In interactive mode, the code server maintains a search path, usually called the code path, consisting of a list of directories, which it searches sequentially when trying to load a module.

Initially, the code path consists of the current working directory and all Erlang object code directories under library directory $OTPROOT/lib, where $OTPROOT is the installation directory of Erlang/OTP, code:root_dir(). Directories can be named Name[-Vsn] and the code server, by default, chooses the directory with the highest version number among those having the same Name. Suffix -Vsn is optional. If an ebin directory exists under Name[-Vsn], this directory is added to the code path.

Environment variable ERL_LIBS (defined in the operating system) can be used to define more library directories to be handled in the same way as the standard OTP library directory described above, except that directories without an ebin directory are ignored.

All application directories found in the additional directories appears before the standard OTP applications, except for the Kernel and STDLIB applications, which are placed before any additional applications. In other words, modules found in any of the additional library directories override modules with the same name in OTP, except for modules in Kernel and STDLIB.

Environment variable ERL_LIBS (if defined) is to contain a colon-separated (for Unix-like systems) or semicolon-separated (for Windows) list of additional libraries.

Example:

On a Unix-like system, ERL_LIBS can be set to the following

On Windows, use semi-colon as separator.

The Erlang archives are ZIP files with extension .ez. Erlang archives can also be enclosed in escript files whose file extension is arbitrary.

Erlang archive files can contain entire Erlang applications or parts of applications. The structure in an archive file is the same as the directory structure for an application. If you, for example, create an archive of mnesia-4.4.7, the archive file must be named mnesia-4.4.7.ez and it must contain a top directory named mnesia-4.4.7. If the version part of the name is omitted, it must also be omitted in the archive. That is, a mnesia.ez archive must contain a mnesia top directory.

An archive file for an application can, for example, be created like this:

zip:create("mnesia-4.4.7.ez",
	["mnesia-4.4.7"],
	[{cwd, code:lib_dir()},
	 {compress, all},
	 {uncompress,[".beam",".app"]}]).

Any file in the archive can be compressed, but to speed up the access of frequently read files, it can be a good idea to store beam and app files uncompressed in the archive.

Normally the top directory of an application is located in library directory $OTPROOT/lib or in a directory referred to by environment variable ERL_LIBS. At startup, when the initial code path is computed, the code server also looks for archive files in these directories and possibly adds ebin directories in archives to the code path. The code path then contains paths to directories that look like $OTPROOT/lib/mnesia.ez/mnesia/ebin or $OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin.

The code server uses module erl_prim_loader in ERTS (possibly through erl_boot_server) to read code files from archives. However, the functions in erl_prim_loader can also be used by other applications to read files from archives. For example, the call erl_prim_loader:list_dir( "/otp/root/lib/mnesia-4.4.7.ez/mnesia-4.4.7/examples/bench)" would list the contents of a directory inside an archive. See erl_prim_loader(3).

An application archive file and a regular application directory can coexist. This can be useful when it is needed to have parts of the application as regular files. A typical case is the priv directory, which must reside as a regular directory to link in drivers dynamically and start port programs. For other applications that do not need this, directory priv can reside in the archive and the files under the directory priv can be read through erl_prim_loader.

When a directory is added to the code path and when the entire code path is (re)set, the code server decides which subdirectories in an application that are to be read from the archive and which that are to be read as regular files. If directories are added or removed afterwards, the file access can fail if the code path is not updated (possibly to the same path as before, to trigger the directory resolution update).

For each directory on the second level in the application archive (ebin, priv, src, and so on), the code server first chooses the regular directory if it exists and second from the archive. Function code:lib_dir/2 returns the path to the subdirectory. For example, code:lib_dir(megaco,ebin) can return /otp/root/lib/megaco-3.9.1.1.ez/megaco-3.9.1.1/ebin while code:lib_dir(megaco,priv) can return /otp/root/lib/megaco-3.9.1.1/priv.

When an escript file contains an archive, there are no restrictions on the name of the escript and no restrictions on how many applications that can be stored in the embedded archive. Single Beam files can also reside on the top level in the archive. At startup, the top directory in the embedded archive and all (second level) ebin directories in the embedded archive are added to the code path. See erts:escript(1).

When the choice of directories in the code path is strict, the directory that ends up in the code path is exactly the stated one. This means that if, for example, the directory $OTPROOT/lib/mnesia-4.4.7/ebin is explicitly added to the code path, the code server does not load files from $OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin.

This behavior can be controlled through command-line flag -code_path_choice Choice. If the flag is set to relaxed, the code server instead chooses a suitable directory depending on the actual file structure. If a regular application ebin directory exists, it is chosen. Otherwise, the directory ebin in the archive is chosen if it exists. If neither of them exists, the original directory is chosen.

Command-line flag -code_path_choice Choice also affects how module init interprets the boot script. The interpretation of the explicit code paths in the boot script can be strict or relaxed. It is particularly useful to set the flag to relaxed when elaborating with code loading from archives without editing the boot script. The default is relaxed. See erts:init(3).

The code for a module can exist in two variants in a system: current code and old code. When a module is loaded into the system for the first time, the module code becomes 'current' and the global export table is updated with references to all functions exported from the module.

If then a new instance of the module is loaded (for example, because of error correction), the code of the previous instance becomes 'old', and all export entries referring to the previous instance are removed. After that, the new instance is loaded as for the first time, and becomes 'current'.

Both old and current code for a module are valid, and can even be evaluated concurrently. The difference is that exported functions in old code are unavailable. Hence, a global call cannot be made to an exported function in old code, but old code can still be evaluated because of processes lingering in it.

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'.

For more information about old and current code, and how to make a process switch from old to current code, see section Compilation and Code Loading in the Erlang Reference Manual.

Module and application names are atoms, while file and directory names are strings. For backward compatibility reasons, some functions accept both strings and atoms, but a future release will probably only allow the arguments that are documented.

As from Erlang/OTP R12B, functions in this module generally fail with an exception if they are passed an incorrect type (for example, an integer or a tuple where an atom is expected). An error tuple is returned if the argument type is correct, but there are some other errors (for example, a non-existing directory is specified to set_path/1).

Functions that load code (such as load_file/1) will return {error,Reason} if the load operation fails. Here follows a description of the common reasons.