Sys and Proc_Lib

## 6 Sys and Proc_Lib

The module `sys` contains functions for simple debugging of processes implemented using behaviours.

There are also functions that, together with functions in the module `proc_lib`, can be used to implement a special process, a process which comply to the OTP design principles without making use of a standard behaviour. They 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 module `sys` contains some functions for simple debugging of processes implemented using behaviours. We use the `code_lock` example from the gen_event chapter to illustrate this:

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

    Eshell V5.2.3.6  (abort with ^G)
    1> code_lock:start_link([1,2,3,4]).
    {ok,<0.32.0>}
    2> sys:statistics(code_lock, true).
    ok
    3> sys:trace(code_lock, true).
    ok
    4> code_lock:button(4).
    *DBG* code_lock got event {button,4} in state closed
    ok
    *DBG* code_lock switched to state closed
    5>  code_lock:button(3).
    *DBG* code_lock got event {button,3} in state closed
    ok
    *DBG* code_lock switched to state closed
    6> code_lock:button(2).
    *DBG* code_lock got event {button,2} in state closed
    ok
    *DBG* code_lock switched to state closed
    7> code_lock:button(1).
    *DBG* code_lock got event {button,1} in state closed
    ok
    OPEN DOOR
    *DBG* code_lock switched to state open
    *DBG* code_lock got event timeout in state open
    CLOSE DOOR
    *DBG* code_lock switched to state closed
    8> sys:statistics(code_lock, get).
    {ok,[{start_time,{{2003,6,12},{14,11,40}}},
         {current_time,{{2003,6,12},{14,12,14}}},
         {reductions,333},
         {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,<0.32.0>,
            {module,gen_fsm},
            [[{'$ancestors',[<0.30.0>]},
              {'$initial_call',{gen,init_it,
                                    [gen_fsm,
                                     <0.30.0>,
                                     <0.30.0>,
                                     {local,code_lock},
                                     code_lock,
                                     [1,2,3,4],
                                     []]}}],
             running,
             <0.30.0>,
             [],
             [code_lock,closed,{[],[1,2,3,4]},code_lock,infinity]]}

### 6.2 Special Processes

This section describes how to write a process which comply to the OTP design principles, without making use of a standard behaviour. Such a process should:

- be started in a way that makes the process fit into a supervision tree,
- support the `sys` debug facilities, and
- take care of system messages.

System messages are messages with 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.

#### 6.2.1 Example

The simple server from the Overview chapter, 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]).

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

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

Example on how the simple debugging functions in `sys` can be used for `ch4` as well:

    % 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]]}

#### 6.2.2 Starting the Process

A function in the `proc_lib` module should be used to start the process. There are several possible functions, 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 will store information that is needed for a process in a supervision tree, for example about the ancestors and initial call.

Also, if the process terminates with another reason than `normal` or `shutdown`, a crash report (see SASL User's Guide) is generated.

In the example, synchronous start is used. The process is started 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 and spawns and links to a new process. The new process starts by executing the given function, in this case `ch4:init(Pid)`, where `Pid` is the pid (`self()`) of the first process, that is the parent process.

In `init`, all initialization including name registration is done. 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` has been called.

#### 6.2.3 Debugging

To support the debug facilites in `sys`, we need a debug structure, a term `Deb` which 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. See `sys(3)` for information about possible options.

Then for each system event that we want to be logged or traced, the following function should be called.

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

- `Deb` is the debug structure.
- `Func` is a tuple `{Module, Name}` (or a fun) and should specify a (user defined) function used to format trace output. For each system event, the format function is called as `Module:Name(Dev, Event, Info)`, where:
  - `Dev` is the IO device to which the output should be printed. See `io(3)`.
  - `Event` and `Info` are passed as-is from `handle_debug`.
- `Info` is used to pass additional 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 should be represented, but typically at least incoming and outgoing messages are considered system events and represented by the tuples `{in,Msg[,From]}` and `{out,Msg,To}`, 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, {ch4, write_debug},
                                        ch4, {in, alloc, From}),
                {Ch, Chs2} = alloc(Chs),
                From ! {ch4, Ch},
                Deb3 = sys:handle_debug(Deb2, {ch4, write_debug},
                                        ch4, {out, {ch4, Ch}, From}),
                loop(Chs2, Parent, Deb3);
            {free, Ch} ->
                Deb2 = sys:handle_debug(Deb, {ch4, write_debug},
                                        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]).

#### 6.2.4 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 should be called:

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

This function does not return. It will handle the system message and then call:

    Module:system_continue(Parent, Deb, State)

if process execution should continue, or:

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

if the process should terminate. Note that a process in a supervision tree is expected to terminate with the same reason as its parent.

- `Request` and `From` should 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`.

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).

If the special process is set to trap exits, note that 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 calling functions in a callback module for handling specific tasks.

If it is desired that the compiler should warn for missing callback functions, as it does for the OTP behaviours, implement and export the function:

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

where each `{Name,Arity}` specifies the name and arity of a callback function.

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

Example:

    %% User-defined behaviour module
    -module(simple_server).
    -export([start_link/2,...]).
    -export([behaviour_info/1]).

    behaviour_info(callbacks) ->
        [{init,1},
         {handle_req,1},
         {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/0, handle_req/1, terminate/0]).

    ...

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