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# Compilation and Code Loading

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How code is compiled and loaded is not a language issue, but is system-dependent. This section describes compilation and code loading in Erlang/OTP with references to relevant parts of the documentation.

## Compilation

Erlang programs must be compiled to object code. The compiler can generate a new file that contains the object code. The current abstract machine, which runs the object code, is called BEAM and therefore the object files get the suffix `.beam`. The compiler can also generate a binary which can be loaded directly.

The compiler is located in the module `compile` in Compiler.

    compile:file(Module)
    compile:file(Module, Options)

The Erlang shell understands the command `c(Module)`, which both compiles and loads `Module`.

There is also a module `make`, which provides a set of functions similar to the UNIX type Make functions, see module `make` in Tools.

The compiler can also be accessed from the OS prompt using the erl executable in ERTS.

    % erl -compile Module1...ModuleN
    % erl -make

The `erlc` program provides a way to compile modules from the OS shell, see the erlc executable in ERTS. It understands a number of flags that can be used to define macros, add search paths for include files, and more.

    % erlc <flags> File1.erl...FileN.erl

## Code Loading

The object code must be loaded into the Erlang runtime system. This is handled by the code server, see module `code` in Kernel.

The code server loads code according to a code loading strategy, which is either interactive (default) or embedded. In interactive mode, code is searched for in a code path and loaded when first referenced. In embedded mode, code is loaded at start-up according to a boot script. This is described in System Principles.

## Code Replacement

Erlang supports change of code in a running system. Code replacement is done on the module level.

The code of a module can exist in two variants in a system: current and old. When a module is loaded into the system for the first time, the code becomes 'current'. If then a new instance of the module is loaded, the code of the previous instance becomes 'old' and the new instance becomes 'current'.

Both old and current code are valid, and can be evaluated concurrently. Fully qualified function calls always refer to current code. Old code can still be evaluated because of processes lingering in the old code.

If a third instance of the module is loaded, the code server removes (purges) the old code and any processes lingering in it are terminated. Then the third instance becomes 'current' and the previously current code becomes 'old'.

To change from old code to current code, a process must make a fully qualified function call.

Example:

    -module(m).
    -export([loop/0]).

    loop() ->
        receive
            code_switch ->
                m:loop();
            Msg ->
                ...
                loop()
        end.

To make the process change code, send the message `code_switch` to it. The process then makes a fully qualified call to `m:loop()` and changes to current code. Notice that `m:loop/0` must be exported.

For code replacement of funs to work, use the syntax `fun Module:FunctionName/Arity`.

## Running a Function When a Module is Loaded

The `-on_load()` directive names a function that is to be run automatically when a module is loaded.

Its syntax is as follows:

    -on_load(Name/0).

It is not necessary to export the function. It is called in a freshly spawned process (which terminates as soon as the function returns).

The function must return `ok` if the module is to become the new current code for the module and become callable.

Returning any other value or generating an exception causes the new code to be unloaded. If the return value is not an atom, a warning error report is sent to the error logger.

If there already is current code for the module, that code will remain current and can be called until the `on_load` function has returned. If the `on_load` function fails, the current code (if any) will remain current. If there is no current code for a module, any process that makes an external call to the module before the `on_load` function has finished will be suspended until the `on_load` function has finished.

#### Change

Before Erlang/OTP 19, if the `on_load` function failed, any previously current code would become old, essentially leaving the system without any working and reachable instance of the module.

In embedded mode, first all modules are loaded. Then all `on_load` functions are called. The system is terminated unless all of the `on_load` functions return `ok`.

Example:

    -module(m).
    -on_load(load_my_nifs/0).

    load_my_nifs() ->
        NifPath = ...,    %Set up the path to the NIF library.
        Info = ...,       %Initialize the Info term
        erlang:load_nif(NifPath, Info).

If the call to `erlang:load_nif/2` fails, the module is unloaded and a warning report is sent to the error logger.

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