diff options
Diffstat (limited to '')
| -rw-r--r-- | doc/info/bus.texinfo | 428 |
1 files changed, 428 insertions, 0 deletions
diff --git a/doc/info/bus.texinfo b/doc/info/bus.texinfo new file mode 100644 index 0000000..ab3c276 --- /dev/null +++ b/doc/info/bus.texinfo @@ -0,0 +1,428 @@ +\input texinfo @c -*-texinfo-*- + +@c %**start of header +@setfilename bus.info +@settitle bus +@afourpaper +@documentencoding UTF-8 +@documentlanguage en +@finalout +@c %**end of header + + +@dircategory Interprorcess Communication +@direntry +* bus: (bus). A simple daemonless system for broadcasting messages locally +@end direntry + + +@copying +Copyright @copyright{} 2015 Mattias Andrée + +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with no +Invariant Sections, with no Front-Cover Texts, and with no Back-Cover +Texts. A copy of the license is included in the section entitled +``GNU Free Documentation License''. +@end quotation +@end copying + +@ifnottex +@node Top +@top bus -- A simple daemonless system for broadcasting messages locally +@insertcopying +@end ifnottex + +@titlepage +@title bus +@subtitle A simple daemonless system for broadcasting messages locally +@author by Mattias Andrée (maandree) + +@page +@vskip 0pt plus 1filll +@insertcopying +@page +@end titlepage + +@contents + + +@iftex +@macro xrm{} +@rm{} +@end macro +@macro xtt{} +@tt{} +@end macro +@end iftex + +@ifnottex +@macro xrm{} +@end macro +@macro xtt{} +@end macro +@end ifnottex + + + +@menu +* Overview:: Brief overview of @command{bus}. +* Standard:: How to use @command{bus} properly. +* Invoking:: Executing @command{bus}. +* Protocol:: How communication over @command{bus} works internally. +* Rationale:: Why @command{bus}? +* GNU Free Documentation License:: Copying and sharing this manual. +@end menu +@c TODO @detailmenu (`C-c C-u m`) + + + +@node Overview +@chapter Overview + +@command{bus} is a stupid-simple, thrilless, daemonless interprocess +communication system for broadcasting messages. It is a lightweight +alternative to a two-phase interprocess flexible barrier. + +@command{bus} uses a System V semaphore array and System V shared +memory. Buses are named; the key of the semaphore array and the +shared memory is stored in a regular file. + +The shared memory used by @command{bus} is always 2048 bytes. +Additionally all messages should be encoded in UTF-8 and not contain +any NULL characters, except they @emph{must} always end with a NULL +byte. Furthermore messages should be prefixed with the process +identifer of the process whence the message originated, followed +by a space. If the process is ephemeral@footnote{The process exits +after the broadcast, or shortly thereafter.}, 0 should be used +instead of the process identifier. + +Communication over @command{bus} is synchronous. The broadcast call +does not return until all listeners have received (and copied) the +message. A malfunctioning program can lock the bus. + +This software package contains a C library and a command line +utility. The package python-bus provides a Python 3 module. + + + +@node Standard +@chapter Standard + +The command @command{bus create} can be used to create new buses. By +convention, buses should be stored in @file{$XDG_RUNTIME_DIR/bus}, +this is what @command{bus create} does if no pathname is given. The +pathname of the bus should be tracked using @env{BUS_X}, where @env{X} +is replaced with either: + +@table @env +@item GENERIC +For the bus used in generic cases. That is all but the cases +of the buses listed below. +@item AUDIO +For the bus used in with the audio subsystem is involved. +@item VIDEO +For the bus used in with the video subsystem is involved. +@item INPUT +For the bus used in with the input subsystem is involved. +@item FILES +For the bus used in with the storage subsystem is involved. +@end table + +This list may be extended in the future. Therefore, and for +other conventions, project-private buses should be tracked +using @env{X_BUS}, where @env{X} is the project name. + +Messages broadcasted on a bus cannot be longer than 2047 bytes, +excluding NUL termination. Message should be encoded in UTF-8, +and most not contain the NUL character. + +Broadcasted message should start with the process ID whence +the message originated, followed by a single regular space. +If the process is ephemeral@footnote{The process exits after +the broadcast, or shortly thereafter.}, 0 should be used instead +of the process identifier. + + + +@node Invoking +@chapter Invoking + +@command{bus} includes the following commands: + +@table @command +@item create +Create a bus. +See @ref{bus create} for more information. +@item remove +Remove a bus. +See @ref{bus remove} for more information. +@item listen +Listen for new message on a bus. +See @ref{bus listen} for more information. +@item wait +Listen for one new message only on a bus. +See @ref{bus wait} for more information. +@item broadcast +Broadcast a message on a bus. +See @ref{bus broadcast} for more information. +@item chmod +Change permissions on a bus. +See @ref{bus chmod} for more information. +@item chown +Change ownership of a bus. +See @ref{bus chown} for more information. +@item chgrp +Change group ownership of a bus. +See @ref{bus chgrp} for more information. +@end table + +Upon successful completion, these commands exit with the value +0. On failure, they exit with the value 1. If the command is +not recognised the exit value is 2. + +@menu +* bus create:: Create a bus. +* bus remove:: Remove a bus. +* bus listen:: Listen for new message on a bus. +* bus wait:: Listen for one new message only on a bus. +* bus broadcast:: Broadcast a message on a bus. +* bus chmod:: Change permissions on a bus. +* bus chown:: Change ownership of a bus. +* bus chgrp:: Change group ownership of a bus. +@end menu + + + +@node bus create +@section @command{bus create} + +The syntax for invocation of @command{bus create} is +@example +bus create [-x] [--] [@var{PATHNAME}] +@end example + +The command creates a bus and stores the key to it in the +file @var{PATHNAME}. If @var{PATHNAME} is omitted, a +random pathname in @file{$XDG_RUNTIME_DIR/bus} will be +used and printed to stdout. + +If @option{-x} is used, the command will fail if +the file @var{PATHNAME} already exists. + + + + +@node bus remove +@section @command{bus remove} + +The syntax for invocation of @command{bus remove} is +@example +bus remove [--] @var{PATHNAME} +@end example + +The command removes the bus whose key is stored in +the file @var{PATHNAME}. The file holding the +key is also unlinked. + + + +@node bus listen +@section @command{bus listen} + +The syntax for invocation of @command{bus command} is +@example +bus listen [--] @var{PATHNAME} @var{COMMAND} +@end example + +The command listens for new messages on the bus whose +key is stored in the file @var{PATHNAME}. Once a message +is received, @var{COMMAND} will be spawned with the +environment variable @env{msg} (lowercased) set to the +received message. @sc{POSIX} shell syntax applies to +@var{COMMAND}. + + +@node bus wait +@section @command{bus wait} + +The syntax for invocation of @command{bus wait} is +@example +bus wait [--] @var{PATHNAME} @var{COMMAND} +@end example + +The command listens for a new message on the bus whose +key is stored in the file @var{PATHNAME}. Once a message +is received, the process will stop listening for more +messages and @var{COMMAND} will be spawned with the +environment variable @env{msg} (lowercased) set to the +received message. @sc{POSIX} shell syntax applies to +@var{COMMAND}. + + + +@node bus broadcast +@section @command{bus broadcast} + +The syntax for invocation of @command{bus broadcast} is +@example +bus broadcast [-n] [--] @var{PATHNAME} @var{MESSAGE} +@end example + +The command broadcasts the message @var{MESSAGE} on the +bus whose key is stored in the file @var{PATHNAME}. + + + +@node bus chmod +@section @command{bus chmod} + +The syntax for invocation of @command{bus chmod} is +@example +bus chmod [--] @var{PERMISSIONS} @var{PATHNAME} +@end example + +This command changes who have access to the bus whose key +is stored in the file @var{PATHNAME}. In the permissions, +the owner, the group, and others (not in tgroup) are +represented by the symbols @code{u}@footnote{@code{u} +stands for `user'.}, @code{g}, and @code{o}, respectively. +The permissions string is imagined to have always be +prefixed with an @code{=}. This symbols means that all user +classes list after it, and only those classes, as permission +to use the bus. Similarly the symbols @code{+} and @code{-} +can be used to grant and revoke access, respectively. The +symbols @code{=}, @code{+}, and @code{-} can be mixed, and +are interpreted from left to right. Alternatively the +permissions string can be a octal number, where the owner +is represented by any bit in 700 (100, 200, or 400, or any +combination thereof), the group is represented by any bit +in 70, and others (not in the group) is represented by any +bit in 7. + +The current permission of the bus can be retrieved by +running @command{stat} over the file @var{PATHNAME}. + + + +@node bus chown +@section @command{bus chown} + +The syntax for invocation of @command{bus chown} is +@example +bus chown [--] @var{OWNER}[:@var{GROUP}] @var{PATHNAME} +@end example + +This command changes the owner, that owns the bus whose +key is stored in the file @var{PATHNAME}, to the specified +owner. The owner can be specified either with a numerical +user identifier or with a user name. If a group is +specified, the bus's owner-group will be set to that group, +otherwise the group will remain unchanged (not changed +to the group of the new owner.) The group can be specified +either with a numerical group identifier or with a group +name. + +The current ownership of the bus can be retrieved by +running @command{stat} over the file @var{PATHNAME}. + + + +@node bus chgrp +@section @command{bus chgrp} + +The syntax for invocation of @command{bus chgrp} is +@example +bus chgrp [--] @var{GROUP} @var{PATHNAME} +@end example + +This command changes the group, that owns the bus whose +key is stored in the file @var{PATHNAME}, to the specified +group. The group can be specified either with a numerical +group identifier or with a group name. + +The current ownership of the bus can be retrieved by +running @command{stat} over the file @var{PATHNAME}. + + + +@node Protocol +@chapter Protocol + +@command{bus} is built upon following three procedures. + +@noindent +@code{create} +@example +@w{@xrm{}Select a filename.@xtt{}} + +@w{@xrm{}Create XSI semaphore array @{@code{S} = 0, @code{W} = 0, @code{X} = 1, @code{Q} = 0, @code{N} = 0@}@xtt{}} +@w{@xrm{}with random key. Store the semaphore array's key in decimal form@xtt{}} +@w{@xrm{}on the first line in the selected file.@xtt{}} + +@w{@xrm{}Create XSI shared memory, with an allocation of 2048 bytes, with@xtt{}} +@w{@xrm{}a random key. Store the shared memory's key in decimal form on@xtt{}} +@w{@xrm{}the second line in the selected file.@xtt{}} +@end example + +@noindent +@code{broadcast} +@example +with P(X): + Z(W) + @w{@xrm{}Write NUL-terminate message to shared memory@xtt{}} + with V(N): -- (1) + Q := 0 + Z(S) + +-- (1) @w{@xrm{}may be omitted if semaphores are known that@xtt{}} + @w{P()@xrm{}, @xtt{}Z()@xrm{}, @xtt{}V()@xrm{} cannot create a race condition@xtt{}} + @w{@xrm{}with a processes running @xtt{}Z()@xrm{}.@xtt{}} +@end example + +@noindent +@code{listen} +@example +with V(S): + forever: + V(Q) + Z(Q) + @w{@xrm{}Read NUL-terminated message from shared memory@xtt{}} + if breaking: + break + with V(W): + with P(S): + Z(S) + Z(N) +@end example + +@noindent +@code{V(a)} means that semaphore a is released.@* +@code{P(a)} means that semaphore a is acquired.@* +@code{Z(a)} means that the process waits for semaphore a to become 0.@* +@code{with P(a)} that @code{P(a)} is done before the entering the scope, +and @code{V(a)} is done when exiting the scope. It also means that +these actions [@code{P(a)} and @code{V(a)}] are undone when the process +exits, or if the call fails.@* +@code{with V(a)} is to @code{V(a)} as @code{with P(a)} is to @code{P(a)}. + + + +@node Rationale +@chapter Rationale + +We need an interprocess communication system similar to message queues. +But we need broadcasting rather than anycasting, so we have a fast, +simple and daemonless system for announcing events to any processes that +might be interested. + + + +@node GNU Free Documentation License +@appendix GNU Free Documentation License +@include fdl.texinfo + +@bye + |