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codecodeErlang Code Server
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 accidentally 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 $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 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.
The environment variable ERL_LIBS (defined in the operating
system) can be used to define additional library directories that
will be handled in the same way as the standard OTP library
directory described above, except that directories that do not
have an ebin directory will be ignored.
All application directories found in the additional directories
will appear before the standard OTP applications, except for the
Kernel and STDLIB applications, which will be placed before any
additional applications. In other words, modules found in any
of the additional library directories will override modules with
the same name in OTP, except for modules in Kernel and
STDLIB.
The environment variable ERL_LIBS (if defined) should contain
a colon-separated (for Unix-like systems) or semicolon-separated
(for Windows) list of additional libraries.
Example: On an Unix-like system, ERL_LIBS could be set to
/usr/local/jungerl:/home/some_user/my_erlang_lib. (On Windows,
use semi-colon as separator.)
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.
Loading of Code From Archive Files
The support for loading of code from archive files is
experimental. The sole purpose of releasing it before it is ready
is to obtain early feedback. The file format, semantics,
interfaces etc. may be changed in a future release. The function
lib_dir/2 and the flag -code_path_choice are also
experimental.
In the current implementation, Erlang archives are ZIP
files with .ez extension. Erlang archives may also be
enclosed in escript files whose file extension is arbitrary.
Erlang archive files may 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 would 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 with the name 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 may for example be
created like this:
Any file in the archive may be compressed, but in order to
speed up the access of frequently read files, it may be a good
idea to store beam and app files uncompressed in
the archive.
Normally the top directory of an application is located either
in the library directory $OTPROOT/lib or in a directory
referred to by the environment variable ERL_LIBS. At
startup when the initial code path is computed, the code server
will also look for archive files in these directories and
possibly add ebin directories in archives to the code path. The
code path will then contain paths to directories that looks like
$OTPROOT/lib/mnesia.ez/mnesia/ebin or
$OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin.
The code server uses the module erl_prim_loader
(possibly via the erl_boot_server) to read code files from
archives. But the functions in erl_prim_loader may 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
may coexist. This may be useful when there is a need of having
parts of the application as regular files. A typical case is the
priv directory which must reside as a regular directory in
order to be able to dynamically link in drivers and start port
programs. For other applications that do not have this need, the
priv directory may reside in the archive and the files
under the priv directory may be read via the
erl_prim_loader.
At the time point when a directory is added to the code path as
well as when the entire code path is (re)set, the code server
will decide which subdirectories in an application that shall be
read from the archive and which that shall be read as regular
files. If directories are added or removed afterwards, the file
access may fail if the code path is not updated (possibly to the
same path as before in order to trigger the directory resolution
update). For each directory on the second level (ebin, priv, src
etc.) in the application archive, the code server will firstly
choose the regular directory if it exists and secondly from the
archive. The function
code:lib_dir/2 returns the path to the subdirectory. For
example code:lib_dir(megaco,ebin) may return
/otp/root/lib/megaco-3.9.1.1.ez/megaco-3.9.1.1/ebin while
code:lib_dir(megaco,priv) may return
/otp/root/lib/megaco-3.9.1.1/priv.
When an escript file contains an archive, there are
neither restrictions on the name of the escript nor on how
many applications that may be stored in the embedded
archive. Single beam files may also reside on the top
level in the archive. At startup, both the top directory in the
embedded archive as well as all (second level) ebin
directories in the embedded archive are added to the code path.
See escript(1)
When the choice of directories in the code path is
strict, the directory that ends up in the code path will
be 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 will not load files from
$OTPROOT/lib/mnesia-4.4.7.ez/mnesia-4.4.7/ebin and vice
versa.
This behavior can be controlled via the command line flag
-code_path_choice Choice. If the flag is set to relaxed,
the code server will instead choose a suitable directory
depending on the actual file structure. If there exists a regular
application ebin directory,situation it will be chosen. But if it does
not exist, the ebin directory in the archive is chosen if it
exists. If neither of them exists the original directory will be
chosen.
The command line flag -code_path_choice Choice does also
affect how init interprets the boot script. The
interpretation of the explicit code paths in the boot
script may be strict or relaxed. It is
particular useful to set the flag to relaxed when you want
to elaborate with code loading from archives without editing the
boot script. The default is relaxed. See init(3)
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.
Argument Types and Invalid Arguments
Generally, 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.
From the R12B release, functions in this module will generally fail with an
exception if they are passed an incorrect type (for instance, an integer or a tuple
where an atom was expected). An error tuple will be returned if type of argument
was correct, but there was some other error (for instance, a non-existing directory
given to set_path/1.
Set the code server search 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.
Return the code server search path
Returns the code path
Add a directory to the end of the code path
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 a directory to the beginning of the code path
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 directories to the end of the code path
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 directories to the beginning of the code path
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.
Delete a directory from the code path
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 a directory with another in the code path
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 a module
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. Also if the loading fails, an error tuple is
returned. See
erlang:load_module/2
for possible values of What.
Load a module, residing in a given file
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 that a module is loaded
Tries to to load a module in the same way as
load_file/1,
unless the module is already loaded.
In embedded mode, however, it does not load a module which is not
already loaded, but returns {error, embedded} instead.
Load object code for a module
This function can be used to load object code on remote
Erlang nodes. The 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. 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.
Removes current code for a 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 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.
Removes old code for a 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.
Returns true if successful and any process needed to
be killed, otherwise false.
Removes old code for a module, unless no process uses it
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.
Check if a module is loadedFilename is an absolute filename
Checks if Module is loaded. If it is,
{file, Loaded} is returned, otherwise false.
Normally, Loaded is the absolute file name
Filename 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.
Get all loaded modulesFilename is an absolute filename
Returns a list of tuples {Module, Loaded} for all
loaded modules. Loaded is normally the absolute file
name, as described for
is_loaded/1.
The object code file of a module
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 the object code for a module
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:
Returns the root directory of Erlang/OTP, which is
the directory where it is installed.
> code:root_dir().
"/usr/local/otp"
Library directory of Erlang/OTP
Returns the library directory, $OTPROOT/lib, where
$OTPROOT is the root directory of Erlang/OTP.
> code:lib_dir().
"/usr/local/otp/lib"
Library directory for an application
This function is mainly intended for finding out the path
for the "library directory", the top directory, for an
application Name located under $OTPROOT/lib or
on a directory referred to via the ERL_LIBS
environment variable.
If there is a regular directory called Name or
Name-Vsn 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 the 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
will be returned. This means that the library directory for
an application is the same, regardless of whether the
application resides in an archive or not.
Returns {error, bad_name} if Name
is not the name of an application under $OTPROOT/lib or
on a directory referred to via the ERL_LIBS
environment variable. Fails with an exception if Name
has the wrong type.
For backward compatibility, Name is also allowed to
be a string. That will probably change in a future release.
subdirectory for an application
Returns the path to a subdirectory directly under the top
directory of an application. Normally the subdirectories
resides 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 may reside
as regular directories while other resides in an archive
file. It is not checked if this directory really exists.
Fails with an exception if Name or SubDir has
the wrong type.
Library directory for the compiler
Returns the compiler library directory. Equivalent to
code:lib_dir(compiler).
Priv directory for an application
Returns the path to the priv directory in an
application. Equivalent to code:lib_dir(Name, priv)..
For backward compatibility, Name is also allowed to
be a string. That will probably change in a future release.
Object code file extension
Returns the object code file extension that corresponds to
the Erlang machine used, namely ".beam".
Mark a directory as sticky
This function marks Dir as sticky.
Returns ok if successful or error if not.
Remove a sticky directory mark
This function unsticks a directory which has been marked as
sticky.
Returns ok if successful or error if not.
Test whether a module is sticky
This function returns true if Module is the
name of a module that has been loaded from a sticky directory
(or in other words: an attempt to reload the module will fail),
or false if Module is not a loaded module or is
not sticky.
Rehash or create code path cache
This function creates or rehashes the code path cache.
Full name of a file located in the code path
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.
Search for modules with identical names.
Searches the entire code space for module names with
identical names and writes a report to stdout.
Test whether a module has native code
This function returns true if Module is
name of a loaded module that has native code loaded, and
false if Module is loaded but does not have
native. If Module is not loaded, this function returns
undefined.
The code_server's mode.
This function returns an atom describing the code_server's mode:
interactive or embedded.
This information is useful when an external entity (for example,
an IDE) provides additional code for a running node. If in interactive
mode, it only needs to add to the code path. If in embedded mode,
the code has to be loaded with load_binary/3