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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 are also expected to understand system messages, such as
debug messages and code change. These functions must be used to implement the use of system messages for
a process; either directly, or through standard behaviors, such as gen_server.
The default time-out is 5000 ms, unless otherwise specified. timeout defines the time 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 references to a debug structure. The debug structure is a list of dbg_opt(), which is
an internal data type used by function handle_system_msg/6. No debugging is performed if it is an empty
list.
SYSTEM MESSAGES
Processes that are not implemented as one of the standard behaviors must still understand system
messages. The following three messages 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 is received,
function handle_system_msg/6 is called to handle the request.
* Shutdown messages. If the process traps exits, it must be able to handle a shutdown 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.
* If the modules used to implement the process change dynamically during runtime, the process must
understand one more message. An example is the gen_event processes. The message is {_Label, {From,
Ref}, get_modules}. The reply to this message is From ! {Ref, 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 that execute a certain module.
The process can later 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 terminal.
Three predefined system events 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 the introduction of this manual page.
dbg_fun() =
fun((FuncState :: term(),
Event :: system_event(),
ProcState :: term()) ->
done | (NewFuncState :: term()))
format_fun() =
fun((Device :: io:device() | file:io_device(),
Event :: system_event(),
Extra :: term()) ->
any())
EXPORTS
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. Argument
Extra is reserved for each process to use as its own. Function Module:system_code_change/4 is
called. OldVsn is the old version of the Module.
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 the 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, as follows:
* For a gen_server process, the returned State is the state of the callback module.
* For a gen_statem process, State is the tuple {CurrentState,CurrentData}.
* For a gen_event process, State is a list of tuples, where each tuple corresponds to an event
handler registered in the process and contains {Module, Id, HandlerState}, as follows:
Module:
The module name of the event handler.
Id:
The ID of the handler (which is false if it was registered without an ID).
HandlerState:
The state of the handler.
If the callback module exports a function system_get_state/1, it is called in the target process
to get its state. Its argument is the same as the Misc value returned by get_status/1,2, and
function Module:system_get_state/1 is expected to extract the state of the callback module from
it. Function system_get_state/1 must return {ok, State}, where State is the state of the callback
module.
If the callback module does not export a system_get_state/1 function, get_state/1,2 assumes that
the Misc value is the state of the callback module and returns it directly instead.
If the callback module's system_get_state/1 function crashes or throws an exception, the caller
exits with error {callback_failed, {Module, system_get_state}, {Class, Reason}}, where Module is
the name of the callback module and Class and Reason indicate details of the exception.
Function system_get_state/1 is primarily useful for user-defined behaviors and modules that
implement OTP special processes. The gen_server, gen_statem, and gen_event OTP behavior modules
export this function, so callback modules for those behaviors need not to supply their own.
For more information about a process, including its state, see get_status/1 and get_status/2.
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 state of the callback module.
* A gen_statem process returns information, such as its current state name and state data.
* A gen_event process returns information about each of its registered handlers.
Callback modules for gen_server, gen_statem, and gen_event can also change the value of Misc by
exporting a function format_status/2, which contributes module-specific information. For details,
see gen_server:format_status/2, gen_statem:format_status/2, and gen_event:format_status/2.
install(Name, FuncSpec) -> ok
install(Name, FuncSpec, Timeout) -> ok
Types:
Name = name()
FuncSpec = {Func, FuncState}
Func = dbg_fun()
FuncState = term()
Timeout = timeout()
Enables installation of alternative debug functions. 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.
For example, turning on low-level tracing.
Func is called whenever a system event is generated. This function is to return done, or a new
Func state. In the first case, the function is removed. It is also removed if the function fails.
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 (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 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 text 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
handle_debug/4). The file is opened with encoding UTF-8.
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 are installed explicitly
with function install/2,3, for example, triggers.
remove(Name, Func) -> ok
remove(Name, Func, Timeout) -> ok
Types:
Name = name()
Func = dbg_fun()
Timeout = timeout()
Removes an installed debug function from the process. Func must be the same as previously
installed.
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 are not to be called from normal
code. They are provided for convenience, allowing developers to avoid having to create their own
custom state replacement functions.
Function StateFun provides a new state for the process. Argument State and the NewState return
value of StateFun vary for different types of processes as follows:
* For a gen_server process, State is the state of the callback module and NewState is a new
instance of that state.
* For a gen_statem process, State is the tuple {CurrentState,CurrentData}, and NewState is a
similar tuple, which can contain a new current state, new state data, or both.
* For a gen_event process, State is the tuple {Module, Id, HandlerState} as follows:
Module:
The module name of the event handler.
Id:
The ID of the handler (which is false if it was registered without an ID).
HandlerState:
The state of the handler.
NewState is a similar tuple where Module and Id are to have the same values as in State, but
the value of HandlerState can be different. Returning a NewState, whose Module or Id values
differ from those of State, leaves the state of the event handler 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 can return its State argument.
If a StateFun function crashes or throws an exception, the original state of the process is
unchanged for gen_server, and gen_statem processes. 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.
If the callback module exports a system_replace_state/2 function, it is called in the target
process to replace its state using StateFun. Its two arguments are StateFun and Misc, where Misc
is the same as the Misc value returned by get_status/1,2. A system_replace_state/2 function is
expected to return {ok, NewState, NewMisc}, where NewState is the new state of the callback
module, obtained by calling StateFun, and NewMisc is a possibly new value used to replace the
original Misc (required as Misc often contains the state of the callback module within it).
If the callback module does not export a system_replace_state/2 function, replace_state/2,3
assumes that Misc is the state of the callback module, passes it to StateFun and uses the return
value as both the new state and as the new value of Misc.
If the callback module's function system_replace_state/2 crashes or throws an exception, the
caller exits with error {callback_failed, {Module, system_replace_state}, {Class, Reason}}, where
Module is the name of the callback module and Class and Reason indicate details of the exception.
If the callback module does not provide a system_replace_state/2 function and StateFun crashes or
throws an exception, the caller exits with error {callback_failed, StateFun, {Class, Reason}}.
Function system_replace_state/2 is primarily useful for user-defined behaviors and modules that
implement OTP special processes. The OTP behavior modules gen_server, gen_statem, and gen_event
export this function, so callback modules for those behaviors need not to supply their own.
resume(Name) -> ok
resume(Name, Timeout) -> ok
Types:
Name = name()
Timeout = timeout()
Resumes a suspended process.
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.
suspend(Name) -> ok
suspend(Name, Timeout) -> ok
Types:
Name = name()
Timeout = timeout()
Suspends the process. When the process is suspended, it only responds to other system messages,
but not other messages.
terminate(Name, Reason) -> ok
terminate(Name, Reason, Timeout) -> ok
Types:
Name = name()
Reason = term()
Timeout = timeout()
Orders the process to terminate with the specified Reason. The termination is done asynchronously,
so it is not guaranteed that the process is terminated when the function returns.
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 handle_debug/4).
PROCESS IMPLEMENTATION FUNCTIONS
The following functions are used when implementing a special process. This is an ordinary process, which
does not use a standard behavior, but a process that 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()
Can be used by a process that initiates a debug structure from a list of options. The values of
argument Opt are the same as for the corresponding functions.
get_debug(Item, Debug, Default) -> term()
Types:
Item = log | statistics
Debug = [dbg_opt()]
Default = term()
Gets the data associated with a debug option. Default is returned if 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 = format_fun()
Extra = term()
Event = system_event()
This function is called by a process when it generates a system event. FormFunc is a formatting
function, called as FormFunc(Device, Event, Extra) to print the events, which is necessary if
tracing is activated. Extra is any extra information that the process needs in the format
function, for example, the process name.
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 to take care of system messages. The process receives a
{system, From, Msg} message and passes Msg and From to this function.
This function never returns. It calls either of the following functions:
* Module:system_continue(Parent, NDebug, Misc), where the process continues the execution.
* Module:system_terminate(Reason, Parent, Debug, Misc), if the process is to terminate.
Module must export the following:
* system_continue/3
* system_terminate/4
* system_code_change/4
* system_get_state/1
* system_replace_state/2
Argument Misc 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.
Module:system_code_change(Misc, Module, OldVsn, Extra) -> {ok, NMisc}
Types:
Misc = term()
OldVsn = undefined | term()
Module = atom()
Extra = term()
NMisc = term()
Called from handle_system_msg/6 when the process is to perform a code change. The code change is
used when the internal data structure has changed. This function converts argument Misc to the new
data structure. OldVsn is attribute vsn of the old version of the Module. If no such attribute is
defined, the atom undefined is sent.
Module:system_continue(Parent, Debug, Misc) -> none()
Types:
Parent = pid()
Debug = [dbg_opt()]
Misc = term()
Called from handle_system_msg/6 when the process is to continue its execution (for example, after
it has been suspended). This function never returns.
Module:system_get_state(Misc) -> {ok, State}
Types:
Misc = term()
State = term()
Called from handle_system_msg/6 when the process is to return a term that reflects its current
state. State is the value returned by get_state/2.
Module:system_replace_state(StateFun, Misc) -> {ok, NState, NMisc}
Types:
StateFun = fun((State :: term()) -> NState)
Misc = term()
NState = term()
NMisc = term()
Called from handle_system_msg/6 when the process is to replace its current state. NState is the
value returned by replace_state/3.
Module:system_terminate(Reason, Parent, Debug, Misc) -> none()
Types:
Reason = term()
Parent = pid()
Debug = [dbg_opt()]
Misc = term()
Called from handle_system_msg/6 when the process is to terminate. For example, this function is
called when the process is suspended and its parent orders shutdown. It gives the process a chance
to do a cleanup. This function never returns.