# 6 sys and proc_lib

The sys module has functions for simple debugging of processes implemented using behaviours. It also has functions that, together with functions in the proc_lib module, can be used to implement a special process that complies to the OTP design principles without using a standard behaviour. These functions can also be used to implement user-defined (non-standard) behaviours.

Both sys and proc_lib belong to the STDLIB application.

###
6.1 Simple Debugging

The sys module has functions for simple debugging of processes implemented using behaviours. The code_lock example from gen_statem Behaviour is used to illustrate this:

    Erlang/OTP 20 [DEVELOPMENT] [erts-9.0] [source-5ace45e] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false]

    Eshell V9.0  (abort with ^G)
    1>  code_lock:start_link([1,2,3,4]).
    Lock
    {ok,<0.63.0>}
    2> sys:statistics(code_lock, true).
    ok
    3>  sys:trace(code_lock, true).
    ok
    4>  code_lock:button(1).
    *DBG* code_lock receive cast {button,1} in state locked
    ok
    *DBG* code_lock consume cast {button,1} in state locked
    5>  code_lock:button(2).
    *DBG* code_lock receive cast {button,2} in state locked
    ok
    *DBG* code_lock consume cast {button,2} in state locked
    6>  code_lock:button(3).
    *DBG* code_lock receive cast {button,3} in state locked
    ok
    *DBG* code_lock consume cast {button,3} in state locked
    7>  code_lock:button(4).
    *DBG* code_lock receive cast {button,4} in state locked
    ok
    Unlock
    *DBG* code_lock consume cast {button,4} in state locked
    *DBG* code_lock receive state_timeout lock in state open
    Lock
    *DBG* code_lock consume state_timeout lock in state open
    8> sys:statistics(code_lock, get).
    {ok,[{start_time,{{2017,4,21},{16,8,7}}},
         {current_time,{{2017,4,21},{16,9,42}}},
         {reductions,2973},
         {messages_in,5},
         {messages_out,0}]}
    9> sys:statistics(code_lock, false).
    ok
    10> sys:trace(code_lock, false).
    ok
    11> sys:get_status(code_lock).
    {status,<0.63.0>,
            {module,gen_statem},
            [[{'$initial_call',{code_lock,init,1}},
              {'$ancestors',[<0.61.0>]}],
             running,<0.61.0>,[],
             [{header,"Status for state machine code_lock"},
              {data,[{"Status",running},
                     {"Parent",<0.61.0>},
                     {"Logged Events",[]},
                     {"Postponed",[]}]},
              {data,[{"State",
                      {locked,#{code => [1,2,3,4],remaining => [1,2,3,4]}}}]}]]}

###
6.2 Special Processes

This section describes how to write a process that complies to the OTP design principles, without using a standard behaviour. Such a process is to:

- Be started in a way that makes the process fit into a supervision tree
- Support the sys debug facilities
- Take care of system messages.

System messages are messages with a special meaning, used in the supervision tree. Typical system messages are requests for trace output, and requests to suspend or resume process execution (used during release handling). Processes implemented using standard behaviours automatically understand these messages.

####
Example

The simple server from Overview, implemented using sys and proc_lib so it fits into a supervision tree:

    -module(ch4).
    -export([start_link/0]).
    -export([alloc/0, free/1]).
    -export([init/1]).
    -export([system_continue/3, system_terminate/4,
             write_debug/3,
             system_get_state/1, system_replace_state/2]).

    start_link() ->
        proc_lib:start_link(ch4, init, [self()]).

    alloc() ->
        ch4 ! {self(), alloc},
        receive
            {ch4, Res} ->
                Res
        end.

    free(Ch) ->
        ch4 ! {free, Ch},
        ok.

    init(Parent) ->
        register(ch4, self()),
        Chs = channels(),
        Deb = sys:debug_options([]),
        proc_lib:init_ack(Parent, {ok, self()}),
        loop(Chs, Parent, Deb).

    loop(Chs, Parent, Deb) ->
        receive
            {From, alloc} ->
                Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                        ch4, {in, alloc, From}),
                {Ch, Chs2} = alloc(Chs),
                From ! {ch4, Ch},
                Deb3 = sys:handle_debug(Deb2, fun ch4:write_debug/3,
                                        ch4, {out, {ch4, Ch}, From}),
                loop(Chs2, Parent, Deb3);
            {free, Ch} ->
                Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                        ch4, {in, {free, Ch}}),
                Chs2 = free(Ch, Chs),
                loop(Chs2, Parent, Deb2);

            {system, From, Request} ->
                sys:handle_system_msg(Request, From, Parent,
                                      ch4, Deb, Chs)
        end.

    system_continue(Parent, Deb, Chs) ->
        loop(Chs, Parent, Deb).

    system_terminate(Reason, _Parent, _Deb, _Chs) ->
        exit(Reason).

    system_get_state(Chs) ->
        {ok, Chs}.

    system_replace_state(StateFun, Chs) ->
        NChs = StateFun(Chs),
        {ok, NChs, NChs}.

    write_debug(Dev, Event, Name) ->
        io:format(Dev, "~p event = ~p~n", [Name, Event]).

Example on how the simple debugging functions in the sys module can also be used for ch4:

    % erl
    Erlang (BEAM) emulator version 5.2.3.6 [hipe] [threads:0]

    Eshell V5.2.3.6  (abort with ^G)
    1> ch4:start_link().
    {ok,<0.30.0>}
    2> sys:statistics(ch4, true).
    ok
    3> sys:trace(ch4, true).
    ok
    4> ch4:alloc().
    ch4 event = {in,alloc,<0.25.0>}
    ch4 event = {out,{ch4,ch1},<0.25.0>}
    ch1
    5> ch4:free(ch1).
    ch4 event = {in,{free,ch1}}
    ok
    6> sys:statistics(ch4, get).
    {ok,[{start_time,{{2003,6,13},{9,47,5}}},
         {current_time,{{2003,6,13},{9,47,56}}},
         {reductions,109},
         {messages_in,2},
         {messages_out,1}]}
    7> sys:statistics(ch4, false).
    ok
    8> sys:trace(ch4, false).
    ok
    9> sys:get_status(ch4).
    {status,<0.30.0>,
            {module,ch4},
            [[{'$ancestors',[<0.25.0>]},{'$initial_call',{ch4,init,[<0.25.0>]}}],
             running,<0.25.0>,[],
             [ch1,ch2,ch3]]}

####
Starting the Process

A function in the proc_lib module is to be used to start the process. Several functions are available, for example, spawn_link/3,4 for asynchronous start and start_link/3,4,5 for synchronous start.

A process started using one of these functions stores information (for example, about the ancestors and initial call) that is needed for a process in a supervision tree.

If the process terminates with another reason than normal or shutdown, a crash report is generated. For more information about the crash report, see the SASL User's Guide.

In the example, synchronous start is used. The process starts by calling ch4:start_link():

    start_link() ->
        proc_lib:start_link(ch4, init, [self()]).

ch4:start_link calls the function proc_lib:start_link. This function takes a module name, a function name, and an argument list as arguments, spawns, and links to a new process. The new process starts by executing the given function, here ch4:init(Pid), where Pid is the pid (self()) of the first process, which is the parent process.

All initialization, including name registration, is done in init. The new process must also acknowledge that it has been started to the parent:

    init(Parent) ->
        ...
        proc_lib:init_ack(Parent, {ok, self()}),
        loop(...).

proc_lib:start_link is synchronous and does not return until proc_lib:init_ack or proc_lib:init_fail has been called, or when the process exits.

####
Debugging

To support the debug facilities in sys, a debug structure is needed. The Deb term is initialized using sys:debug_options/1:

    init(Parent) ->
        ...
        Deb = sys:debug_options([]),
        ...
        loop(Chs, Parent, Deb).

sys:debug_options/1 takes a list of options as argument. Here the list is empty, which means no debugging is enabled initially. For information about the possible options, see the sys(3) manual page in STDLIB.

Then, for each system event to be logged or traced, the following function is to be called.

    sys:handle_debug(Deb, Func, Info, Event) => Deb1

Here:

- Deb is the debug structure.
- Func is a fun specifying a (user-defined) function used to format trace output. For each system event, the format function is called as Func(Dev, Event, Info), where:
  - Dev is the I/O device to which the output is to be printed. See the io(3) manual page in STDLIB.
  - Event and Info are passed as is from handle_debug.
- Info is used to pass more information to Func. It can be any term and is passed as is.
- Event is the system event. It is up to the user to define what a system event is and how it is to be represented. Typically at least incoming and outgoing messages are considered system events and represented by the tuples {in,Msg[,From]} and {out,Msg,To[,State]}, respectively.

handle_debug returns an updated debug structure Deb1.

In the example, handle_debug is called for each incoming and outgoing message. The format function Func is the function ch4:write_debug/3, which prints the message using io:format/3.

    loop(Chs, Parent, Deb) ->
        receive
            {From, alloc} ->
                Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                        ch4, {in, alloc, From}),
                {Ch, Chs2} = alloc(Chs),
                From ! {ch4, Ch},
                Deb3 = sys:handle_debug(Deb2, fun ch4:write_debug/3,
                                        ch4, {out, {ch4, Ch}, From}),
                loop(Chs2, Parent, Deb3);
            {free, Ch} ->
                Deb2 = sys:handle_debug(Deb, fun ch4:write_debug/3,
                                        ch4, {in, {free, Ch}}),
                Chs2 = free(Ch, Chs),
                loop(Chs2, Parent, Deb2);
            ...
        end.

    write_debug(Dev, Event, Name) ->
        io:format(Dev, "~p event = ~p~n", [Name, Event]).

####
Handling System Messages

System messages are received as:

    {system, From, Request}

The content and meaning of these messages do not need to be interpreted by the process. Instead the following function is to be called:

    sys:handle_system_msg(Request, From, Parent, Module, Deb, State)

This function does not return. It handles the system message and then either calls the following if process execution is to continue:

    Module:system_continue(Parent, Deb, State)

Or calls the following if the process is to terminate:

    Module:system_terminate(Reason, Parent, Deb, State)

A process in a supervision tree is expected to terminate with the same reason as its parent.

- Request and From are to be passed as is from the system message to the call to handle_system_msg.
- Parent is the pid of the parent.
- Module is the name of the module.
- Deb is the debug structure.
- State is a term describing the internal state and is passed to system_continue/system_terminate/ system_get_state/system_replace_state.

If the process is to return its state, handle_system_msg calls:

    Module:system_get_state(State)

If the process is to replace its state using the fun StateFun, handle_system_msg calls:

    Module:system_replace_state(StateFun, State)

In the example:

    loop(Chs, Parent, Deb) ->
        receive
            ...

            {system, From, Request} ->
                sys:handle_system_msg(Request, From, Parent,
                                      ch4, Deb, Chs)
        end.

    system_continue(Parent, Deb, Chs) ->
        loop(Chs, Parent, Deb).

    system_terminate(Reason, Parent, Deb, Chs) ->
        exit(Reason).

    system_get_state(Chs) ->
        {ok, Chs, Chs}.

    system_replace_state(StateFun, Chs) ->
        NChs = StateFun(Chs),
        {ok, NChs, NChs}.

If the special process is set to trap exits and if the parent process terminates, the expected behavior is to terminate with the same reason:

    init(...) ->
        ...,
        process_flag(trap_exit, true),
        ...,
        loop(...).

    loop(...) ->
        receive
            ...

            {'EXIT', Parent, Reason} ->
                ..maybe some cleaning up here..
                exit(Reason);
            ...
        end.

###
6.3 User-Defined Behaviours

To implement a user-defined behaviour, write code similar to code for a special process, but call functions in a callback module for handling specific tasks.

If the compiler is to warn for missing callback functions, as it does for the OTP behaviours, add -callback attributes in the behaviour module to describe the expected callbacks:

    -callback Name1(Arg1_1, Arg1_2, ..., Arg1_N1) -> Res1.
    -callback Name2(Arg2_1, Arg2_2, ..., Arg2_N2) -> Res2.
    ...
    -callback NameM(ArgM_1, ArgM_2, ..., ArgM_NM) -> ResM.

NameX are the names of the expected callbacks. ArgX_Y and ResX are types as they are described in Types and Function Specifications. The whole syntax of the -spec attribute is supported by the -callback attribute.

Callback functions that are optional for the user of the behaviour to implement are specified by use of the -optional_callbacks attribute:

    -optional_callbacks([OptName1/OptArity1, ..., OptNameK/OptArityK]).

where each OptName/OptArity specifies the name and arity of a callback function. Note that the -optional_callbacks attribute is to be used together with the -callback attribute; it cannot be combined with the behaviour_info() function described below.

Tools that need to know about optional callback functions can call Behaviour:behaviour_info(optional_callbacks) to get a list of all optional callback functions.

Note

We recommend using the -callback attribute rather than the behaviour_info() function. The reason is that the extra type information can be used by tools to produce documentation or find discrepancies.

As an alternative to the -callback and -optional_callbacks attributes you may directly implement and export behaviour_info():

    behaviour_info(callbacks) ->
        [{Name1, Arity1},...,{NameN, ArityN}].

where each {Name, Arity} specifies the name and arity of a callback function. This function is otherwise automatically generated by the compiler using the -callback attributes.

When the compiler encounters the module attribute -behaviour(Behaviour). in a module Mod, it calls Behaviour:behaviour_info(callbacks) and compares the result with the set of functions actually exported from Mod, and issues a warning if any callback function is missing.

Example:

    %% User-defined behaviour module
    -module(simple_server).
    -export([start_link/2, init/3, ...]).

    -callback init(State :: term()) -> 'ok'.
    -callback handle_req(Req :: term(), State :: term()) -> {'ok', Reply :: term()}.
    -callback terminate() -> 'ok'.
    -callback format_state(State :: term()) -> term().

    -optional_callbacks([format_state/1]).

    %% Alternatively you may define:
    %%
    %% -export([behaviour_info/1]).
    %% behaviour_info(callbacks) ->
    %%     [{init,1},
    %%      {handle_req,2},
    %%      {terminate,0}].

    start_link(Name, Module) ->
        proc_lib:start_link(?MODULE, init, [self(), Name, Module]).

    init(Parent, Name, Module) ->
        register(Name, self()),
        ...,
        Dbg = sys:debug_options([]),
        proc_lib:init_ack(Parent, {ok, self()}),
        loop(Parent, Module, Deb, ...).

    ...

In a callback module:

    -module(db).
    -behaviour(simple_server).

    -export([init/1, handle_req/2, terminate/0]).

    ...

The contracts specified with -callback attributes in behaviour modules can be further refined by adding -spec attributes in callback modules. This can be useful as -callback contracts are usually generic. The same callback module with contracts for the callbacks:

    -module(db).
    -behaviour(simple_server).

    -export([init/1, handle_req/2, terminate/0]).

    -record(state, {field1 :: [atom()], field2 :: integer()}).

    -type state()   :: #state{}.
    -type request() :: {'store', term(), term()};
                       {'lookup', term()}.

    ...

    -spec handle_req(request(), state()) -> {'ok', term()}.

    ...

Each -spec contract is to be a subtype of the respective -callback contract.