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NAME
sys - A Functional Interface to System Messages
DESCRIPTION
This module contains functions for sending system messages used by programs, and messages
used for debugging purposes.
Functions used for implementation of processes should also understand system messages such
as debugging messages and code change. These functions must be used to implement the use
of system messages for a process; either directly, or through standard behaviours, such as
gen_server.
The default timeout is 5000 ms, unless otherwise specified. The timeout defines the time
period to wait for the process to respond to a request. If the process does not respond,
the function evaluates exit({timeout, {M, F, A}}).
The functions make reference to a debug structure. The debug structure is a list of
dbg_opt(). dbg_opt() is an internal data type used by the handle_system_msg/6 function. No
debugging is performed if it is an empty list.
SYSTEM MESSAGES
Processes which are not implemented as one of the standard behaviours must still
understand system messages. There are three different messages which must be understood:
* Plain system messages. These are received as {system, From, Msg}. The content and
meaning of this message are not interpreted by the receiving process module. When a
system message has been received, the function sys:handle_system_msg/6 is called in
order to handle the request.
* Shutdown messages. If the process traps exits, it must be able to handle an shut-down
request from its parent, the supervisor. The message {'EXIT', Parent, Reason} from the
parent is an order to terminate. The process must terminate when this message is
received, normally with the same Reason as Parent.
* There is one more message which the process must understand if the modules used to
implement the process change dynamically during runtime. An example of such a process
is the gen_event processes. This message is {get_modules, From}. The reply to this
message is From ! {modules, Modules}, where Modules is a list of the currently active
modules in the process.
This message is used by the release handler to find which processes execute a certain
module. The process may at a later time be suspended and ordered to perform a code
change for one of its modules.
SYSTEM EVENTS
When debugging a process with the functions of this module, the process generates
system_events which are then treated in the debug function. For example, trace formats the
system events to the tty.
There are three predefined system events which are used when a process receives or sends a
message. The process can also define its own system events. It is always up to the process
itself to format these events.
DATA TYPES
name() = pid() | atom() | {global, atom()}
system_event() = {in, Msg :: term()}
| {in, Msg :: term(), From :: term()}
| {out, Msg :: term(), To :: term()}
| term()
dbg_opt()
See above.
dbg_fun() =
fun((FuncState :: term(),
Event :: system_event(),
ProcState :: term()) ->
done | (NewFuncState :: term()))
EXPORTS
log(Name, Flag) -> ok | {ok, [system_event()]}
log(Name, Flag, Timeout) -> ok | {ok, [system_event()]}
Types:
Name = name()
Flag = true | {true, N :: integer() >= 1} | false | get | print
Timeout = timeout()
Turns the logging of system events On or Off. If On, a maximum of N events are kept
in the debug structure (the default is 10). If Flag is get, a list of all logged
events is returned. If Flag is print, the logged events are printed to standard_io.
The events are formatted with a function that is defined by the process that
generated the event (with a call to sys:handle_debug/4).
log_to_file(Name, Flag) -> ok | {error, open_file}
log_to_file(Name, Flag, Timeout) -> ok | {error, open_file}
Types:
Name = name()
Flag = (FileName :: string()) | false
Timeout = timeout()
Enables or disables the logging of all system events in textual format to the file.
The events are formatted with a function that is defined by the process that
generated the event (with a call to sys:handle_debug/4).
statistics(Name, Flag) -> ok | {ok, Statistics}
statistics(Name, Flag, Timeout) -> ok | {ok, Statistics}
Types:
Name = name()
Flag = true | false | get
Statistics = [StatisticsTuple] | no_statistics
StatisticsTuple = {start_time, DateTime1}
| {current_time, DateTime2}
| {reductions, integer() >= 0}
| {messages_in, integer() >= 0}
| {messages_out, integer() >= 0}
DateTime1 = DateTime2 = file:date_time()
Timeout = timeout()
Enables or disables the collection of statistics. If Flag is get, the statistical
collection is returned.
trace(Name, Flag) -> ok
trace(Name, Flag, Timeout) -> ok
Types:
Name = name()
Flag = boolean()
Timeout = timeout()
Prints all system events on standard_io. The events are formatted with a function
that is defined by the process that generated the event (with a call to
sys:handle_debug/4).
no_debug(Name) -> ok
no_debug(Name, Timeout) -> ok
Types:
Name = name()
Timeout = timeout()
Turns off all debugging for the process. This includes functions that have been
installed explicitly with the install function, for example triggers.
suspend(Name) -> ok
suspend(Name, Timeout) -> ok
Types:
Name = name()
Timeout = timeout()
Suspends the process. When the process is suspended, it will only respond to other
system messages, but not other messages.
resume(Name) -> ok
resume(Name, Timeout) -> ok
Types:
Name = name()
Timeout = timeout()
Resumes a suspended process.
change_code(Name, Module, OldVsn, Extra) -> ok | {error, Reason}
change_code(Name, Module, OldVsn, Extra, Timeout) ->
ok | {error, Reason}
Types:
Name = name()
Module = module()
OldVsn = undefined | term()
Extra = term()
Timeout = timeout()
Reason = term()
Tells the process to change code. The process must be suspended to handle this
message. The Extra argument is reserved for each process to use as its own. The
function Module:system_code_change/4 is called. OldVsn is the old version of the
Module.
get_status(Name) -> Status
get_status(Name, Timeout) -> Status
Types:
Name = name()
Timeout = timeout()
Status =
{status, Pid :: pid(), {module, Module :: module()}, [SItem]}
SItem = (PDict :: [{Key :: term(), Value :: term()}])
| (SysState :: running | suspended)
| (Parent :: pid())
| (Dbg :: [dbg_opt()])
| (Misc :: term())
Gets the status of the process.
The value of Misc varies for different types of processes. For example, a
gen_server process returns the callback module's state, a gen_fsm process returns
information such as its current state name and state data, and a gen_event process
returns information about each of its registered handlers. Callback modules for
gen_server, gen_fsm, and gen_event can also customise the value of Misc by
exporting a format_status/2 function that contributes module-specific information;
see gen_server:format_status/2, gen_fsm:format_status/2, and
gen_event:format_status/2 for more details.
get_state(Name) -> State
get_state(Name, Timeout) -> State
Types:
Name = name()
Timeout = timeout()
State = term()
Gets the state of the process.
Note:
These functions are intended only to help with debugging. They are provided for
convenience, allowing developers to avoid having to create their own state
extraction functions and also avoid having to interactively extract state from the
return values of get_status/1 or get_status/2 while debugging.
The value of State varies for different types of processes. For a gen_server
process, the returned State is simply the callback module's state. For a gen_fsm
process, State is the tuple {CurrentStateName, CurrentStateData}. For a gen_event
process, State a list of tuples, where each tuple corresponds to an event handler
registered in the process and contains {Module, Id, HandlerState}, where Module is
the event handler's module name, Id is the handler's ID (which is the value false
if it was registered without an ID), and HandlerState is the handler's state.
To obtain more information about a process, including its state, see get_status/1
and get_status/2.
replace_state(Name, StateFun) -> NewState
replace_state(Name, StateFun, Timeout) -> NewState
Types:
Name = name()
StateFun = fun((State :: term()) -> NewState :: term())
Timeout = timeout()
NewState = term()
Replaces the state of the process, and returns the new state.
Note:
These functions are intended only to help with debugging, and they should not be be
called from normal code. They are provided for convenience, allowing developers to
avoid having to create their own custom state replacement functions.
The StateFun function provides a new state for the process. The State argument and
NewState return value of StateFun vary for different types of processes. For a
gen_server process, State is simply the callback module's state, and NewState is a
new instance of that state. For a gen_fsm process, State is the tuple
{CurrentStateName, CurrentStateData}, and NewState is a similar tuple that may
contain a new state name, new state data, or both. For a gen_event process, State
is the tuple {Module, Id, HandlerState} where Module is the event handler's module
name, Id is the handler's ID (which is the value false if it was registered without
an ID), and HandlerState is the handler's state. NewState is a similar tuple where
Module and Id shall have the same values as in State but the value of HandlerState
may be different. Returning a NewState whose Module or Id values differ from those
of State will result in the event handler's state remaining unchanged. For a
gen_event process, StateFun is called once for each event handler registered in the
gen_event process.
If a StateFun function decides not to effect any change in process state, then
regardless of process type, it may simply return its State argument.
If a StateFun function crashes or throws an exception, then for gen_server and
gen_fsm processes, the original state of the process is unchanged. For gen_event
processes, a crashing or failing StateFun function means that only the state of the
particular event handler it was working on when it failed or crashed is unchanged;
it can still succeed in changing the states of other event handlers registered in
the same gen_event process.
install(Name, FuncSpec) -> ok
install(Name, FuncSpec, Timeout) -> ok
Types:
Name = name()
FuncSpec = {Func, FuncState}
Func = dbg_fun()
FuncState = term()
Timeout = timeout()
This function makes it possible to install other debug functions than the ones
defined above. An example of such a function is a trigger, a function that waits
for some special event and performs some action when the event is generated. This
could, for example, be turning on low level tracing.
Func is called whenever a system event is generated. This function should return
done, or a new func state. In the first case, the function is removed. It is
removed if the function fails.
remove(Name, Func) -> ok
remove(Name, Func, Timeout) -> ok
Types:
Name = name()
Func = dbg_fun()
Timeout = timeout()
Removes a previously installed debug function from the process. Func must be the
same as previously installed.
PROCESS IMPLEMENTATION FUNCTIONS
The following functions are used when implementing a special process. This is an ordinary
process which does not use a standard behaviour, but a process which understands the
standard system messages.
EXPORTS
debug_options(Options) -> [dbg_opt()]
Types:
Options = [Opt]
Opt = trace
| log
| {log, integer() >= 1}
| statistics
| {log_to_file, FileName}
| {install, FuncSpec}
FileName = file:name()
FuncSpec = {Func, FuncState}
Func = dbg_fun()
FuncState = term()
This function can be used by a process that initiates a debug structure from a list
of options. The values of the Opt argument are the same as the corresponding
functions.
get_debug(Item, Debug, Default) -> term()
Types:
Item = log | statistics
Debug = [dbg_opt()]
Default = term()
This function gets the data associated with a debug option. Default is returned if
the Item is not found. Can be used by the process to retrieve debug data for
printing before it terminates.
handle_debug(Debug, FormFunc, Extra, Event) -> [dbg_opt()]
Types:
Debug = [dbg_opt()]
FormFunc = dbg_fun()
Extra = term()
Event = system_event()
This function is called by a process when it generates a system event. FormFunc is
a formatting function which is called as FormFunc(Device, Event, Extra) in order to
print the events, which is necessary if tracing is activated. Extra is any extra
information which the process needs in the format function, for example the name of
the process.
handle_system_msg(Msg, From, Parent, Module, Debug, Misc) ->
no_return()
Types:
Msg = term()
From = {pid(), Tag :: term()}
Parent = pid()
Module = module()
Debug = [dbg_opt()]
Misc = term()
This function is used by a process module that wishes to take care of system
messages. The process receives a {system, From, Msg} message and passes the Msg and
From to this function.
This function never returns. It calls the function Module:system_continue(Parent,
NDebug, Misc) where the process continues the execution, or
Module:system_terminate(Reason, Parent, Debug, Misc) if the process should
terminate. The Module must export system_continue/3, system_terminate/4, and
system_code_change/4 (see below).
The Misc argument can be used to save internal data in a process, for example its
state. It is sent to Module:system_continue/3 or Module:system_terminate/4
print_log(Debug) -> ok
Types:
Debug = [dbg_opt()]
Prints the logged system events in the debug structure using FormFunc as defined
when the event was generated by a call to handle_debug/4.
Mod:system_continue(Parent, Debug, Misc) -> none()
Types:
Parent = pid()
Debug = [dbg_opt()]
Misc = term()
This function is called from sys:handle_system_msg/6 when the process should
continue its execution (for example after it has been suspended). This function
never returns.
Mod:system_terminate(Reason, Parent, Debug, Misc) -> none()
Types:
Reason = term()
Parent = pid()
Debug = [dbg_opt()]
Misc = term()
This function is called from sys:handle_system_msg/6 when the process should
terminate. For example, this function is called when the process is suspended and
its parent orders shut-down. It gives the process a chance to do a clean-up. This
function never returns.
Mod:system_code_change(Misc, Module, OldVsn, Extra) -> {ok, NMisc}
Types:
Misc = term()
OldVsn = undefined | term()
Module = atom()
Extra = term()
NMisc = term()
Called from sys:handle_system_msg/6 when the process should perform a code change.
The code change is used when the internal data structure has changed. This function
converts the Misc argument to the new data structure. OldVsn is the vsn attribute
of the old version of the Module. If no such attribute was defined, the atom
undefined is sent.