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# error_handler (kernel v10.6.3)

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Default system error handler.

This module defines what happens when certain types of errors occur.

You can change the error handler of a process by calling `erlang:process_flag(error_handler, NewErrorHandler)`.

## Notes

The code in `error_handler` is complex. Do not change it without fully understanding the interaction between the error handler, the `init` process of the code server, and the I/O mechanism of the code.

Code changes that seem small can cause a deadlock, as unforeseen consequences can occur. The use of `input` is dangerous in this type of code.

## Summary

### Functions

raise_undef_exception(Module, Function, Args)

Raises an `undef` exception with a stacktrace, indicating that `Module:Function/N` is undefined.

undefined_function(Module, Function, Args)

This function is called by the runtime system if a call is made to `Module:Function(Arg1,.., ArgN)` and `Module:Function/N` is undefined. Notice that this function is evaluated inside the process making the original call.

undefined_lambda(Module, Fun, Args)

This function is evaluated if a call is made to `Fun(Arg1,.., ArgN)` when the module defining the fun is not loaded. The function is evaluated inside the process making the original call.

## Functions

# raise_undef_exception(Module, Function, Args)

(since OTP R16B)

    -spec raise_undef_exception(Module, Function, Args) -> no_return()
                                   when Module :: atom(), Function :: atom(), Args :: list().

Raises an `undef` exception with a stacktrace, indicating that `Module:Function/N` is undefined.

# undefined_function(Module, Function, Args)

    -spec undefined_function(Module, Function, Args) -> any()
                                when Module :: atom(), Function :: atom(), Args :: list().

This function is called by the runtime system if a call is made to `Module:Function(Arg1,.., ArgN)` and `Module:Function/N` is undefined. Notice that this function is evaluated inside the process making the original call.

This function first attempts to autoload `Module`. If that is not possible, an `undef` exception is raised.

If it is possible to load `Module` and function `Function/N` is exported, it is called.

Otherwise, if function `'$handle_undefined_function'/2` is exported, it is called as `'$handle_undefined_function'(`Function, Args).

#### Warning

Defining `'$handle_undefined_function'/2` in ordinary application code is highly discouraged. It is very easy to make subtle errors that can take a long time to debug. Furthermore, none of the tools for static code analysis (such as Dialyzer and Xref) supports the use of `'$handle_undefined_function'/2` and no such support will be added. Only use this function after having carefully considered other, less dangerous, solutions. One example of potential legitimate use is creating stubs for other sub-systems during testing and debugging.

Otherwise an `undef` exception is raised.

# undefined_lambda(Module, Fun, Args)

    -spec undefined_lambda(Module, Fun, Args) -> term() when Module :: atom(), Fun :: fun(), Args :: list().

This function is evaluated if a call is made to `Fun(Arg1,.., ArgN)` when the module defining the fun is not loaded. The function is evaluated inside the process making the original call.

If `Module` is interpreted, the interpreter is invoked and the return value of the interpreted `Fun(Arg1,.., ArgN)` call is returned.

Otherwise, it returns, if possible, the value of `apply(Fun, Args)` after an attempt is made to autoload `Module`. If this is not possible, the call fails with exit reason `undef`.