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NAME
code - 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 embedded or interactive mode. Which one is decided by command-line
flag -mode:
% erl -mode interactive
The modes are as follows:
* In embedded mode, all code is loaded during system startup according to the boot script. (Code can
also be loaded later by explicitly ordering the code server to do so).
* In interactive mode, which is default, only some code is loaded during system startup, basically the
modules needed by the runtime system. 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 accidentally reloading of modules affecting the Erlang runtime system, directories kernel,
stdlib, and compiler 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
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 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
/usr/local/jungerl:/home/some_user/my_erlang_lib
On Windows, use semi-colon as separator.
LOADING OF CODE FROM ARCHIVE FILES
Warning:
The support for loading code from archive files is experimental. The purpose of releasing it before it is
ready is to obtain early feedback. The file format, semantics, interfaces, and so on, can be changed in a
future release. The function lib_dir/2 and flag -code_path_choice are also experimental.
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(3erl).
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(3erl).
CURRENT AND OLD CODE
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.
ARGUMENT TYPES AND INVALID ARGUMENTS
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.
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).
ERROR REASONS FOR CODE-LOADING FUNCTIONS
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.
badfile:
The object code has an incorrect format or the module name in the object code is not the expected
module name.
nofile:
No file with object code was found.
not_purged:
The object code could not be loaded because an old version of the code already existed.
on_load_failure:
The module has an -on_load function that failed when it was called.
sticky_directory:
The object code resides in a sticky directory.
DATA TYPES
load_ret() =
{error, What :: load_error_rsn()} |
{module, Module :: module()}
load_error_rsn() =
badfile |
nofile |
not_purged |
on_load_failure |
sticky_directory
prepared_code()
An opaque term holding prepared code.
EXPORTS
set_path(Path) -> true | {error, What}
Types:
Path = [Dir :: file:filename()]
What = bad_directory
Sets the code path to the list of directories Path.
Returns:
true:
If successful
{error, bad_directory}:
If any Dir is not a directory name
get_path() -> Path
Types:
Path = [Dir :: file:filename()]
Returns the code path.
add_path(Dir) -> add_path_ret()
add_pathz(Dir) -> add_path_ret()
Types:
Dir = file:filename()
add_path_ret() = true | {error, 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) -> add_path_ret()
Types:
Dir = file:filename()
add_path_ret() = true | {error, bad_directory}
Adds Dir to the beginning of the code path. If Dir 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 :: file:filename()]
Adds the directories in Dirs to the end of the code path. If a Dir exists, it is not added.
Always returns ok, regardless of the validity of each individual Dir.
add_pathsa(Dirs) -> ok
Types:
Dirs = [Dir :: file:filename()]
Traverses Dirs and adds each Dir to the beginning of the code path. This means that the order of
Dirs is reversed in the resulting code path. For example, if you add [Dir1,Dir2], the resulting
path will be [Dir2,Dir1|OldCodePath].
If a Dir already exists in the code path, it is removed from the old position.
Always returns ok, regardless of the validity of each individual Dir.
del_path(NameOrDir) -> boolean() | {error, What}
Types:
NameOrDir = Name | Dir
Name = atom()
Dir = file:filename()
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. Also, the complete
directory name Dir can be specified as argument.
Returns:
true:
If successful
false:
If the directory is not found
{error, bad_name}:
If the argument is invalid
replace_path(Name, Dir) -> true | {error, What}
Types:
Name = atom()
Dir = file:filename()
What = bad_directory | bad_name | {badarg, term()}
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 is to be used if a new version of the
directory (library) is added to a running system.
Returns:
true:
If successful
{error, bad_name}:
If Name is not found
{error, bad_directory}:
If Dir does not exist
{error, {badarg, [Name, Dir]}}:
If Name or Dir is invalid
load_file(Module) -> load_ret()
Types:
Module = module()
load_ret() =
{error, What :: load_error_rsn()} |
{module, Module :: module()}
Tries to load the Erlang module Module, using the code path. It looks for the object code file
with an extension corresponding 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, Reason} if loading fails. See Error Reasons for
Code-Loading Functions for a description of the possible error reasons.
load_abs(Filename) -> load_ret()
Types:
Filename = file:filename()
load_ret() =
{error, What :: load_error_rsn()} |
{module, Module :: module()}
loaded_filename() =
(Filename :: file:filename()) | loaded_ret_atoms()
loaded_ret_atoms() = cover_compiled | preloaded
Same as load_file(Module), but Filename is an absolute or relative filename. The code path is not
searched. It returns a value in the same way as load_file/1. Notice that Filename must not contain
the extension (for example, .beam) because load_abs/1 adds the correct extension.
ensure_loaded(Module) -> {module, Module} | {error, What}
Types:
Module = module()
What = embedded | badfile | nofile | on_load_failure
Tries to load a module in the same way as load_file/1, unless the module is already loaded.
However, in embedded mode it does not load a module that is not already loaded, but returns
{error, embedded} instead. See Error Reasons for Code-Loading Functions for a description of other
possible error reasons.
load_binary(Module, Filename, Binary) ->
{module, Module} | {error, What}
Types:
Module = module()
Filename = loaded_filename()
Binary = binary()
What = badarg | load_error_rsn()
loaded_filename() =
(Filename :: file:filename()) | loaded_ret_atoms()
loaded_ret_atoms() = cover_compiled | preloaded
This function can be used to load object code on remote Erlang nodes. Argument 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. Thus, Filename is not opened and read by the code server.
Returns {module, Module} if successful, or {error, Reason} if loading fails. See Error Reasons for
Code-Loading Functions for a description of the possible error reasons.
atomic_load(Modules) -> ok | {error, [{Module, What}]}
Types:
Modules = [Module | {Module, Filename, Binary}]
Module = module()
Filename = file:filename()
Binary = binary()
What =
badfile |
nofile |
on_load_not_allowed |
duplicated |
not_purged |
sticky_directory |
pending_on_load
Tries to load all of the modules in the list Modules atomically. That means that either all
modules are loaded at the same time, or none of the modules are loaded if there is a problem with
any of the modules.
Loading can fail for one the following reasons:
badfile:
The object code has an incorrect format or the module name in the object code is not the
expected module name.
nofile:
No file with object code exists.
on_load_not_allowed:
A module contains an -on_load function.
duplicated:
A module is included more than once in Modules.
not_purged:
The object code can not be loaded because an old version of the code already exists.
sticky_directory:
The object code resides in a sticky directory.
pending_on_load:
A previously loaded module contains an -on_load function that never finished.
If it is important to minimize the time that an application is inactive while changing code, use
prepare_loading/1 and finish_loading/1 instead of atomic_load/1. Here is an example:
{ok,Prepared} = code:prepare_loading(Modules),
%% Put the application into an inactive state or do any
%% other preparation needed before changing the code.
ok = code:finish_loading(Prepared),
%% Resume the application.
prepare_loading(Modules) ->
{ok, Prepared} | {error, [{Module, What}]}
Types:
Modules = [Module | {Module, Filename, Binary}]
Module = module()
Filename = file:filename()
Binary = binary()
Prepared = prepared_code()
What = badfile | nofile | on_load_not_allowed | duplicated
Prepares to load the modules in the list Modules. Finish the loading by calling
finish_loading(Prepared).
This function can fail with one of the following error reasons:
badfile:
The object code has an incorrect format or the module name in the object code is not the
expected module name.
nofile:
No file with object code exists.
on_load_not_allowed:
A module contains an -on_load function.
duplicated:
A module is included more than once in Modules.
finish_loading(Prepared) -> ok | {error, [{Module, What}]}
Types:
Prepared = prepared_code()
Module = module()
What = not_purged | sticky_directory | pending_on_load
Tries to load code for all modules that have been previously prepared by prepare_loading/1. The
loading occurs atomically, meaning that either all modules are loaded at the same time, or none of
the modules are loaded.
This function can fail with one of the following error reasons:
not_purged:
The object code can not be loaded because an old version of the code already exists.
sticky_directory:
The object code resides in a sticky directory.
pending_on_load:
A previously loaded module contains an -on_load function that never finished.
ensure_modules_loaded(Modules :: [Module]) ->
ok | {error, [{Module, What}]}
Types:
Module = module()
What = badfile | nofile | on_load_failure
Tries to load any modules not already loaded in the list Modules in the same way as load_file/1.
Returns ok if successful, or {error,[{Module,Reason}]} if loading of some modules fails. See Error
Reasons for Code-Loading Functions for a description of other possible error reasons.
delete(Module) -> boolean()
Types:
Module = module()
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 no external function calls can
be made to it.
Returns true if successful, or false if there is old code for Module that must be purged first, or
if Module is not a (loaded) module.
purge(Module) -> boolean()
Types:
Module = module()
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.
Note:
As of ERTS version 9.0, a process is only considered to be lingering in the code if it has direct
references to the code. For more information see documentation of erlang:check_process_code/3,
which is used in order to determine this.
Returns true if successful and any process is needed to be killed, otherwise false.
soft_purge(Module) -> boolean()
Types:
Module = module()
Purges the code for Module, that is, removes code marked as old, but only if no processes linger
in it.
Note:
As of ERTS version 9.0, a process is only considered to be lingering in the code if it has direct
references to the code. For more information see documentation of erlang:check_process_code/3,
which is used in order to determine this.
Returns false if the module cannot be purged because of processes lingering in old code, otherwise
true.
is_loaded(Module) -> {file, Loaded} | false
Types:
Module = module()
Loaded = loaded_filename()
loaded_filename() =
(Filename :: file:filename()) | loaded_ret_atoms()
Filename is an absolute filename.
loaded_ret_atoms() = cover_compiled | preloaded
Checks if Module is loaded. If it is, {file, Loaded} is returned, otherwise false.
Normally, Loaded is the absolute filename Filename from which the code is obtained. If the module
is preloaded (see script(5)), Loaded==preloaded. If the module is Cover-compiled (see
cover(3erl)), Loaded==cover_compiled.
all_loaded() -> [{Module, Loaded}]
Types:
Module = module()
Loaded = loaded_filename()
loaded_filename() =
(Filename :: file:filename()) | loaded_ret_atoms()
Filename is an absolute filename.
loaded_ret_atoms() = cover_compiled | preloaded
Returns a list of tuples {Module, Loaded} for all loaded modules. Loaded is normally the absolute
filename, as described for is_loaded/1.
which(Module) -> Which
Types:
Module = module()
Which = file:filename() | loaded_ret_atoms() | non_existing
loaded_ret_atoms() = cover_compiled | preloaded
If the module is not loaded, this function searches the code path for the first file containing
object code for Module and returns the absolute filename.
If the module is loaded, it returns the name of the file containing the loaded object code.
If the module is preloaded, preloaded is returned.
If the module is Cover-compiled, cover_compiled is returned.
If the module cannot be found, non_existing is returned.
get_object_code(Module) -> {Module, Binary, Filename} | error
Types:
Module = module()
Binary = binary()
Filename = file:filename()
Searches the code path for the object code of module Module. Returns {Module, Binary, Filename} if
successful, otherwise error. 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() -> file:filename()
Returns the root directory of Erlang/OTP, which is the directory where it is installed.
Example:
> code:root_dir().
"/usr/local/otp"
lib_dir() -> file:filename()
Returns the library directory, $OTPROOT/lib, where $OTPROOT is the root directory of Erlang/OTP.
Example:
> code:lib_dir().
"/usr/local/otp/lib"
lib_dir(Name) -> file:filename() | {error, bad_name}
Types:
Name = atom()
Returns the path for the "library directory", the top directory, for an application Name located
under $OTPROOT/lib or on a directory referred to with environment variable ERL_LIBS.
If a regular directory called Name or Name-Vsn exists in the code path with an ebin subdirectory,
the path to this directory is returned (not the ebin directory).
If the directory refers to a directory in an archive, the archive name is stripped away before the
path is returned. For example, if directory /usr/local/otp/lib/mnesia-4.2.2.ez/mnesia-4.2.2/ebin
is in the path, /usr/local/otp/lib/mnesia-4.2.2/ebin is returned. This means that the library
directory for an application is the same, regardless if the application resides in an archive or
not.
Example:
> code:lib_dir(mnesia).
"/usr/local/otp/lib/mnesia-4.2.2"
Returns {error, bad_name} if Name is not the name of an application under $OTPROOT/lib or on a
directory referred to through environment variable ERL_LIBS. Fails with an exception if Name has
the wrong type.
Warning:
For backward compatibility, Name is also allowed to be a string. That will probably change in a
future release.
lib_dir(Name, SubDir) -> file:filename() | {error, bad_name}
Types:
Name = SubDir = atom()
Returns the path to a subdirectory directly under the top directory of an application. Normally
the subdirectories reside under the top directory for the application, but when applications at
least partly resides in an archive, the situation is different. Some of the subdirectories can
reside as regular directories while other reside in an archive file. It is not checked whether
this directory exists.
Example:
> code:lib_dir(megaco, priv).
"/usr/local/otp/lib/megaco-3.9.1.1/priv"
Fails with an exception if Name or SubDir has the wrong type.
compiler_dir() -> file:filename()
Returns the compiler library directory. Equivalent to code:lib_dir(compiler).
priv_dir(Name) -> file:filename() | {error, bad_name}
Types:
Name = atom()
Returns the path to the priv directory in an application. Equivalent to code:lib_dir(Name, priv).
Warning:
For backward compatibility, Name is also allowed to be a string. That will probably change in a
future release.
objfile_extension() -> nonempty_string()
Returns the object code file extension corresponding to the Erlang machine used, namely .beam.
stick_dir(Dir) -> ok | error
Types:
Dir = file:filename()
Marks Dir as sticky.
Returns ok if successful, otherwise error.
unstick_dir(Dir) -> ok | error
Types:
Dir = file:filename()
Unsticks a directory that is marked as sticky.
Returns ok if successful, otherwise error.
is_sticky(Module) -> boolean()
Types:
Module = module()
Returns true if Module is the name of a module that has been loaded from a sticky directory (in
other words: an attempt to reload the module will fail), or false if Module is not a loaded module
or is not sticky.
where_is_file(Filename) -> non_existing | Absname
Types:
Filename = Absname = file:filename()
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.
clash() -> ok
Searches all directories in the code path for module names with identical names and writes a
report to stdout.
module_status(Module :: module()) ->
not_loaded | loaded | modified | removed
Returns:
not_loaded:
If Module is not currently loaded.
loaded:
If Module is loaded and the object file exists and contains the same code.
removed:
If Module is loaded but no corresponding object file can be found in the code path.
modified:
If Module is loaded but the object file contains code with a different MD5 checksum.
Preloaded modules are always reported as loaded, without inspecting the contents on disk. Cover
compiled modules will always be reported as modified if an object file exists, or as removed
otherwise. Modules whose load path is an empty string (which is the convention for auto-generated
code) will only be reported as loaded or not_loaded.
For modules that have native code loaded (see is_module_native/1), the MD5 sum of the native code
in the object file is used for the comparison, if it exists; the Beam code in the file is ignored.
Reversely, for modules that do not currently have native code loaded, any native code in the file
will be ignored.
See also modified_modules/0.
modified_modules() -> [module()]
Returns the list of all currently loaded modules for which module_status/1 returns modified. See
also all_loaded/0.
is_module_native(Module) -> true | false | undefined
Types:
Module = module()
Returns:
true:
If Module is the name of a loaded module that has native code loaded
false:
If Module is loaded but does not have native code
undefined:
If Module is not loaded
get_mode() -> embedded | interactive
Returns an atom describing the mode of the code server: interactive or embedded.
This information is useful when an external entity (for example, an IDE) provides additional code
for a running node. If the code server is in interactive mode, it only has to add the path to the
code. If the code server is in embedded mode, the code must be loaded with load_binary/3.