A Functional Interface to System Messages

This module contains functions for sending system messages used by programs, and messages used for debugging purposes.

Functions used for implementation of processes should also understand system messages such as debugging messages and code change. These functions must be used to implement the use of system messages for a process; either directly, or through standard behaviours, such as gen_server.

The default timeout is 5000 ms, unless otherwise specified. The timeout defines the time period 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 reference to a debug structure. The debug structure is a list of dbg_opt(). dbg_opt() is an internal data type used by the handle_system_msg/6 function. No debugging is performed if it is an empty list.

System Messages

Processes which are not implemented as one of the standard behaviours must still understand system messages. There are three different messages which 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 has been received, the function sys:handle_system_msg/6 is called in order to handle the request.

Shutdown messages. If the process traps exits, it must be able to handle an shut-down 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.

There is one more message which the process must understand if the modules used to implement the process change dynamically during runtime. An example of such a process is the gen_event processes. This message is {get_modules, From}. The reply to this message is From ! {modules, 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 execute a certain module. The process may at a later time 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 tty.

There are three predefined system events which 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.

See above.




Turns the logging of system events On or Off. If On, a maximum of N events are kept in the debug structure (the 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 sys:handle_debug/4).

log_to_file(Name, Flag) -> ok | {error, open_file}

log_to_file(Name, Flag, Timeout) -> ok | {error, open_file}

Enables or disables the logging of all system events in textual 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 sys:handle_debug/4).

statistics(Name, Flag) -> ok | {ok, Statistics}

statistics(Name, Flag, Timeout) -> ok | {ok, Statistics}

Enables or disables the collection of statistics. If Flag is get, the statistical collection is returned.

trace(Name, Flag) -> ok

trace(Name, Flag, Timeout) -> ok

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 sys:handle_debug/4).

no_debug(Name) -> ok

no_debug(Name, Timeout) -> ok

Turns off all debugging for the process. This includes functions that have been installed explicitly with the install function, for example triggers.

suspend(Name) -> Void

suspend(Name, Timeout) -> Void

Suspends the process. When the process is suspended, it will only respond to other system messages, but not other messages.

resume(Name) -> Void

resume(Name, Timeout) -> Void

Resumes a suspended process.

change_code(Name, Module, OldVsn, Extra) -> ok | {error, Reason}


Tells the process to change code. The process must be suspended to handle this message. The Extra argument is reserved for each process to use as its own. The function Module:system_code_change/4 is called. OldVsn is the old version of the Module.

get_status(Name) -> Status

get_status(Name, Timeout) -> Status

Gets the status of the process.

The value of Misc varies for different types of processes. For example, a gen_server process returns the callback module's state, and a gen_fsm process returns information such as its current state name. Callback modules for gen_server and gen_fsm can also customise the value of Misc by exporting a format_status/2 function that contributes module-specific information; see gen_server:format_status/2 and gen_fsm:format_status/2 for more details.

install(Name, FuncSpec) -> Void

install(Name, FuncSpec, Timeout) -> Void

This function makes it possible to install other debug functions than the ones defined above. 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. This could, for example, be turning on low level tracing.

Func is called whenever a system event is generated. This function should return done, or a new func state. In the first case, the function is removed. It is removed if the function fails.

remove(Name, Func) -> Void

remove(Name, Func, Timeout) -> Void

Removes a previously installed debug function from the process. Func must be the same as previously installed.

Process Implementation Functions

The following functions are used when implementing a special process. This is an ordinary process which does not use a standard behaviour, but a process which understands the standard system messages.



This function can be used by a process that initiates a debug structure from a list of options. The values of the Opt argument are the same as the corresponding functions.

get_debug(Item, Debug, Default) -> term()

This function gets the data associated with a debug option. Default is returned if the Item is not found. Can be used by the process to retrieve debug data for printing before it terminates.


This function is called by a process when it generates a system event. FormFunc is a formatting function which is called as FormFunc(Device, Event, Extra) in order to print the events, which is necessary if tracing is activated. Extra is any extra information which the process needs in the format function, for example the name of the process.

handle_system_msg(Msg, From, Parent, Module, Debug, Misc) -> Void

This function is used by a process module that wishes to take care of system messages. The process receives a {system, From, Msg} message and passes the Msg and From to this function.

This function never returns. It calls the function Module:system_continue(Parent, NDebug, Misc) where the process continues the execution, or Module:system_terminate(Reason, Parent, Debug, Misc) if the process should terminate. The Module must export system_continue/3, system_terminate/4, and system_code_change/4 (see below).

The Misc argument 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

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.

Mod:system_continue(Parent, Debug, Misc) -> none()

  • Parent = pid()
  • Debug = [dbg_opt()]
  • Misc = term()

This function is called from sys:handle_system_msg/6 when the process should continue its execution (for example after it has been suspended). This function never returns.

Mod:system_terminate(Reason, Parent, Debug, Misc) -> none()

  • Reason = term()
  • Parent = pid()
  • Debug = [dbg_opt()]
  • Misc = term()

This function is called from sys:handle_system_msg/6 when the process should terminate. For example, this function is called when the process is suspended and its parent orders shut-down. It gives the process a chance to do a clean-up. This function never returns.

Mod:system_code_change(Misc, Module, OldVsn, Extra) -> {ok, NMisc}

  • Misc = term()
  • OldVsn = undefined | term()
  • Module = atom()
  • Extra = term()
  • NMisc = term()

Called from sys:handle_system_msg/6 when the process should perform a code change. The code change is used when the internal data structure has changed. This function converts the Misc argument to the new data structure. OldVsn is the vsn attribute of the old version of the Module. If no such attribute was defined, the atom undefined is sent.

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