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# proc_lib

### MODULE

proc_lib

### MODULE SUMMARY

Functions for asynchronous and synchronous start of processes adhering to the OTP design principles.

### DESCRIPTION

This module is used to start processes adhering to the OTP Design Principles. Specifically, the functions in this module are used by the OTP standard behaviors (gen_server, gen_fsm, ...) when starting new processes. The functions can also be used to start special processes, user defined processes which comply to the OTP design principles. See Sys and Proc_Lib in OTP Design Principles for an example.

Some useful information is initialized when a process starts. The registered names, or the process identifiers, of the parent process, and the parent ancestors, are stored together with information about the function initially called in the process.

While in "plain Erlang" a process is said to terminate normally only for the exit reason normal, a process started using proc_lib is also said to terminate normally if it exits with reason shutdown or {shutdown,Term}. shutdown is the reason used when an application (supervision tree) is stopped.

When a process started using proc_lib terminates abnormally -- that is, with another exit reason than normal, shutdown, or {shutdown,Term} -- a crash report is generated, which is written to terminal by the default SASL event handler. That is, the crash report is normally only visible if the SASL application is started. See sasl(6) and SASL User's Guide.

The crash report contains the previously stored information such as ancestors and initial function, the termination reason, and information regarding other processes which terminate as a result of this process terminating.

### EXPORTS

spawn(Fun) -> pid()
spawn(Node, Fun) -> pid()
spawn(Module, Function, Args) -> pid()
spawn(Node, Module, Function, Args) -> pid()

Types:

Node = node()
Fun = fun() -> void()
Module = Function = atom()
Args = [term()]

Spawns a new process and initializes it as described above. The process is spawned using the spawn BIFs.

spawn_link(Fun) -> pid()
spawn_link(Node, Fun) -> pid()
spawn_link(Module, Function, Args) -> pid()
spawn_link(Node, Module, Function, Args) -> pid()

Types:

Node = node()
Fun = fun() -> void()
Module = Function = atom()
Args = [term()]

Spawns a new process and initializes it as described above. The process is spawned using the spawn_link BIFs.

spawn_opt(Fun, SpawnOpts) -> pid()
spawn_opt(Node, Fun, SpawnOpts) -> pid()
spawn_opt(Module, Function, Args, SpawnOpts) -> pid()
spawn_opt(Node, Module, Func, Args, SpawnOpts) -> pid()

Types:

Node = node()
Fun = fun() -> void()
Module = Function = atom()
Args = [term()]
SpawnOpts -- see erlang:spawn_opt/2,3,4,5

Spawns a new process and initializes it as described above. The process is spawned using the spawn_opt BIFs.

Note

Using the spawn option monitor is currently not allowed, but will cause the function to fail with reason badarg.

start(Module, Function, Args) -> Ret
start(Module, Function, Args, Time) -> Ret
start(Module, Function, Args, Time, SpawnOpts) -> Ret
start_link(Module, Function, Args) -> Ret
start_link(Module, Function, Args, Time) -> Ret
start_link(Module, Function, Args, Time, SpawnOpts) -> Ret

Types:

Module = Function = atom()
Args = [term()]
Time = int() >= 0 | infinity
SpawnOpts -- see erlang:spawn_opt/2,3,4,5
Ret = term() | {error, Reason}

Starts a new process synchronously. Spawns the process and waits for it to start. When the process has started, it must call init_ack(Parent,Ret) or init_ack(Ret), where Parent is the process that evaluates this function. At this time, Ret is returned.

If the start_link/3,4,5 function is used and the process crashes before it has called init_ack/1,2, {error, Reason} is returned if the calling process traps exits.

If Time is specified as an integer, this function waits for Time milliseconds for the new process to call init_ack, or {error, timeout} is returned, and the process is killed.

The SpawnOpts argument, if given, will be passed as the last argument to the spawn_opt/2,3,4,5 BIF.

Note

Using the spawn option monitor is currently not allowed, but will cause the function to fail with reason badarg.

init_ack(Parent, Ret) -> void()
init_ack(Ret) -> void()

Types:

Parent = pid()
Ret = term()

This function must used by a process that has been started by a start[_link]/3,4,5 function. It tells Parent that the process has initialized itself, has started, or has failed to initialize itself.

The init_ack/1 function uses the parent value previously stored by the start function used.

If this function is not called, the start function will return an error tuple (if a link and/or a timeout is used) or hang otherwise.

The following example illustrates how this function and proc_lib:start_link/3 are used.

    -module(my_proc).
    -export([start_link/0]).
    -export([init/1]).

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

    init(Parent) ->
        case do_initialization() of
            ok ->
                proc_lib:init_ack(Parent, {ok, self()});
            {error, Reason} ->
                exit(Reason)
        end,
        loop().

    ...

format(CrashReport) -> string()

Types:

CrashReport = term()

This function can be used by a user defined event handler to format a crash report. The crash report is sent using error_logger:error_report(crash_report, CrashReport). That is, the event to be handled is of the format {error_report, GL, {Pid, crash_report, CrashReport}} where GL is the group leader pid of the process Pid which sent the crash report.

initial_call(Process) -> {Module,Function,Args} | false

Types:

Process = pid() | {X,Y,Z} | ProcInfo
X = Y = Z = int()
ProcInfo = term()
Module = Function = atom()
Args = [atom()]

Extracts the initial call of a process that was started using one of the spawn or start functions described above. Process can either be a pid, an integer tuple (from which a pid can be created), or the process information of a process Pid fetched through an erlang:process_info(Pid) function call.

Note

The list Args no longer contains the actual arguments, but the same number of atoms as the number of arguments; the first atom is always 'Argument__1', the second 'Argument__2', and so on. The reason is that the argument list could waste a significant amount of memory, and if the argument list contained funs, it could be impossible to upgrade the code for the module.

If the process was spawned using a fun, initial_call/1 no longer returns the actual fun, but the module, function for the local function implementing the fun, and the arity, for instance {some_module,-work/3-fun-0-,0} (meaning that the fun was created in the function some_module:work/3). The reason is that keeping the fun would prevent code upgrade for the module, and that a significant amount of memory could be wasted.

translate_initial_call(Process) -> {Module,Function,Arity} | Fun

Types:

Process = pid() | {X,Y,Z} | ProcInfo
X = Y = Z = int()
ProcInfo = term()
Module = Function = atom()
Arity = int()
Fun = fun() -> void()

This function is used by the c:i/0 and c:regs/0 functions in order to present process information.

Extracts the initial call of a process that was started using one of the spawn or start functions described above, and translates it to more useful information. Process can either be a pid, an integer tuple (from which a pid can be created), or the process information of a process Pid fetched through an erlang:process_info(Pid) function call.

If the initial call is to one of the system defined behaviors such as gen_server or gen_event, it is translated to more useful information. If a gen_server is spawned, the returned Module is the name of the callback module and Function is init (the function that initiates the new server).

A supervisor and a supervisor_bridge are also gen_server processes. In order to return information that this process is a supervisor and the name of the call-back module, Module is supervisor and Function is the name of the supervisor callback module. Arity is 1 since the init/1 function is called initially in the callback module.

By default, {proc_lib,init_p,5} is returned if no information about the initial call can be found. It is assumed that the caller knows that the process has been spawned with the proc_lib module.

hibernate(Module, Function, Args)

Types:

Module = Function = atom()
Args = [term()]

This function does the same as (and does call) the BIF hibernate/3, but ensures that exception handling and logging continues to work as expected when the process wakes up. Always use this function instead of the BIF for processes started using proc_lib functions.

### SEE ALSO

error_logger(3)