace([Node, OtherNode],
[{mod, foo, []}, {mod, bar, 2}],
{all, call},
[{file, File}, {handler,{fun myhandler/4, S}}]).</input></pre>
<p> is equivalent to:</p>
<pre>
> <input>ttb:start_trace([Node, OtherNode],
[{file, File}, {handler,{fun myhandler/4, S}}]),
ttb:tpl(mod, foo, []),
ttb:tpl(mod, bar, 2, []),
ttb:p(all, call).</input></pre>
</desc>
</func>
<func>
<name>tracer() -> Result</name>
<fsummary>Equivalent to tracer(node()).</fsummary>
<desc>
<p>Equivalent to <c>tracer(node())</c>.</p>
</desc>
</func>
<func>
<name>tracer(Shortcut) -> Result</name>
<fsummary>Handy shortcuts for common tracing settings.</fsummary>
<type>
<v>Shortcut = shell | dbg</v>
</type>
<desc>
<p>Handy shortcuts for common tracing settings.</p>
<p><c>shell</c> is equivalent to <c>tracer(node(),[{file, {local, "ttb"}}, shell])</c>.</p>
<p><c>dbg</c> is equivalent to <c>tracer(node(),[{shell, only}])</c>.</p>
</desc>
</func>
<func>
<name>tracer(Nodes) -> Result</name>
<fsummary>Equivalent to tracer(Nodes,[]).</fsummary>
<desc>
<p>Equivalent to <c>tracer(Nodes,[])</c>.</p>
</desc>
</func>
<func>
<name>tracer(Nodes,Opts) -> Result</name>
<fsummary>Start a trace port on each specified node.</fsummary>
<type>
<v>Result = {ok, ActivatedNodes} | {error,Reason}</v>
<v>Nodes = atom() | [atom()] | all | existing | new</v>
<v>Opts = Opt | [Opt]</v>
<v>Opt = {file,Client} | {handler, FormatHandler} | {process_info,PI} |
shell | {shell, ShellSpec} | {timer, TimerSpec} |
{overload_check, {MSec, Module, Function}} |
{flush, MSec} | resume | {resume, FetchTimeout} |
{queue_size, QueueSize}</v>
<v>TimerSpec = MSec | {MSec, StopOpts}</v>
<v>MSec = FetchTimeout = integer()</v>
<v>Module = Function = atom() </v>
<v>StopOpts = see stop/2</v>
<v>Client = File | {local, File}</v>
<v>File = Filename | Wrap</v>
<v>Filename = string()</v>
<v>Wrap = {wrap,Filename} | {wrap,Filename,Size,Count}</v>
<v>FormatHandler = See format/2</v>
<v>PI = true | false </v>
<v>ShellSpec = true | false | only</v>
<v>QueueSize = non_neg_integer()</v>
</type>
<desc>
<p>Starts a file trace port on all specified nodes
and points the system tracer for sequential tracing to
the same port.
</p>
<p><em>Options:</em></p>
<taglist>
<tag><c>Filename</c></tag>
<item><p>The specified <c>Filename</c> is prefixed with the node name.
Default <c>Filename</c> is <c>ttb</c>.</p></item>
<tag><c>File={wrap,Filename,Size,Count}</c></tag>
<item><p>Can be used if the size of the trace logs must be limited.
Default values are
<c>Size=128*1024</c> and <c>Count=8</c>.</p></item>
<tag><c>Client</c></tag>
<item><p>When tracing diskless nodes, <c>ttb</c> must be started
from an external "trace control node" with disk access, and
<c>Client</c> must be <c>{local, File}</c>. All
trace information is then sent to the trace control node where
it is written to file.</p></item>
<tag><c>queue_size</c></tag>
<item><p>When tracing to shell or <c>{local,File}</c>, an ip
trace driver is used internally. The ip trace driver has a
queue of maximum <c>QueueSize</c> messages waiting to be
delivered. If the driver cannot deliver messages as fast as
they are produced, the queue size might be exceeded and
messages are dropped. This parameter is optional, and is
only useful if many <c>{drop,N}</c> trace messages are
received by the trace handler. It has no meaning if shell
or <c>{local,File}</c> is not used. See
<seealso marker="runtime_tools:dbg#trace_port/2">dbg:trace_port/2</seealso>
for more information about the ip trace driver.</p></item>
<tag><c>process_info</c></tag>
<item><p>Indicates if process
information is to be collected. If <c>PI = true</c> (which is
default), each process identifier <c>Pid</c> is replaced by a
tuple <c>{Pid,ProcessInfo,Node}</c>, where <c>ProcessInfo</c>
is the registered process name, its globally registered name,
or its initial function. To turn off this functionality,
set <c>PI = false</c>.</p></item>
<tag><c>{shell, ShellSpec}</c></tag>
<item><p>Indicates that trace messages are to be printed on the
console as they are received by the tracing process. This implies
trace client <c>{local, File}</c>. If <c>ShellSpec</c>
is <c>only</c> (instead of <c>true</c>), no trace logs are stored.</p></item>
<tag><c>shell</c></tag>
<item><p>Shortcut for <c>{shell, true}</c>.</p></item>
<tag><c>timer</c></tag>
<item><p>Indicates that the trace is to be
automatically stopped after <c>MSec</c> milliseconds. <c>StopOpts</c>
are passed to command <c>ttb:stop/2</c> if specified (default is <c>[]</c>).
Notice that the timing is approximate, as delays related to
network communication are always present. The timer starts after
<c>ttb:p/2</c> is issued, so you can set up your trace patterns before.</p></item>
<tag><c>overload_check</c></tag>
<item><p>Allows to enable overload
checking on the nodes under trace. <c>Module:Function(check)</c>
is performed each <c>MSec</c> millisecond. If the check returns
<c>true</c>, the tracing is disabled on a specified node.</p>
<p><c>Module:Function</c> must be able to handle at least three
atoms: <c>init</c>, <c>check</c>, and <c>stop</c>. <c>init</c> and
<c>stop</c> allows you to initialize and clean
up the check environment.</p>
<p>When a node gets overloaded, it is not possible to issue <c>ttb:p/2</c>
or any command from the <c>ttb:tp/2,3,4</c> family, as it would lead to
inconsistent tracing state (different trace specifications on
different nodes).</p></item>
<tag><c>flush</c></tag>
<item><p>Periodically flushes all file trace
port clients (see
<seealso marker="runtime_tools:dbg#flush_trace_port/1">
<c>dbg:flush_trace_port/1</c></seealso>). When enabled,
the buffers are freed each <c>MSec</c> millisecond. This option is
not allowed with <c>{file, {local, File}}</c> tracing.</p></item>
<tag><c>{resume, FetchTimeout}</c></tag>
<item><p>Enables the autoresume feature.
When enabled, remote nodes try to reconnect to the controlling node
if they are restarted. The feature requires application Runtime_Tools
to be started (so it has to be present in the <c>.boot</c>
scripts if the traced nodes run with embedded Erlang). If this is
not possible, resume can be performed manually by starting
<c>Runtime_Tools</c> remotely using
<seealso marker="kernel:rpc#call/4"><c>rpc:call/4</c></seealso>.</p>
<p><c>ttb</c> tries to fetch all logs from a reconnecting node before
reinitializing the trace. This must finish within <c>FetchTimeout</c>
milliseconds or is aborted.</p>
<p>By default, autostart information is stored in a file named
<c>ttb_autostart.bin</c> on each node. If this is not desired
(for example, on diskless nodes), a custom module handling autostart
information storage and retrieval can be provided by specifying
environment variable <c>ttb_autostart_module</c> for the application
Runtime_Tools. The module must respond to the following API:</p>
<taglist>
<tag><c>write_config(Data) -> ok</c></tag>
<item><p>Stores the provided data for further retrieval. It is
important to realize that the data storage used must not
be affected by the node crash.</p></item>
<tag><c>read_config() -> {ok, Data} | {error, Error}</c></tag>
<item><p>Retrieves configuration stored with <c>write_config(Data)</c>.</p></item>
<tag><c>delete_config() -> ok</c></tag>
<item><p>Deletes configuration stored with <c>write_config(Data)</c>.
Notice that after this call any subsequent calls to <c>read_config</c>
must return <c>{error, Error}</c>.</p>
</item>
</taglist>
<p><c>resume</c> implies the default <c>FetchTimeout</c>, which is
10 seconds</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>p(Item,Flags) -> Return</name>
<fsummary>Set the specified trace flags on the specified processes or ports.</fsummary>
<type>
<v>Return = {ok,[{Item,MatchDesc}]}</v>
<v>Items = Item | [Item]</v>
<v>Item = pid() | port() | RegName | {global,GlobalRegName} |
all | processes | ports |
existing | existing_processes | existing_ports |
new | new_processes | new_ports</v>
<v>RegName = atom()</v>
<v>GlobalRegName = term()</v>
<v>Flags = Flag | [Flag]</v>
</type>
<desc>
<p>Sets the specified trace flags on the specified processes
or ports. Flag <c>timestamp</c> is always turned on.
</p>
<p>See the Reference Manual for module
<seealso marker="runtime_tools:dbg"><c>dbg</c></seealso>
for the possible trace flags. Parameter
<c>MatchDesc</c> is the same as returned from
<c>dbg:p/2</c>.</p>
<p>Processes can be specified as registered names, globally
registered names, or process identifiers. Ports can be
specified as registered names or port identifiers. If a
registered name is specified, the flags are set on
processes/ports with this name on all active nodes.</p>
<p>Issuing this command starts the timer for this trace if option
<c>timer</c> is specified with <c>tracer/2</c>.
</p>
</desc>
</func>
<func>
<name>tp, tpl, tpe, ctp, ctpl, ctpg, ctpe</name>
<fsummary>Set and clear trace patterns.</fsummary>
<desc>
<p>These functions are to be used with trace
flag <c>call</c>, <c>send</c>, and <c>'receive'</c> for
setting and clearing trace patterns.</p>
<p>When trace flag <c>call</c> is set on a process,
function calls are traced on that process if a trace
pattern is set for the called function.</p>
<p>The <c>send</c> and <c>'receive'</c> flags enable tracing
of all messages sent and received by the process/port. Trace
patterns set with <c>tpe</c> may limit traced messages based
on the message content, the sender, and/or the receiver.</p>
<p>Trace patterns specify how to trace a function or a message
by using match specifications. Match specifications are
described in the
<seealso marker="erts:match_spec"><c>ERTS User's Guide</c></seealso>.
</p>
<p>These functions are equivalent to the corresponding
functions in module
<seealso marker="runtime_tools:dbg">dbg</seealso>,
but all calls are stored in the
history. The history buffer makes it easy to create configuration
files; the same trace environment can be set up many
times, for example, to compare two test runs. It also
reduces the amount of typing when using <c>ttb</c> from the
Erlang shell.
</p>
<taglist>
<tag><c>tp</c></tag>
<item><p>Sets trace patterns on global function calls.</p></item>
<tag><c>tpl</c></tag>
<item><p>Sets trace patterns on local and global function calls.</p></item>
<tag><c>tpe</c></tag>
<item><p>Sets trace patterns on messages.</p></item>
<tag><c>ctp</c></tag>
<item><p>Clears trace patterns on local and global function
calls.</p></item>
<tag><c>ctpl</c></tag>
<item><p>Clears trace patterns on local function calls.</p></item>
<tag><c>ctpg</c></tag>
<item><p>Clears trace patterns on global function calls.</p></item>
<tag><c>ctpe</c></tag>
<item><p>Clears trace patterns on messages.</p></item>
</taglist>
<p>With <c>tp</c> and <c>tpl</c>, one of the match specification shortcuts
can be used (for example, <c>ttb:tp(foo_module, caller)</c>).</p>
<p>The shortcuts are as follows:</p>
<list type="bulleted">
<item><c>return</c> - for <c>[{'_',[],[{return_trace}]}]</c>
(report the return value from a traced function)</item>
<item><c>caller</c> - for <c>[{'_',[],[{message,{caller}}]}]</c>
(report the calling function)</item>
<item><c>{codestr, Str}</c> - for <c>dbg:fun2ms/1</c> arguments
passed as strings (example: <c>"fun(_) -> return_trace() end"</c>)
</item>
</list>
</desc>
</func>
<func>
<name>list_history() -> History</name>
<fsummary>Return all calls stored in history.</fsummary>
<type>
<v>History = [{N,Func,Args}]</v>
</type>
<desc>
<p>All calls to <c>ttb</c> is stored in the history. This
function returns the current content of the history. Any entry
can be reexecuted with <c>run_history/1</c> or stored in a
configuration file with <c>write_config/2,3</c>.</p>
</desc>
</func>
<func>
<name>run_history(N) -> ok | {error, Reason}</name>
<fsummary>Execute one entry of the history.</fsummary>
<type>
<v>N = integer() | [integer()]</v>
</type>
<desc>
<p>Executes the specified entry or entries from the history
list. To list history, use <c>list_history/0</c>.</p>
</desc>
</func>
<func>
<name>write_config(ConfigFile,Config)</name>
<fsummary>Equivalent to write_config(ConfigFile,Config,[]).</fsummary>
<desc>
<p>Equivalent to <c>write_config(ConfigFile,Config,[])</c>.</p>
</desc>
</func>
<func>
<name>write_config(ConfigFile,Config,Opts) -> ok | {error,Reason}</name>
<fsummary>Create a configuration file.</fsummary>
<type>
<v>ConfigFile = string()</v>
<v>Config = all | [integer()] | [{Mod,Func,Args}]</v>
<v>Mod = atom()</v>
<v>Func = atom()</v>
<v>Args = [term()]</v>
<v>Opts = Opt | [Opt]</v>
<v>Opt = append</v>
</type>
<desc>
<p>Creates or extends a configuration file, which can be
used for restoring a specific configuration later.
</p>
<p>The contents of the configuration file can either be fetched from
the history or specified directly as a list of
<c>{Mod,Func,Args}</c>.
</p>
<p>If the complete history is to be stored in the configuration file,
<c>Config</c> must be <c>all</c>. If only a selected number
of entries from the history are to be stored, <c>Config</c>
must be a list of integers pointing out the entries to be
stored.
</p>
<p>If <c>Opts</c> is not specified or if it is <c>[]</c>,
<c>ConfigFile</c> is deleted and a new file is created. If
<c>Opts = [append]</c>, <c>ConfigFile</c> is not deleted.
The new information is appended at the end of the file.</p>
</desc>
</func>
<func>
<name>run_config(ConfigFile) -> ok | {error,Reason}</name>
<fsummary>Execute all entries in a configuration file.</fsummary>
<type>
<v>ConfigFile = string()</v>
</type>
<desc>
<p>Executes all entries in the specified configuration file.
Notice that the history of the last trace is always available
in file <c>ttb_last_config</c>.</p>
</desc>
</func>
<func>
<name>run_config(ConfigFile,NumList) -> ok | {error,Reason}</name>
<fsummary>Execute selected entries from a configuration file.</fsummary>
<type>
<v>ConfigFile = string()</v>
<v>NumList = [integer()]</v>
</type>
<desc>
<p>Executes selected entries from the specified configuration
file. <c>NumList</c> is a list of integers pointing out the
entries to be executed.
</p>
<p>To list the contents of a configuration file, use
<c>list_config/1</c>.</p>
<p>Notice that the history of the last trace is always available
in file <c>ttb_last_config</c>.</p>
</desc>
</func>
<func>
<name>list_config(ConfigFile) -> Config | {error,Reason}</name>
<fsummary>List all entries in a configuration file.</fsummary>
<type>
<v>ConfigFile = string()</v>
<v>Config = [{N,Func,Args}]</v>
</type>
<desc>
<p>Lists all entries in the specified configuration file.</p>
</desc>
</func>
<func>
<name>write_trace_info(Key,Info) -> ok</name>
<fsummary>Write any information to file <c>.ti</c>.</fsummary>
<type>
<v>Key = term()</v>
<v>Info = Data | fun() -> Data</v>
<v>Data = term()</v>
</type>
<desc>
<p>File <c>.ti</c> contains <c>{Key,ValueList}</c>
tuples. This function adds <c>Data</c> to the <c>ValueList</c>
associated with <c>Key</c>. All information written with this
function is included in the call to the format handler.</p>
</desc>
</func>
<func>
<name>seq_trigger_ms() -> MatchSpec</name>
<fsummary>Equivalent to seq_trigger_ms(all).</fsummary>
<desc>
<p>Equivalent to <c>seq_trigger_ms(all)</c>.</p>
</desc>
</func>
<func>
<name>seq_trigger_ms(Flags) -> MatchSpec</name>
<fsummary>Return a match_spec() which starts sequential tracing.</fsummary>
<type>
<v>MatchSpec = match_spec()</v>
<v>Flags = all | SeqTraceFlag | [SeqTraceFlag]</v>
<v>SeqTraceFlag = atom()</v>
</type>
<desc>
<p>A match specification can turn on or off sequential
tracing. This function returns a match specification, which
turns on sequential tracing with the specified <c>Flags</c>.
</p>
<p>This match specification can be specified as the last argument
to <c>tp</c> or <c>tpl</c>. The activated <c>Item</c>
then becomes a <em>trigger</em> for sequential tracing. This
means that if the item is called on a process with trace flag
<c>call</c> set, the process is "contaminated"
with token <c>seq_trace</c>.
</p>
<p>If <c>Flags = all</c>, all possible flags are set.
</p>
<p>The possible values for <c>SeqTraceFlag</c> are available in
<seealso marker="kernel:seq_trace"><c>seq_trace</c></seealso>.</p>
<p>For a description of the <c>match_spec()</c> syntax,
see section
<seealso marker="erts:match_spec"><c>Match Specifications in Erlang</c></seealso>
in ERTS, which explains the general match specification "language".
</p>
<note>
<p>The <em>system tracer</em> for sequential tracing is
automatically initiated by <c>ttb</c> when a trace port is
started with <c>ttb:tracer/0,1,2</c>.</p>
</note>
<p>An example of how to use function <c>seq_trigger_ms/0,1</c> follows:</p>
<pre>
(tiger@durin)5> <input>ttb:tracer().</input>
{ok,[tiger@durin]}
(tiger@durin)6> <input>ttb:p(all,call).</input>
{ok,{[all],[call]}}
(tiger@durin)7> <input>ttb:tp(mod,func,ttb:seq_trigger_ms()).</input>
{ok,[{matched,1},{saved,1}]}
(tiger@durin)8></pre>
<p>Whenever <c>mod:func(...)</c> is called after this,
token <c>seq_trace</c> is set on the executing process.</p>
</desc>
</func>
<func>
<name>stop()</name>
<fsummary>Equivalent to stop([]).</fsummary>
<desc>
<p>Equivalent to <c>stop([])</c>.</p>
</desc>
</func>
<func>
<name>stop(Opts) -> stopped | {stopped, Dir}</name>
<fsummary>Stop tracing and fetch/format logs from all nodes.</fsummary>
<type>
<v>Opts = Opt | [Opt]</v>
<v>Opt = nofetch | {fetch_dir, Dir} | format | {format, FormatOpts} | return_fetch_dir</v>
<v>Dir = string()</v>
<v>FormatOpts = see format/2</v>
</type>
<desc>
<p>Stops tracing on all nodes. Logs and
trace information files are sent to the trace control
node and stored in a directory named
<c>ttb_upload_FileName-Timestamp</c>, where <c>Filename</c> is
the one provided with <c>{file, File}</c> during trace setup
and <c>Timestamp</c> is of the
form <c>yyyymmdd-hhmmss</c>. Even logs from nodes on the same
machine as the trace control node are moved to this directory.
The history list is saved to a file named <c>ttb_last_config</c>
for further reference (as it is no longer accessible
through history and configuration management functions, like
<c>ttb:list_history/0</c>).
</p>
<p><em>Options:</em></p>
<taglist>
<tag><c>nofetch</c></tag>
<item><p>Indicates that trace logs are not to be
collected after tracing is stopped.</p></item>
<tag><c>{fetch, Dir}</c></tag>
<item><p>Allows specification of the directory
to fetch the data to. If the directory already exists, an
error is thrown.</p></item>
<tag><c>format</c></tag>
<item><p>Indicates the trace logs to be formatted after tracing
is stopped. All logs in the fetch directory are merged.</p></item>
<tag><c>return_fetch_dir</c></tag>
<item><p>Indicates the return value
to be <c>{stopped, Dir}</c> and not just <c>stopped</c>.
This implies <c>fetch</c>.</p></item>
</taglist>
</desc>
</func>
<func>
<name since="OTP R15B">get_et_handler()</name>
<fsummary>Return the <c>et</c> handler.</fsummary>
<desc>
<p>Returns the <c>et</c> handler, which can be used with <c>format/2</c>
or <c>tracer/2</c>.</p>
<p>Example: <c>ttb:format(Dir, [{handler, ttb:get_et_handler()}])</c>.</p>
</desc>
</func>
<func>
<name>format(File)</name>
<fsummary>Equivalent to <c>format(File,[])</c>.</fsummary>
<desc>
<p>Equivalent to <c>format(File,[])</c>.</p>
</desc>
</func>
<func>
<name>format(File,Options) -> ok | {error, Reason}</name>
<fsummary>Format a binary trace log.</fsummary>
<type>
<v>File = string() | [string()]</v>
<d>This can be the name of a binary log, a list of such logs,
or the name of a directory containing one or more binary logs.</d>
<v>Options = Opt | [Opt]</v>
<v>Opt = {out,Out} | {handler,FormatHandler} | disable_sort</v>
<v>Out = standard_io | string()</v>
<v>FormatHandler = {Function, InitialState}</v>
<v>Function = fun(Fd,Trace,TraceInfo,State) -> State</v>
<v>Fd = standard_io | FileDescriptor</v>
<d>File descriptor of the destination file <c>Out</c>.</d>
<v>Trace = tuple()</v>
<d>The trace message. For details, see the Reference Manual for
module <c>erlang</c>.</d>
<v>TraceInfo = [{Key,ValueList}]</v>
<d>Includes the keys <c>flags</c>, <c>client</c>, and <c>node</c>.
If <c>handler</c> is specified as option to the tracer function, this
is also included. Also, all information written with function
<c>write_trace_info/2</c> is included.</d>
</type>
<desc>
<p>Reads the specified binary trace log(s). The logs are processed
in the order of their time stamps as long as option <c>disable_sort</c>
is not specified.
</p>
<p>If <c>FormatHandler = {Function,InitialState}</c>,
<c>Function</c> is called for each trace message.</p>
<p>If <c>FormatHandler = get_et_handler()</c>, <c>et_viewer</c> in
application ET is used for presenting
the trace log graphically. <c>ttb</c> provides a few different
filters that can be selected from menu <em>Filters and scaling</em>
in the <c>et_viewer</c>.</p>
<p>If <c>FormatHandler</c> is not specified, a
default handler is used presenting each trace message as a
text line.
</p>
<p>The state returned from each call of <c>Function</c> is passed to
the next call, even if the next call is to format a message from another
log file.
</p>
<p>If <c>Out</c> is specified, <c>FormatHandler</c> gets the
file descriptor to <c>Out</c> as the first parameter.
</p>
<p><c>Out</c> is ignored if the <c>et</c> format handler is used.
</p>
<p>Wrap logs can be formatted one by one or all at once. To
format one of the wrap logs in a set, specify the exact file name.
To format the whole set of wrap logs, specify the name
with <c>*</c> instead of the wrap count. For examples, see the
<seealso marker="ttb_ug#format"><c>User's Guide</c></seealso>.
</p>
</desc>
</func>
</funcs>
</erlref>
