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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.