12 Compilation and Code Loading

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

12.1 Compilation

Erlang programs must be compiled to object code. The compiler can generate a new file which contains the object code. The current abstract machine which runs the object code is called BEAM, 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 Kernel module compile, see compile(3).

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 make(3).

The compiler can also be accessed from the OS prompt, see erl(1).

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

The erlc program provides an even better way to compile modules from the shell, see erlc(1). 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

12.2 Code Loading

The object code must be loaded into the Erlang runtime system. This is handled by the code server, see code(3).

The code server loads code according to a code loading strategy which is either interactive (default) or embedded. In interactive mode, code are 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.

12.3 Code Replacement

Erlang supports change of code in a running system. Code replacement is done on 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 is valid, and may be evaluated concurrently. Fully qualified function calls always refer to current code. Old code may still be evaluated because of processes lingering in the old code.

If a third instance of the module is loaded, the code server will remove (purge) the old code and any processes lingering in it will be 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 will make a fully qualified call to m:loop() and change to current code. Note that m:loop/0 must be exported.

For code replacement of funs to work, the tuple syntax {Module,FunctionName} must be used to represent the fun.