\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}. * Interface:: Using @command{libbus}. * Protocol:: How communication over @command{bus} works internally. * Rationale:: Why @command{bus}? * Examples:: Usecase examples and API-demonstration. * 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 Interface @chapter Interface To use @command{libbus} in your C program, include the header file @file{} and link with the flag @option{-lbus}. With exception to @code{bus_poll} and @code{bus_poll_timed}, all functions return @code{0} upon successful completion, and @code{-1} in case of failure. @code{bus_poll} and @code{bus_poll_timed} return @code{NULL} on failure. On failure on all functions set @code{errno} to indicate what went wrong. @file{} defines the following functions: @table @code @item int bus_create(const char *file, int flags, char **out_file) This function creates a bus with the asscoiated pathname specifed by the value of the parameter @code{file}. If @code{file} is @code{NULL} a random pathname is selected. This pathname adheres to the convention set forth by in @ref{Standard}. If @code{file} is not @code{NULL} the function fails if the file already exists if @code{flags} contains @code{BUS_EXCL}. Otherwise if @code{file} is not @code{NULL}, the function does nothing if the file already exists. If @code{flags} contains @code{BUS_INTR}, the function fails if it is interrupted. Unless @code{out_file} is NULL, the pathname of the bus should be stored in a new char array stored in @code{*out_file}. The caller must free the allocated stored in @code{*out_file}. If the processes cannot allocate enough memory to perform the action, the function sets @code{errno} to @code{ENOMEM} and fails. It may also fail and set @code{errno} to any of the errors specified for the system calls @code{open} and @code{write}. @item int bus_unlink(const char *file) This function removes the bus assoicated with the pathname stored in the parameter @code{file}. The function also unlinks the file. The function may set @code{errno} to any of the following values and fail for the specified reasons: @table @code @item EINVAL The bus does not exist. @item EACCES Operation permission is denied to the calling process. @item EPERM The user does not have permission to remove the bus. @end table @noindent It may also fail and set @code{errno} to any of the errors specified for the system calls @code{unlink} and @code{open}, and the functions @code{semget} and @code{shmget}. @item int bus_open(bus_t *bus, const char *file, int flags) This function acquires resources required for the process to use the bus associated with the filename stored in the parameter @code{file}. The function also stores the resources in @code{bus} for use by other @command{bus} functions. Values for @code{flags} are constructed by a bitwise inclusive @sc{or} of flags from the following list. @table @code @item BUS_RDONLY The process will only be using the bus for receiving messages. @item BUS_WRONLY The process will only be using the bus for sending messages. @item BUS_RDWR The process will use the bus for both receiving and sending messages. @end table The function may set @code{errno} to any of the following values and fail for the specified reasons: @table @code @item ENOMEM The process cannot allocate enough memory to perform the action. @item EACCES Operation permission is denied to the calling process. @item EINVAL The described bus does not exist. @end table @noindent It may also fail and set @code{errno} to any of the errors specified for the system call @code{open}. @item int bus_close(bus_t *bus) This function disposes of resources allocated to the process, as referenced in the parameter @code{bus}. The function fails and sets @code{errno} to @code{EINVAL} if the bus does not exist. @item int bus_write(const bus_t *bus, const char *message, int flags) This function broadcasts a message on the bus whose information is stored in the parameter @code{bus}. The message read by the function is stored in the parameter @code{message}. It may not exceeed 2048 bytes, including NUL termination. The function shall fail, and set @code{errno} to @code{EAGAIN}, if the call would suspend the process and @code{flags} contains @code{BUS_NOWAIT}. The function may fail and set @code{errno} to any of the errors specified for the function @code{semop}. @item int bus_write_timed(const bus_t *bus, const char *message, const struct timespec *timeout, clockid_t clockid) This function behaves like @code{bus_write}, except if it is not able to write the message within the specified time, it will fail and set @code{errno} to @code{EAGAIN}. The time is specified as an absolute time using the parameter @code{timeout}. The behaviour is unspecified if @code{timeout} is @code{NULL}. @code{timeout} is measured with the clock whose identifier is specified by the parameter @code{clockid}. This clock must be a predicitable clock@footnote{There are probably other, undocumented, seemingly arbitrary restrictions too.}. The function may fail and set @code{errno} to any of the errors specified for the functions @code{semop} and @code{clock_gettime}. @item int bus_read(const bus_t *bus, int (*callback)(const char *message, void *user_data), void *user_data) This function waits for new message to be sent on the bus specified in the @code{bus} parameter, as provieded by a previous call to the function @code{bus_open}. Once a message is received, the parameter-function @code{callback} is invoked. The parameter @code{message} in @code{callback} is the received message, and @code{user_data} in @code{callback} should be @code{user_data} from @code{bus_read}. However, once the function [@code{bus_read}] has ensured that it will receive any message sent on the bus, it shall invoke the parameter-function @code{callback} with @code{message} set to @code{NULL}, to notify the process that it can perform any action that requires that it is listening on the bus. After @code{callback} returns, @code{message} may be override. Therefore @code{callback} should copy message and start a new thread that uses the copy of @code{message}. @code{callback} shall return @code{-1} on failure, @code{0} if the function [@code{bus_read}] should stop listening, or @code{1} if the function should continue listening. The function may fail and set @code{errno} to any of the errors specified for the function @code{semop}. @item int bus_read_timed(const bus_t *bus, int (*callback)(const char *message, void *user_data), void *user_data, const struct timespec *timeout, clockid_t clockid) This function behaves like @code{bus_read}, except it will automatically fail and set @code{errno} to @code{EAGAIN} when the specified time has passed. The time is specified as an absolute time using the parameter @code{timeout}. The behaviour is unspecified if @code{timeout} is @code{NULL}. @code{timeout} is measured with the clock whose identifier is specified by the parameter @code{clockid}. This clock must be a predicitable clock@footnote{There are probably other, undocumented, seemingly arbitrary restrictions too.}. The function may fail and set @code{errno} to any of the errors specified for the functions @code{semop} and @code{clock_gettime}. @item int bus_poll_start(bus_t *bus) @itemx int bus_poll_stop(const bus_t *bus) @itemx const char *bus_poll(bus_t *bus, int flags) @itemx const char *bus_poll_timed(bus_t *bus, const struct timespec *timeout, clockid_t clockid) The function @code{bus_poll} waits for a message to be broadcasted on the bus, and return the message it receives. The function fails if @code{flags} contains @code{BUS_NOWAIT} and there is not already a message waiting on the bus. Received messages shall be copied and parsed, and acted upon, in a separate thread, and the function @code{bus_poll} or the function @code{bus_poll_stop} called again as soon as possible. The funcion @code{bus_poll_start} must be called before @code{bus_poll} is called for the first time. When the process is done listening on the bus, it must call the function @code{bus_poll_stop}. The function @code{bus_poll_timed} behaves like the function @code{bus_poll}, except if it is not able to read a message within the specified time, it will fail and set @code{errno} to @code{EAGAIN}. The time is specified as an absolute time using the parameter @code{timeout}. The behaviour is unspecified if @code{timeout} is @code{NULL}. @code{timeout} is measured with the clock whose identifier is specified by the parameter @code{clockid}. This clock must be a predicitable clock@footnote{There are probably other, undocumented, seemingly arbitrary restrictions too.}. Upon successful completion, the functions @code{bus_poll} and @code{bus_poll_timed} returns the received message. These functions may fail and set @code{errno} to any of the errors specified for the function @code{semop}. The function @code{bus_poll_timed} may also set @code{errno} to any of the errors specified for @code{clock_gettime}. @item int bus_chown(const char *file, uid_t owner, gid_t group) This function changes the owner and the group of the bus, associated with the file whose pathname is stored in the parameter @code{file}, to the owner and group specified by the parameters @code{owner} and @code{group}, respectively. The current ownership of a bus can be retrieved by calling @code{stat} over the pathname of the bus. The function may fail and set @code{errno} to any of the errors specified for the functions @code{bus_open}, @code{chown}, @code{semget}, @code{shmget}, and @code{shmctl} as well as any errors specified for the commands @code{IPC_STAT} and @code{IPC_SET} for the function @code{semctl}. @item int bus_chmod(const char *file, mode_t mode) This function gives access to the bus associated with the file whose pathname is stored in the parameter @code{file} according to the following rules: @itemize @bullet{} @item If @code{mode} contains any of the bits @code{S_IRWXU} contains, the owner should be given full access to the bus. Otherwise the owner should have no access. @item If @code{mode} contains any of the bits @code{S_IRWXG} contains, the group should be given read and write access to the bus. Otherwise the group should have no access. @item If @code{mode} contains any of the bits @code{S_IRWXO} contains, users that are neither the owner nor member of the group should be given read and write access to the bus. Otherwise they should have no access. @end itemize The current permissions of a bus can be retrieved by calling @code{stat} over the pathname of the bus. The function may fail and set @code{errno} to any of the errors specified for the functions @code{bus_open}, @code{chmode}, @code{semget}, @code{shmget}, and @code{shmctl} as well as any errors specified for the commands @code{IPC_STAT} and @code{IPC_SET} for the function @code{semctl}. @end table There is not reason for poking around in @code{bus_t} (@code{struct bus}). It should be considered opaque. You can read the documentation in @file{} if you want to know what is in it. @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 Examples @chapter Examples This chapter contains usecase examples and API-demonstrations. You will find that they are (on a standard installation) installed on your system. @menu * Audio-volume control:: * Telephony and music:: * Timed:: * Nonblocking:: * Daemon-dependencies:: @end menu @node Audio-volume control @section Audio-volume control Assume you have program that display the audio volume. This program checks every second third if the volume have changed. Also assume that you use @command{amixer} to change the volume, most often by using keybindings via @command{xbindkeys}. To reduce the delay, you want to send a signal to the monitor program that the volume have changed. For this more primitive IPC is sufficient, but lets assume there are other programs interested in this information too. To accomplish this, you create a wrapper for @command{amixer} that broadcasts updates on a bus. This wrapper is installed as @command{~/.local/bin/amixer}, and @command{~/.local/bin/} is included in @env{$PATH} before @command{/usr/bin}. @* @noindent Before starting run @command{./init}, this code is should be run from your profile file if you want to implement this on your system. After running @command{./init}, you can start one or more listeners by running @command{./alsa-monitor}. To change the volume run @code{./amixer -c 0 -- set Master 5%+} or similar. When you are done run @command{./cleanup}. @subsubheading @file{./amixer} @example #!/bin/sh /usr/bin/amixer "$@@" for arg in "$@@"; do if [ "$@{arg@}" = "set" ] || \ [ "$@{arg@}" = "sset" ] || \ [ "$@{arg@}" = "cset" ]; then exec bus broadcast "/tmp/example-bus" '0 volume-changed *' fi done @end example @subsubheading @file{./cleanup} @example #!/bin/sh exec bus remove "/tmp/example-bus" @end example @subsubheading @file{./init} @example #!/bin/sh bus create "/tmp/example-bus" # @w{@xrm{}The following code is more suitable in the real world,@xtt{}} # @w{@xrm{}if used, the other files should use @xtt{}"$@{BUS_AUDIO@}"} # @w{@xrm{}instead of @xtt{}"/tmp/example-bus"@xrm{}.@xtt{}} # # export BUS_AUDIO="$@{XDG_RUNTIME_DIR@}/bus/audio" # if [ ! -f "$@{BUS_AUDIO@}" ]; then # bus create "$@{BUS_AUDIO@}" # fi @end example @subsubheading @file{./monitor} @example #!/bin/sh if [ $# = 1 ]; then if [ "$(echo "$@{1@}" | cut -d ' ' -f 2)" = "volume-changed" ]; then printf '\e[H\e[2J' amixer get Master fi exit 0 fi exec 2>/dev/null printf '\e[?1049h\e[H\e[2J' trap -- "printf '\e[?1049l'" SIGINT bus listen "/tmp/example-bus" \'"$@{0/\'/\'\\\'\'@}"\'' "$@{msg@}"' @end example @node Telephony and music @section Telephony and music Assume you have a music player and a telephony program. You might like it if the music player pauses whenever you make or receive a call. You may also like it, if the music resumed when the call ended. In this example we will assume you the have @command{mocp} (@command{moc} package) running. And we will use the shell to simulate a telephony program. @* @noindent First of, run make to build this example. Before starting run @command{./init}. And when you are done run @command{./cleanup}. In one terminal run @command{./monitor}. This program will pause @command{mocp} when you make or receive a call, it will also resume @command{mocp} when all calls have ended if it did pause @command{mocp}. Then start any positive number of terminals. We will pretend that each of them are telephony programs. To make or receive a call, run @command{./receive-or-make-call}, when you want to end the pretend call, run @command{./end-call} from the terminal (or more accurately, from the same process.) @subsubheading @file{./Makefile} @example COMMANDS = init cleanup monitor end-call receive-or-make-call all: $@{COMMANDS@} %: %.c $@{CC@} -Wall -Wextra -pedantic -std=c99 -lbus -o $@@ $< clean: -rm $@{COMMANDS@} .PHONY: all clean @end example @subsubheading @file{./cleanup.c} @example #include #include int main() @{ return bus_unlink("/tmp/example-bus") && (perror("cleanup"), 1); @} @end example @subsubheading @file{./end-call.c} @example #include #include #include #include #define t(stmt) if (stmt) goto fail static char message[BUS_MEMORY_SIZE]; int main() @{ bus_t bus; sprintf(message, "%ji unforce-pause", (intmax_t)getppid()); /* @w{@xrm{}Yes, PPID; in this example we pretend the shell is the telephony process.@xtt{}} */ t (bus_open(&bus, "/tmp/example-bus", BUS_WRONLY)); t (bus_write(&bus, message, 0)); bus_close(&bus); return 0; fail: perror("end-call"); bus_close(&bus); return 1; @} @end example @subsubheading @file{./init.c} @example #include #include int main() @{ return bus_create("/tmp/example-bus", 0, NULL) && (perror("init"), 1); @} @end example @subsubheading @file{./monitor.c} @example #include #include #include #include static size_t pauser_count = 0; static size_t pausers_size = 0; static char* pausers = NULL; static int is_moc_playing(void) @{ return !WEXITSTATUS(system("env LANG=C mocp -i 2>/dev/null |" "grep 'State: PLAY' >/dev/null")); @} /* @w{@xrm{}In a proper implementation, message whould be copyied, and then@xtt{}} * @w{@xrm{}a new thread would be created that parsed the copy. But that is@xtt{}} * @w{@xrm{}too much for an example, especially since it would also require@xtt{}} * @w{@xrm{}a mutex to make sure two threads do not modify data at the same@xtt{}} * @w{@xrm{}time, causing chaos.@xtt{}} */ static int callback(const char *message, void *user_data) @{ char *msg = NULL; size_t len = 0; if (message == 0) return 1; while ((len < 2047) && message[len]) len++; msg = malloc((len + 1) * sizeof(char)); t (msg == NULL); memcpy(msg, message, len * sizeof(char)); msg[len] = 0; /* @w{@xrm{}BEGIN run as in a separate thread@xtt{}} */ if (pauser_count || is_moc_playing()) @{ char *begin = strchr(msg, ' '); ssize_t pid; int requests_pause; if (begin == NULL) goto done; *begin++ = 0; pid = (ssize_t)atoll(msg); if (pid < 1) /* @w{@xrm{}We need a real PID, too bad there is@xtt{}} * @w{@xrm{}no convient way to detect if it dies.@xtt{}} */ goto done; if ((strstr(begin, "force-pause ") == begin) || !strcmp(begin, "force-pause")) requests_pause = 1; else if ((strstr(begin, "unforce-pause ") == begin) || !strcmp(begin, "unforce-pause")) requests_pause = 0; else goto done; if ((size_t)pid >= pausers_size) @{ pausers = realloc(pausers, (size_t)(pid + 1) * sizeof(char)); t (pausers == NULL); /* @w{@xrm{}Let's ignore the memory leak.@xtt{}} */ memset(pausers + pausers_size, 0, ((size_t)(pid + 1) - pausers_size) * sizeof(char)); pausers_size = (size_t)(pid + 1); @} if (pausers[pid] ^ requests_pause) @{ pauser_count += requests_pause ? 1 : -1; pausers[pid] = requests_pause; if (pauser_count == (size_t)requests_pause) system(requests_pause ? "mocp -P" : "mocp -U"); @} @} /* @w{@xrm{}END run as in a separate thread@xtt{}} */ goto done; (void) user_data; fail: perror("monitor"); return -1; done: free(msg); return 1; @} int main() @{ bus_t bus; t (bus_open(&bus, "/tmp/example-bus", BUS_RDONLY)); t (bus_read(&bus, callback, NULL)); bus_close(&bus); free(pausers); return 0; fail: perror("monitor"); bus_close(&bus); free(pausers); return 1; @} @end example @subsubheading @file{./receive-or-make-call.c} @example #include #include #include #include #define t(stmt) if (stmt) goto fail static char message[BUS_MEMORY_SIZE]; int main() @{ bus_t bus; sprintf(message, "%ji force-pause", (intmax_t)getppid()); /* @w{@xrm{}Yes, PPID; in this example we pretend the shell is the telephony process.@xtt{}} */ t (bus_open(&bus, "/tmp/example-bus", BUS_WRONLY)); t (bus_write(&bus, message, 0)); bus_close(&bus); return 0; fail: perror("receive-or-make-call"); bus_close(&bus); return 1; @} @end example @node Timed @section Timed This example shows how to use timed operations. First of, run make to build this example. To start the example run @command{./init}. When you are done run @command{./cleanup}. Running instances of @command{./poll} will wait for new messages continuously, but with one second timeouts. @command{./slow-poll} works like @command{./poll}, except it will sleep for one second every time it receives a message. Running instances of @command{./read} will read for ten seconds and then time out. @command{./poll}, @command{./read}, and @command{./slow-poll} will stop if the message "stop" is broadcasted. @command{./write} will wait for atmost a tenth of a seconds before failing. This means that if two instances of @command{./write} is started at the same time one of them will fail if @command{./slow-poll} is running. @command{./poll}, @command{./read}, @command{./init} and @command{./cleanup} are run without any additional arguments. @command{./write} is run with the message as the second argument. @subsubheading @file{./Makefile} @example COMMANDS = init cleanup write poll read slow-poll all: $@{COMMANDS@} %: %.c $@{CC@} -Wall -Wextra -pedantic -std=c99 -lbus -o $@@ $< clean: -rm $@{COMMANDS@} .PHONY: all clean @end example @subsubheading @file{./cleanup.c} @example #include #include int main() @{ return bus_unlink("/tmp/example-bus") && (perror("cleanup"), 1); @} @end example @subsubheading @file{./init.c} @example #include #include int main() @{ return bus_create("/tmp/example-bus", 0, NULL) && (perror("init"), 1); @} @end example @subsubheading @file{./poll.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail int main() @{ bus_t bus; const char *message; long long tick = 0; struct timespec timeout; t (bus_open(&bus, "/tmp/example-bus", BUS_RDONLY)); t (bus_poll_start(&bus)); for (;;) @{ t (clock_gettime(CLOCK_MONOTONIC, &timeout)); timeout.tv_sec += 1; message = bus_poll_timed(&bus, &timeout, CLOCK_MONOTONIC); if (message == NULL) @{ t (errno != EAGAIN); printf("waiting... %lli\n", ++tick); continue; @} tick = 0; message = strchr(message, ' ') + 1; if (!strcmp(message, "stop")) break; printf("\033[01m%s\033[21m\n", message); @} t (bus_poll_stop(&bus)); bus_close(&bus); return 0; fail: perror("poll"); bus_poll_stop(&bus); bus_close(&bus); return 1; @} @end example @subsubheading @file{./read.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail static int callback(const char *message, void *user_data) @{ (void) user_data; if (message == NULL) return 1; message = strchr(message, ' ') + 1; if (!strcmp(message, "stop")) return 0; printf("%s\n", message); return 1; @} int main() @{ bus_t bus; struct timespec timeout; t (bus_open(&bus, "/tmp/example-bus", BUS_RDONLY)); t (clock_gettime(CLOCK_MONOTONIC, &timeout)); timeout.tv_sec += 10; t (bus_read_timed(&bus, callback, NULL, &timeout, CLOCK_MONOTONIC)); bus_close(&bus); return 0; fail: perror("poll"); bus_poll_stop(&bus); bus_close(&bus); return 1; @} @end example @subsubheading @file{./slow-poll.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail int main() @{ bus_t bus; const char *message; long long tick = 0; struct timespec timeout; t (bus_open(&bus, "/tmp/example-bus", BUS_RDONLY)); t (bus_poll_start(&bus)); for (;;) @{ t (clock_gettime(CLOCK_MONOTONIC, &timeout)); timeout.tv_sec += 1; message = bus_poll_timed(&bus, &timeout, CLOCK_MONOTONIC); if (message == NULL) @{ t (errno != EAGAIN); printf("waiting... %lli\n", ++tick); continue; @} tick = 0; message = strchr(message, ' ') + 1; if (!strcmp(message, "stop")) break; printf("\033[01m%s\033[21m\n", message); sleep(1); @} t (bus_poll_stop(&bus)); bus_close(&bus); return 0; fail: perror("poll"); bus_poll_stop(&bus); bus_close(&bus); return 1; @} @end example @subsubheading @file{./write.c} @example #include #include #include #include #define t(stmt) if (stmt) goto fail static char message[BUS_MEMORY_SIZE]; int main(int argc, char *argv[]) @{ bus_t bus; struct timespec timeout; if (argc < 2) @{ fprintf(stderr, "%s: USAGE: %s message\n", argv[0], argv[0]); return 2; @} sprintf(message, "0 %s", argv[1]); t (bus_open(&bus, "/tmp/example-bus", BUS_WRONLY)); t (clock_gettime(CLOCK_MONOTONIC, &timeout)); timeout.tv_nsec += 100000000L; t (bus_write_timed(&bus, message, &timeout, CLOCK_MONOTONIC)); bus_close(&bus); return 0; fail: perror("write"); bus_close(&bus); return 1; @} @end example @node Nonblocking @section Nonblocking This example shows how to use bus_poll instead of bus_read, and how to do non-blocking polling and non-blocking writing. First of, run make to build this example. To start the example run @command{./init}. When you are done run @command{./cleanup}. Running instances of @command{./poll} will check every second if there is a new inbound message. Between these checks @command{./write} will wait for all @command{./poll}:s to receive the message. This means that @command{./write} blocks while @command{./poll} sleeps. If two or more instances of @command{./write} is started at approximately the same time, only one will continue to write a message on the bus, the others will fail. @command{./poll} will stop if the message ``stop'' is broadcasted. @command{./poll}, @command{./init} and @command{./cleanup} are run without any additional arguments. @command{./write} is run with the message as the second argument. @subsubheading @file{./Makefile} @example COMMANDS = init cleanup write poll all: $@{COMMANDS@} %: %.c $@{CC@} -Wall -Wextra -pedantic -std=c99 -lbus -o $@@ $< clean: -rm $@{COMMANDS@} .PHONY: all clean @end example @subsubheading @file{./cleanup.c} @example #include #include int main() @{ return bus_unlink("/tmp/example-bus") && (perror("cleanup"), 1); @} @end example @subsubheading @file{./init.c} @example #include #include int main() @{ return bus_create("/tmp/example-bus", 0, NULL) && (perror("init"), 1); @} @end example @subsubheading @file{./poll.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail int main() @{ bus_t bus; const char *message; long long tick = 0; t (bus_open(&bus, "/tmp/example-bus", BUS_RDONLY)); t (bus_poll_start(&bus)); for (;;) @{ message = bus_poll(&bus, BUS_NOWAIT); if (message == NULL) @{ t (errno != EAGAIN); printf("waiting... %lli\n", ++tick); sleep(1); continue; @} tick = 0; message = strchr(message, ' ') + 1; if (!strcmp(message, "stop")) break; printf("\033[01m%s\033[21m\n", message); @} t (bus_poll_stop(&bus)); bus_close(&bus); return 0; fail: perror("poll"); bus_poll_stop(&bus); bus_close(&bus); return 1; @} @end example @subsubheading @file{./write.c} @example #include #include #include #include #define t(stmt) if (stmt) goto fail static char message[BUS_MEMORY_SIZE]; int main(int argc, char *argv[]) @{ bus_t bus; if (argc < 2) @{ fprintf(stderr, "%s: USAGE: %s message\n", argv[0], argv[0]); return 2; @} sprintf(message, "0 %s", argv[1]); t (bus_open(&bus, "/tmp/example-bus", BUS_WRONLY)); t (bus_write(&bus, message, BUS_NOWAIT)); bus_close(&bus); return 0; fail: perror("write"); bus_close(&bus); return 1; @} @end example @node Daemon-dependencies @section Daemon-dependencies This example shows how bus can be used in an init system to provide ``aggressivly'' parallel startup of daemons. First of, run make to build this example. To start the example run @command{./init}. It will print in red export-statement you may want to run i other terminals. You will need to select at least one daemon, for example you can run @code{./init d-ntp}. The available pretend daemons are: @command{d-network}, @command{d-ntp}, and @command{d-ssh}. When you are done run @command{./cleanup} with @env{BUS_INIT} exported with the value printed by @command{./init}. @subsubheading @file{./Makefile} @example COMMANDS = announce await-ready await-started cleanup \ init require start-daemon test-daemon all: $@{COMMANDS@} %: %.c $@{CC@} -Wall -Wextra -pedantic -std=c99 -lbus -o $@@ $< clean: -rm $@{COMMANDS@} -rm -r run .PHONY: all clean @end example @subsubheading @file{./announce.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail static char arg[4098]; int main(int argc, char *argv[]) @{ bus_t bus; if (argc < 3) return fprintf(stderr, "USAGE: %s state daemon", *argv), 2; t (bus_open(&bus, getenv("BUS_INIT"), BUS_WRONLY)); sprintf(arg, "%ji %s %s", (intmax_t)getppid(), argv[1], argv[2]); t (bus_write(&bus, arg, 0)); t (bus_close(&bus)); return 0; fail: perror("announce"); return 1; @} @end example @subsubheading @file{./await-ready.c} @example #include #include #include #include #include #include #include #define t(stmt) if (stmt) goto fail static char arg[4098]; static int argc; static char **argv; static int remaining = 0; static char *started = NULL; static char msg[BUS_MEMORY_SIZE]; static void announce_wait(pid_t pid) @{ bus_t bus; int i; t (bus_open(&bus, getenv("BUS_INIT"), BUS_WRONLY)); for (i = 1; i < argc; i++) @{ if (!started[i]) @{ sprintf(arg, "%ji awaiting-ready %s", (intmax_t)pid, argv[i]); t (bus_write(&bus, arg, 0)); @} @} t (bus_close(&bus)); return; fail: perror("await-ready"); @} static int callback(const char *message, void *user_data) @{ int i; char *arg2; char *arg3; pid_t pid; pid_t ppid; if (!message) @{ ppid = getppid(); pid = fork(); if (pid == 0) @{ if (fork() == 0) announce_wait(ppid); exit(0); @} else @{ (void) waitpid(pid, NULL, 0); /* @w{@xrm{}Let's pretend everything will go swimmingly.@xtt{}} */ @} return 1; @} strncpy(msg, message, BUS_MEMORY_SIZE - 1); msg[BUS_MEMORY_SIZE - 1] = 0; arg2 = strchr(msg, ' '); if (!arg2) return 1; arg3 = strchr(++arg2, ' '); if (!arg3) return 1; *arg3++ = 0; if (strcmp(arg2, "ready")) return 1; for (i = 1; i < argc; i++) if (!started[i] && !strcmp(argv[i], arg3)) started[i] = 1, remaining--; return !!remaining; (void) user_data; @} int main(int argc_, char *argv_[]) @{ bus_t bus; int i; argc = argc_; argv = argv_; if (argc < 2) return fprintf(stderr, "USAGE: %s daemon...", *argv), 2; t (bus_open(&bus, getenv("BUS_INIT"), BUS_RDONLY)); started = calloc(argc, sizeof(char)); t (started == NULL); started[0] = 1; for (i = 1; i < argc; i++) @{ sprintf(arg, "grep '^%s$'" " <\"$@{XDG_RUNTIME_DIR@}/ready-daemons\"" " >/dev/null", argv[i]); if (!WEXITSTATUS(system(arg))) started[i] = 1; else remaining++; @} if (remaining) bus_read(&bus, callback, NULL); bus_close(&bus); free(started); return 0; fail: perror("await-ready"); bus_close(&bus); free(started); return 1; @} @end example @subsubheading @file{./await-started.c} @example #include #include #include #include #include #include #include #define t(stmt) if (stmt) goto fail static char arg[4098]; static int argc; static char **argv; static int remaining = 0; static char *started = NULL; static char msg[BUS_MEMORY_SIZE]; static void announce_wait(pid_t pid) @{ bus_t bus; int i; t (bus_open(&bus, getenv("BUS_INIT"), BUS_WRONLY)); for (i = 1; i < argc; i++) @{ if (!started[i]) @{ sprintf(arg, "%ji awaiting-started %s", (intmax_t)pid, argv[i]); t (bus_write(&bus, arg, 0)); @} @} t (bus_close(&bus)); return; fail: perror("await-started"); @} static int callback(const char *message, void *user_data) @{ int i; char *arg2; char *arg3; pid_t pid; pid_t ppid; if (!message) @{ ppid = getppid(); pid = fork(); if (pid == 0) @{ if (fork() == 0) announce_wait(ppid); exit(0); @} else @{ (void) waitpid(pid, NULL, 0); /* @w{@xrm{}Let's pretend everything will go swimmingly.@xtt{}} */ @} return 1; @} strncpy(msg, message, BUS_MEMORY_SIZE - 1); msg[BUS_MEMORY_SIZE - 1] = 0; arg2 = strchr(msg, ' '); if (!arg2) return 1; arg3 = strchr(++arg2, ' '); if (!arg3) return 1; *arg3++ = 0; if (strcmp(arg2, "started") && strcmp(arg2, "ready")) return 1; for (i = 1; i < argc; i++) if (!started[i] && !strcmp(argv[i], arg3)) started[i] = 1, remaining--; return !!remaining; (void) user_data; @} int main(int argc_, char *argv_[]) @{ bus_t bus; int i; argc = argc_; argv = argv_; if (argc < 2) return fprintf(stderr, "USAGE: %s daemon...", *argv), 2; t (bus_open(&bus, getenv("BUS_INIT"), BUS_RDONLY)); started = calloc(argc, sizeof(char)); t (started == NULL); started[0] = 1; for (i = 1; i < argc; i++) @{ sprintf(arg, "grep '^%s$'" " <\"$@{XDG_RUNTIME_DIR@}/started-daemons\"" " >/dev/null", argv[i]); if (!WEXITSTATUS(system(arg))) @{ started[i] = 1; @} else @{ sprintf(arg, "grep '^%s$'" " <\"$@{XDG_RUNTIME_DIR@}/ready-daemons\"" " >/dev/null", argv[i]); if (!WEXITSTATUS(system(arg))) started[i] = 1; else remaining++; @} @} if (remaining) bus_read(&bus, callback, NULL); bus_close(&bus); free(started); return 0; fail: perror("await-started"); bus_close(&bus); free(started); return 1; @} @end example @subsubheading @file{./cleanup.c} @example #include #include #include #define t(stmt) if (stmt) goto fail int main() @{ char *bus_address = getenv("BUS_INIT"); if (!bus_address || !*bus_address) @{ fprintf(stderr, "$BUS_INIT has not been set, its export statement " "should have been printed in bold red by ./init\n"); return 1; @} t (bus_unlink(bus_address)); return 0; fail: perror("cleanup"); return 1; @} @end example @subsubheading @file{./d-network} @example #!/bin/sh PATH=.:$PATH d=d-network echo $d: starting sleep 2 echo $d: ready announce ready $d @end example @subsubheading @file{./d-ntp} @example #!/bin/sh PATH=.:$PATH d=d-ntp require d-network echo $d: started announce started $d await-ready d-network echo $d: ready announce ready $d @end example @subsubheading @file{./d-ssh} @example #!/bin/sh PATH=.:$PATH d=d-ssh require d-network echo $d: starting sleep 1 echo $d: started announce started $d sleep 1 echo $d: ready announce ready $d @end example @subsubheading @file{./init.c} @example #include #include #include #include #include #include #define t(stmt) if (stmt) goto fail #define _2(...) __VA_ARGS__, __VA_ARGS__ static char msg[BUS_MEMORY_SIZE]; static int argc; static char **argv; static char arg[4098]; static void start_daemons() @{ int i; for (i = 1; i < argc; i++) if (fork() == 0) execl("./start-daemon", "./start-daemon", argv[i], NULL); @} static int callback(const char *message, void *user_data) @{ pid_t pid; char *arg2; char *arg3; if (!message) @{ pid = fork(); t (pid == -1); if (pid == 0) @{ if (fork() == 0) @{ start_daemons(); @} exit(0); @} else @{ (void) waitpid(pid, NULL, 0); /* @w{@xrm{}Let's pretend everything will go swimmingly.@xtt{}} */ @} return 1; @} strncpy(msg, message, BUS_MEMORY_SIZE - 1); msg[BUS_MEMORY_SIZE - 1] = 0; pid = fork(); t (pid == -1); if (pid == 0) @{ if (fork() == 0) @{ arg2 = strchr(msg, ' '); if (arg2 == NULL) exit(0); arg3 = strchr(++arg2, ' '); if (arg3 == NULL) exit(0); *arg3++ = 0; if (!strcmp(arg2, "require")) @{ execl(_2("./start-daemon"), arg3, NULL); @} else if (!strcmp(arg2, "awaiting-started")) @{ execl(_2("./test-daemon"), arg3, "started", NULL); @} else if (!strcmp(arg2, "awaiting-ready") || !strcmp(arg2, "awaiting")) @{ execl(_2("./test-daemon"), arg3, "ready", NULL); @} else if (!strcmp(arg2, "started")) @{ sprintf(arg, "grep '^%s\\$' < \"%s\" >/dev/null || echo %s >> \"%s\"", _2(arg3, "$@{XDG_RUNTIME_DIR@}/started-daemons")); execlp(_2("sh"), "-c", arg, NULL); @} else if (!strcmp(arg2, "ready")) @{ sprintf(arg, "grep '^%s\\$' < \"%s\" >/dev/null || echo %s >> \"%s\"", _2(arg3, "$@{XDG_RUNTIME_DIR@}/ready-daemons")); execlp(_2("sh"), "-c", arg, NULL); @} @} exit(0); @} else @{ (void) waitpid(pid, NULL, 0); /* @w{@xrm{}Let's pretend everything will go swimmingly.@xtt{}} */ @} return 1; (void) user_data; fail: perror("init"); return -1; @} int main(int argc_, char *argv_[]) @{ char *bus_address = NULL; bus_t bus; argv = argv_; argc = argc_; if (argc < 2) @{ fprintf(stderr, "USAGE: %s daemon...\n", *argv); return 1; @} t (setenv("XDG_RUNTIME_DIR", "./run", 1)); /* @w{@xrm{}Real init systems with not have the period.@xtt{}} */ system("mkdir -p -- \"$@{XDG_RUNTIME_DIR@}\""); system("truncate -s 0 -- \"$@{XDG_RUNTIME_DIR@}/started-daemons\""); system("truncate -s 0 -- \"$@{XDG_RUNTIME_DIR@}/ready-daemons\""); t (bus_create(NULL, 1, &bus_address)); fprintf(stderr, "\033[00;01;31mexport BUS_INIT=%s\033[00m\n", bus_address); fprintf(stderr, "\033[00;31mexport XDG_RUNTIME_DIR=./run\033[00m\n"); t (setenv("BUS_INIT", bus_address, 1)); t (bus_open(&bus, bus_address, BUS_RDONLY)); t (bus_read(&bus, callback, NULL)); bus_close(&bus); free(bus_address); return 0; fail: perror("init"); bus_close(&bus); free(bus_address); return 1; @} @end example @subsubheading @file{./require.c} @example #include #include #include #include #include #define t(stmt) if (stmt) goto fail static char arg[4098]; int main(int argc, char *argv[]) @{ bus_t bus; int i; if (argc < 2) return fprintf(stderr, "USAGE: %s daemon...", *argv), 2; t (bus_open(&bus, getenv("BUS_INIT"), BUS_WRONLY)); for (i = 1; i < argc; i++) @{ sprintf(arg, "grep '^%s$' <\"%s\" >/dev/null", argv[i], "$@{XDG_RUNTIME_DIR@}/started-daemons"); if (WEXITSTATUS(system(arg))) @{ sprintf(arg, "%ji require %s", (intmax_t)getppid(), argv[i]); t (bus_write(&bus, arg, 0)); @} @} bus_close(&bus); return 0; fail: perror("require"); bus_close(&bus); return 1; @} @end example @subsubheading @file{./start-daemon.c} @example #include #include #include #include #include #include static char arg[4098]; int main(int argc, char *argv[]) @{ if (argc != 2) @{ fprintf(stderr, "This program should be called from ./init\n"); return 2; @} sprintf(arg, "grep '^%s$' <\"%s\" >/dev/null", argv[1], "$@{XDG_RUNTIME_DIR@}/started-daemons"); if (!WEXITSTATUS(system(arg))) return 0; sprintf(arg, "grep '^%s$' <\"%s\" >/dev/null", argv[1], "$@{XDG_RUNTIME_DIR@}/ready-daemons"); if (!WEXITSTATUS(system(arg))) return 0; sprintf(arg, "./%s", argv[1]); execlp(arg, arg, NULL); perror("start-daemon"); return 1; @} @end example @subsubheading @file{./test-daemon.c} @example #include #include #include #include #define t(stmt) if (stmt) goto fail static char arg[4098]; int main(int argc, char *argv[]) @{ bus_t bus; if (argc != 3) @{ fprintf(stderr, "This program should be called from ./init\n"); return 2; @} retry: sprintf(arg, "grep '^%s$'" " <\"$@{XDG_RUNTIME_DIR@}/%s-daemons\"" " >/dev/null", argv[1], argv[2]); if (!WEXITSTATUS(system(arg))) @{ t (bus_open(&bus, getenv("BUS_INIT"), BUS_WRONLY)); sprintf(arg, "0 %s %s", argv[2], argv[1]); t (bus_write(&bus, arg, 0)); bus_close(&bus); @} else if (!strcmp(argv[2], "started")) @{ argv[2] = "ready"; goto retry; @} return 0; fail: perror("test-daemon"); return 1; @} @end example @node GNU Free Documentation License @appendix GNU Free Documentation License @include fdl.texinfo @bye