# erl_prim_loader

### MODULE

erl_prim_loader

### MODULE SUMMARY

The Low Level Erlang Loader.

### DESCRIPTION

The `erl_prim_loader` is used to load all Erlang modules into the system. The start script is also fetched with the low level loader.

The `erl_prim_loader` knows about the environment and how to fetch modules. The loader could, for example, fetch files using the file system (with absolute file names as input), or a database (where the binary format of a module is stored).

The `-loader Loader` command line flag can be used to choose the method used by the `erl_prim_loader`. Two `Loader` methods are supported by the Erlang runtime system: `efile` and `inet`. If another loader is required, then it has to be implemented by the user. The `Loader` provided by the user must fulfill the protocol defined below, and it is started with the `erl_prim_loader` by evaluating `open_port({spawn,Loader},[binary])`.

### EXPORTS

`start(Id,Loader,Hosts) -> {ok, Pid} | {error, What}`

Types:

`Id = term()`
`Loader = atom() | string()`
`Hosts = [Host]`
`Host = atom()`
`Pid = pid()`
`What = term()`

Starts the Erlang low level loader. This function is called by the `init` process (and module). The `init` process reads the command line flags `-id Id`, `-loader Loader`, and `-hosts Hosts`. These are the arguments supplied to the `start/3` function.

If `-loader` is not given, the default loader is `efile` which tells the system to read from the file system.

If `-loader` is `inet`, the `-id Id`, `-hosts Hosts`, and `-setcookie Cookie` flags must also be supplied. `Hosts` identifies hosts which this node can contact in order to load modules. One Erlang runtime system with a `erl_boot_server` process must be started on each of hosts given in `Hosts` in order to answer the requests. See `erl_boot_server(3)`.

If `-loader` is something else, the given port program is started. The port program is supposed to follow the protocol specified below.

`get_file(File) -> {ok, Bin, FullName} | error`

Types:

`File = string()`
`Bin = binary()`
`FullName = string()`

This function fetches a file using the low level loader. `File` is either an absolute file name or just the name of the file, for example `"lists.beam"`. If an internal path is set to the loader, this path is used to find the file. If a user supplied loader is used, the path can be stripped off if it is obsolete, and the loader does not use a path. `FullName` is the complete name of the fetched file. `Bin` is the contents of the file as a binary.

`get_path() -> {ok, Path}`

Types:

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

This function gets the path set in the loader. The path is set by the `init` process according to information found in the start script.

`set_path(Path) -> ok`

Types:

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

This function sets the path of the loader if `init` interprets a `path` command in the start script.

### Protocol

The following protocol must be followed if a user provided loader port program is used. The `Loader` port program is started with the command `open_port({spawn,Loader},[binary])`. The protocol is as follows:

    Function          Send               Receive
    -------------------------------------------------------------
    get_file          [102 | FileName]   [121 | BinaryFile] (on success)
                                         [122]              (failure)

    stop              eof                terminate

### Command Line Flags

The `erl_prim_loader` module interprets the following flags:

-loader Loader
  Specifies the name of the loader used by `erl_prim_loader`. `Loader` can be `efile` (use the local file system), or `inet` (load using the `boot_server` on another Erlang node). If `Loader` is user defined, the defined `Loader` port program is started.
  If the `-loader` flag is omitted, it defaults to `efile`.
-hosts Hosts
  Specifies which other Erlang nodes the `inet` loader can use. This flag is mandatory if the `-loader inet` flag is present. On each host, there must be on Erlang node with the `erl_boot_server` which handles the load requests. `Hosts` is a list of IP addresses (hostnames are not acceptable).
-id Id
  Specifies the identity of the Erlang runtime system. If the system runs as a distributed node, `Id` must be identical to the name supplied with the `-sname` or `-name` distribution flags.
-setcookie Cookie
  Specifies the cookie of the Erlang runtime system. This flag is mandatory if the `-loader inet` flag is present.

### SEE ALSO

init(3), erl_boot_server(3)

### AUTHORS

Magnus Fröberg - support@erlang.ericsson.se

kernel 2.7.2
Copyright © 1991-2002 Ericsson Utvecklings AB