# code

### MODULE

code

### MODULE SUMMARY

Erlang Code Server

### DESCRIPTION

This module contains the interface to the Erlang code server, which deals with the loading of compiled code into a running Erlang runtime system.

The runtime system can be started in either embedded or interactive mode. Which one is decided by the command line flag `-mode`.

    % erl -mode interactive

Default mode is `interactive`.

- In embedded mode, all code is loaded during system start-up according to the boot script. (Code can also be loaded later by explicitly ordering the code server to do so).
- In interactive mode, only some code is loaded during system startup-up, basically the modules needed by the runtime system itself. Other code is dynamically loaded when first referenced. When a call to a function in a certain module is made, and the module is not loaded, the code server searches for and tries to load the module.

To prevent accidently reloading modules affecting the Erlang runtime system itself, the `kernel`, `stdlib` and `compiler` directories are considered sticky. This means that the system issues a warning and rejects the request if a user tries to reload a module residing in any of them. The feature can be disabled by using the command line flag `-nostick`.

### Code Path

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 the library directory `$ROOT/lib`, where `$ROOT` 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 which have the same `Name`. The `-Vsn` suffix is optional. If an `ebin` directory exists under `Name[-Vsn]`, it is this directory which is added to the code path.

### Code Path Cache

The code server incorporates a code path cache. The cache functionality is disabled by default. To activate it, start the emulator with the command line flag `-code_path_cache` or call `code:rehash()`. When the cache is created (or updated), the code server searches for modules in the code path directories. This may take some time if the the code path is long. After the cache creation, the time for loading modules in a large system (one with a large directory structure) is significantly reduced compared to having the cache disabled. The code server is able to look up the location of a module from the cache in constant time instead of having to search through the code path directories.

Application resource files (`.app` files) are also stored in the code path cache. This feature is used by the application controller (see application(3)) to load applications efficiently in large systems.

Note that when the code path cache is created (or updated), any relative directory names in the code path are converted to absolute.

### Current and Old Code

The code of a module can exists in two variants in a system: current code and old code. When a module is loaded into the system for the first time, the code of the module 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 (perhaps because of the correction of an error), then 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 if it was loaded for the first time, as described above, and becomes 'current'.

Both old and current code for a module are valid, and may even be evaluated concurrently. The difference is that exported functions in old code are unavailable. Hence there is no way to make a global call to an exported function in old code, but old code may still be evaluated because of processes lingering in it.

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'.

For more information about old and current code, and how to make a process switch from old to current code, refer to Erlang Reference Manual.

### EXPORTS

`set_path(Path) -> true | {error, What}`

Types:

`Path = [Dir]`
`Dir = string()`
`What = bad_directory | bad_path`

Sets the code path to the list of directories `Path`.

Returns `true` if successful, or `{error, bad_directory}` if any `Dir` is not the name of a directory, or `{error, bad_path}` if the argument is invalid.

`get_path() -> Path`

Types:

`Path = [Dir]`
`Dir = string()`

Returns the code path

`add_path(Dir) -> true | {error, What}`
`add_pathz(Dir) -> true | {error, What}`

Types:

`Dir = string()`
`What = bad_directory`

Adds `Dir` to the code path. The directory is added as the last directory in the new path. If `Dir` already exists in the path, it is not added.

Returns `true` if successful, or `{error, bad_directory}` if `Dir` is not the name of a directory.

`add_patha(Dir) -> true | {error, What}`

Types:

`Dir = string()`
`What = bad_directory`

Adds `Dir` to the beginning of the code path. If `Dir` already exists, it is removed from the old position in the code path.

Returns `true` if successful, or `{error, bad_directory}` if `Dir` is not the name of a directory.

`add_paths(Dirs) -> ok`
`add_pathsz(Dirs) -> ok`

Types:

`Dirs = [Dir]`
`Dir = string()`

Adds the directories in `Dirs` to the end of the code path. If a `Dir` already exists, it is not added. This function always returns `ok`, regardless of the validity of each individual `Dir`.

`add_pathsa(Dirs) -> ok`

Types:

`Dirs = [Dir]`
`Dir = string()`

Adds the directories in `Dirs` to the beginning of the code path. If a `Dir` already exists, it is removed from the old position in the code path. This function always returns `ok`, regardless of the validity of each individual `Dir`.

`del_path(Name | Dir) -> true | false | {error, What}`

Types:

`Name = atom()`
`Dir = string()`
`What = bad_name`

Deletes a directory from the code path. The argument can be an atom `Name`, in which case the directory with the name `.../Name[-Vsn][/ebin]` is deleted from the code path. It is also possible to give the complete directory name `Dir` as argument.

Returns `true` if successful, or `false` if the directory is not found, or `{error, bad_name}` if the argument is invalid.

`replace_path(Name, Dir) -> true | {error, What}`

Types:

`Name = atom()`
`Dir = string()`
`What = bad_name | bad_directory | {badarg, term()}`

This function replaces an old occurrence of a directory named `.../Name[-Vsn][/ebin]`, in the code path, with `Dir`. If `Name` does not exist, it adds the new directory `Dir` last in the code path. The new directory must also be named `.../Name[-Vsn][/ebin]`. This function should be used if a new version of the directory (library) is added to a running system.

Returns `true` if successful, or `{error, bad_name}` if `Name` is not found, or `{error, bad_directory}` if `Dir` does not exist, or `{error, {badarg, [Name, Dir]}}` if `Name` or `Dir` is invalid.

`load_file(Module) -> {module, Module} | {error, What}`

Types:

`Module = atom()`
`What = nofile | sticky_directory | badarg | term()`

Tries to load the Erlang module `Module`, using the code path. It looks for the object code file with an extension that corresponds to the Erlang machine used, for example `Module.beam`. The loading fails if the module name found in the object code differs from the name `Module`. load_binary/3 must be used to load object code with a module name that is different from the file name.

Returns `{module, Module}` if successful, or `{error, nofile}` if no object code is found, or `{error, sticky_directory}` if the object code resides in a sticky directory, or `{error, badarg}` if the argument is invalid. Also if the loading fails, an error tuple is returned. See erlang:load_module/2 for possible values of `What`.

`load_abs(Filename) -> {module, Module} | {error, What}`

Types:

`Filename = string()`
`Module = atom()`
`What = nofile | sticky_directory | badarg | term()`

Does the same as `load_file(Module)`, but `Filename` is either an absolute file name, or a relative file name. The code path is not searched. It returns a value in the same way as load_file/1. Note that `Filename` should not contain the extension (for example `".beam"`); `load_abs/1` adds the correct extension itself.

`ensure_loaded(Module) -> {module, Module} | {error, What}`

Types:

`Module = atom()`
`What = nofile | sticky_directory | embedded | badarg | term()`

Tries to to load a module in the same way as load_file/1. In embedded mode, however, it does not load a module which is not already loaded, but returns `{error, embedded}` instead.

`load_binary(Module, Filename, Binary) -> {module, Module} | {error, What}`

Types:

`Module = atom()`
`Filename = string()`
`What = sticky_directory | badarg | term()`

This function can be used to load object code on remote Erlang nodes. It can also be used to load object code where the file name and module name differ. This, however, is a very unusual situation and not recommended. The parameter `Binary` must contain object code for `Module`. `Filename` is only used by the code server to keep a record of from which file the object code for `Module` comes. Accordingly, `Filename` is not opened and read by the code server.

Returns `{module, Module}` if successful, or `{error, sticky_directory}` if the object code resides in a sticky directory, or `{error, badarg}` if any argument is invalid. Also if the loading fails, an error tuple is returned. See erlang:load_module/2 for possible values of `What`.

`delete(Module) -> true | false`

Types:

`Module = atom()`

Removes the current code for `Module`, that is, the current code for `Module` is made old. This means that processes can continue to execute the code in the module, but that no external function calls can be made to it.

Returns `true` if successful, or `false` if there is old code for `Module` which must be purged first, or if `Module` is not a (loaded) module.

`purge(Module) -> true | false`

Types:

`Module = atom()`

Purges the code for `Module`, that is, removes code marked as old. If some processes still linger in the old code, these processes are killed before the code is removed.

Returns `true` if successful and any process needed to be killed, otherwise `false`.

`soft_purge(Module) -> true | false`

Types:

`Module = atom()`

Purges the code for `Module`, that is, removes code marked as old, but only if no processes linger in it.

Returns `false` if the module could not be purged due to processes lingering in old code, otherwise `true`.

`is_loaded(Module) -> {file, Loaded} | false`

Types:

`Module = atom()`
`Loaded = Absname | preloaded | cover_compiled`
`Absname = string()`

Checks if `Module` is loaded. If it is, `{file, Loaded}` is returned, otherwise `false`.

Normally, `Loaded` is the absolute file name `Absname` from which the code was obtained. If the module is preloaded (see script(4)), `Loaded==preloaded`. If the module is Cover compiled (see cover(3)), `Loaded==cover_compiled`.

`all_loaded() -> [{Module, Loaded}]`

Types:

`Module = atom()`
`Loaded = Absname | preloaded | cover_compiled`
`Absname = string()`

Returns a list of tuples `{Module, Loaded}` for all loaded modules. `Loaded` is normally the absolute file name, as described for is_loaded/1.

`which(Module) -> Which`

Types:

`Module = atom()`
`Which = Filename | non_existing | preloaded | cover_compiled`
`Filename = string()`

If the module is not loaded, this function searches the code path for the first file which contains object code for `Module` and returns the absolute file name. If the module is loaded, it returns the name of the file which contained the loaded object code. If the module is pre-loaded, `preloaded` is returned. If the module is Cover compiled, `cover_compiled` is returned. `non_existing` is returned if the module cannot be found.

`get_object_code(Module) -> {Module, Binary, Filename} | error`

Types:

`Module = atom()`
`Binary = binary()`
`Filename = string()`

Searches the code path for the object code of the module `Module`. It returns `{Module, Binary, Filename}` if successful, and `error` if not. `Binary` is a binary data object which contains the object code for the module. This can be useful if code is to be loaded on a remote node in a distributed system. For example, loading module `Module` on a node `Node` is done as follows:

    ...
    {_Module, Binary, Filename} = code:get_object_code(Module),
    rpc:call(Node, code, load_binary, [Module, Filename, Binary]),
    ...

`root_dir() -> RootDir`

Types:

`RootDir = string()`

Returns the root directory of Erlang/OTP, which is the directory where it is installed.

`lib_dir() -> LibDir`

Types:

`LibDir = string()`

Returns the library directory, `$ROOT/lib`, where `$ROOT` is the root directory of Erlang/OTP.

`lib_dir(Name) -> Dir | {error, What}`

Types:

`Name = atom()`
`Dir = string()`
`What = bad_name`

Searches the code path for a directory named `.../Name[-Vsn][/ebin]` and returns the directory `.../Name[-Vsn]`. Mainly intended for finding the library directory of the Erlang/OTP application called `Name`.

    > code:lib_dir(mnesia).
    /usr/local/otp/lib/mnesia-4.2.2"

Returns `{error, bad_name}` if `Name` is not found or if the argument is not valid.

`compiler_dir() -> Dir`

Types:

`Dir = string()`

Searches the code path for the compiler library directory. Equivalent of calling `code:lib_dir(compiler)`.

`priv_dir(Name) -> PrivDir | {error, What}`

Types:

`Name = atom()`
`PrivDir = string()`
`What = bad_name`

Searches the code path for a directory named `.../Name[-Vsn][/ebin]` and returns the directory `.../Name[-Vsn]/priv`. It is not checked if this directory really exists. Mainly intended for finding the `priv` directory of the Erlang/OTP application called `Name`.

    > code:priv_dir(mnesia).
    /usr/local/otp/lib/mnesia-4.2.2/priv"

`objfile_extension() -> Ext`

Types:

`Ext = string()`

Returns the object code file extension that corresponds to the Erlang machine used, normally ".beam".

`stick_dir(Dir) -> ok | {error, What}`

Types:

`Dir = string()`
`What = term()`

This function marks `Dir` as sticky.

Returns `ok` if successful, and an error tuple otherwise.

`unstick_dir(Dir) -> ok | {error, What}`

Types:

`Dir = string()`
`What = term()`

This function unsticks a directory which has been marked as sticky.

Returns `ok` if successful, and an error tuple otherwise.

`rehash() -> ok`

This function creates or rehashes the code path cache.

`where_is_file(Filename) -> Absname | non_existing`

Types:

`Filename = Absname = string()`

Searches the code path for `Filename`, a file of arbitrary type. If found, the full name is returned. `non_existing` is returned if the file cannot be found. The function can be useful, for example, to locate application resource files. If the code path cache is used, the code server will efficiently read the full name from the cache, provided that `Filename` is an object code file or an `.app` file.

`clash() -> ok`

Searches the entire code space for module names with identical names and writes a report to `stdout`.

### AUTHORS

Joe Armstrong - support@erlang.ericsson.se
Magnus Fröberg - support@erlang.ericsson.se
Robert Virding - support@erlang.ericsson.se
Claes Wikström - support@erlang.ericsson.se

kernel 2.11.2
Copyright © 1991-2006 Ericsson AB