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{{Short description|Inter-process communication facility}}
'''Doors''' is an [[Inter-process communication|inter-process communication]] facility for [[Software:Unix|Unix]] computer systems. They provide a form of [[Subroutine|procedure call]].

== History ==
Doors were developed by [[Company:Sun Microsystems|Sun Microsystems]] as a core part of the [[Software:Spring (operating system)|Spring operating system]], then added to [[Software:Solaris (operating system)|Solaris]] in version 2.5 as an undocumented internal interface.<ref>{{cite web|url=https://blogs.oracle.com/meem/entry/head_title_dsvclockd_1m_using |title=dsvclockd(1M): Using Doors to Implement Inter-Process Readers/Writer Locks |website=meem simplex |date=June 14, 2005|accessdate=2012-03-20 |url-status=dead |archiveurl=https://web.archive.org/web/20121022135943/https://blogs.oracle.com/meem/entry/head_title_dsvclockd_1m_using |archivedate=2012-10-22 }}</ref> They became a documented feature in Solaris 2.6. Recent versions of Solaris use Doors in many places, including <code>nscd</code> (the [[Name server|name service]] cache daemon) and [[Software:Syslog|syslog]].

A [[Software:Linux|Linux]] port of Doors was released in 2003, but is only available for version 2.4.18.<ref>{{Cite web|url=http://www.rampant.org/doors/index.html|title=Namefs and Solaris Doors for Linux}}</ref> <!-- http://sourceforge.net/projects/ldoor/ is also defunct -->

== Overview ==
The Doors subsystem is implemented as a [[User space and kernel space|user-space]] library with some [[Software:Kernel (operating system)|kernel]] support, and relies heavily on [[Thread (computing)|threads]]. It is designed for low overhead, and the Solaris implementation uses some assembly code for maximum efficiency.

Doors are created by server processes (which must use threads) and called by client processes. It is possible for one process to both create and call a door. When creating a door, the server must specify a server procedure, which will be called by the Doors library on behalf of clients. Unlike most [[Remote procedure call|remote procedure call]] systems, each door has only one server procedure. A server can "attach" a door to a file, enabling clients to connect to that door simply by opening that file. The [[Software:Ls|<code>ls -l</code>]] command will then show the file with a 'type' of "D" (not to be confused with "d" for a directory) — for example:
{{sxhl|2=console|
$ ls -l
total 9
Drw-r--r-- 1 jmorrison dev 876 Dec  8 19:43 myfile
}}

Clients use <code>door_call()</code> to invoke the door's server procedure, passing a contiguous region of memory and a list of [[File descriptor|file descriptor]]s as arguments, and getting back another contiguous region and list of file descriptors. Either region may be empty, as may either list. Usually, two [[C (programming language)|C]] <code>struct</code>s will be defined, one for the input data and one for the output data. (Alternatively, [[Tagged union|tagged union]]s may be used, allowing a door procedure to provide multiple actions in much the same way as the [[Ioctl|ioctl]] system call.) Every file descriptor is accompanied by a flags word. The <code>DOOR_RELEASE</code> flag requests that a file descriptor be closed in the sending process after being duplicated in the receiving process. If a file descriptor that refers to a door is sent, the system records the properties of that door in the flags word.

As well as representing a procedure, or a group of procedures, a door can represent a stateful data [[Object (computer science)|object]], making it possible to pass references to such objects between processes. Such a door would typically take a [[Tagged union|tagged union]] as input data with each tag value denoting a different method.

The Doors system also provides a way for clients and servers to get information about each other. For example, a server can check the client's user or process ID to implement [[Access-control list|access control]].

The Doors library normally creates and manages a pool of threads in the server process to handle calls, but it is possible to override this behavior. The Doors system does not provide any form of synchronization, but servers can use the normal thread-level synchronization primitives. Doors can be used to synchronize access to shared memory segments, allowing single-copy data transfer.<ref>{{dead link|date=February 2015}}http://developers.sun.com/solaris/articles/fastSockets.html</ref>

The Doors concept is very similar to X/Open XATMI API specification, where client processes invoke server processes' exposed functions: <code>door_call()</code> is analogous to <code>tpcall()</code> in XATMI clients, while <code>door_return()</code> is analogous to <code>tpreturn()</code> in XATMI servers.

== See also ==
*Local Procedure Call
*Enduro/X
*[[Software:Tuxedo|Tuxedo]]
*[[Software:D-Bus|D-Bus]]

== References ==
{{Reflist}}

== External links ==
*[http://download.oracle.com/docs/cd/E18752_01/html/816-5171/index.html Solaris 10 man pages section 3: Realtime Library Functions] (man pages for the door-related functions)

{{Solaris}}

[[Category:Remote procedure call]]

{{Sourceattribution|Doors (computing)}}