@Admin: import from ancient backup raw files

AUTHORS

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+Karl N. Redman (A.K.A. Parasyte).

Condition_Variable_FAQ

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+Condition Variable Mini-FAQ (under construction)
+
+by Karl N. Redman 
+contact: kredman@sleepingstill.com
+
+Last updated: 1-3-2001
+
+
+What is a condition variable?
+
+     A condition variable is an atomic waiting and signaling mechanism
+     which allows a process or thread to temporarily stop execution
+     until a signal is received, indicating the change of a shared
+     variable within a predicate, from some other process or thread. 
+
+     What this means is that a condition variable provides a way to
+     temporarily "stop" the execution of a program until it is
+     signaled to continue via another process or thread as a result of
+     the change in the value of one or more variables. The process of
+     checking the predicate for a change and waiting for a signal
+     happens uninterrupted (atomically) within a process or thread.  
+
+     The term "condition variable" is sometimes considered misleading
+     because the mechanism does not rely on a variable but rather on
+     the process of signaling at the system level. The term comes from
+     the use of the mechanism rather than the actual
+     operation. Condition variables are most often used as a way for 
+     one process or thread to "notify" another process or thread of
+     the change in the value of a variable which is shared between 
+     processes and/or threads.  
+
+
+     Practical uses for the condition variable mechanism:
+
+	       Notify a writer thread that a reader thread has filled
+	       it's data set.
+
+	       Notify consumer processes that a producer thread has
+	       updated a shared data set.
+
+		Anytime a process or thread needs to wait for the
+		change of one or more shared variables which are
+		updated by another process or thread
+
+
+     It is my experience that condition variables are use rarely. It
+     is also my experience that the operation of many programs may be
+     made more efficient through the use of condition variables. Very
+     often semaphores are used in place of condition variables as
+     well; as a result of not being aware of the behavior of the
+     condition variable mechanism.
+
+
+What's the difference between a semaphore and a condition variable?
+
+       A semaphore is used to synchronize processes and threads. It is
+       not intended to be as an asynchronous blocking or waiting
+       mechanism. A condition variable is most often used as an
+       asynchronous blocking or waiting mechanism. It's purpose is to
+       allow multiple processes and/or threads to wait on some
+       predicate (condition) independently.
+
+
+
+Why not use a semaphore?
+
+    A semaphore may cause some or all processes or threads to wait for
+    the release of a semaphore before continuing operation. The very
+    nature of a semaphore guarantees that no other process will
+    continue until some condition is met. There are some possibly
+    multi platform conditions where the use of a semaphore would cause
+    the wrong processes to wait or cease execution temporarily.
+
+
+	An Example where the use of a condition variable is more
+	appropriate than a semaphore follows:
+
+	I came across this problem in the real world while working
+	for a financial data provider company. The situation is that
+	there is a time critical (real time) "producer" process which
+	receives data from a socket, unpacks it, and deposits the
+	data into a shared memory buffer. At the same time, there are
+	one or more "consumer" processes that read the data from the
+	shared buffer simultaneously.
+
+	The specifications for this real time data delivery system are
+	as follows:
+
+	   There is a shared "ring buffer" that the producer puts data
+	   into and the consumers read from. 
+
+	   No consumer may read from a memory region where the
+	   producer has not deposited data; consumers may not "pass
+	   up" the producer.
+
+	   The producer is only allowed to block on a socket; it must
+	   otherwise never be stopped or slowed down by any other
+	   process or thread.
+
+	   Consumer processes may run much slower than the producer or
+	   may halt or "hang up" during execution; the producer may
+	   "pass up" any and/or all consumers.
+
+       The producer program is a daemon that puts data into a memory
+       region that is shared and readable by other programs on a
+       computer system. The producer program will put data into the
+       first memory "slot" or "bin" of a set and continue filling
+       successive "bins" until it reaches the last. Once the last
+       memory slot is filled, the producer then starts over at the
+       beginning and overwrites the data in the first "bin" with new
+       data; this is referred to as a ring buffer.
+
+       The problem where a condition variable comes in is when a
+       consumer process has read from all the memory "bins" and has
+       caught up with the producer. We do not want the consumer to
+       pass the producer, for it may then read data that is out of
+       date or just bad altogether. Regardless of why the producer may
+       have stopped putting data into memory "slots", the consumer
+       must never check a "slot" past the producer. 
+
+       There are a few solutions that we could use in this
+       scenario. One would be to continuously poll or check the
+       position that the producer has last written to in a loop. The
+       problem with this solution is that the consumer program would
+       then cause over usage of the processor (more than one consumer
+       polling a shared value would bring just about any PC today to
+       it's knees). 
+
+	    Consumer code Example:
+		----------- code snip-it -----------------
+
+		...
+
+		x = csv; //current slot to check
+
+		y = shared_mem_producer_slot //producer's current slot
+
+		while(x < y)
+		{
+			//check the shared memory slot
+			y = shared_mem_producer_slot //producer's current slot
+		}
+
+		//get data from slot 'x' for processing
+		memcopy(data[x], shared_memory_region[x],data_length);
+
+		...
+
+		----------- end code snip-it --------------
+
+
+       Another solution might be the use of a semaphore. In this
+       scenario the producer would, when "waiting" for whatever
+       reason, increase the value of a semaphore by the number of
+       consumer processes currently running. The reason for using the
+       number of consumers is so that each consumer, when or if it
+       catches up with the producer, gets a turn at reading the data
+       when the producer continues. One problem with this solution is
+       that under certain implementations and uses of this scenario
+       the producer would end up waiting for all consumers to
+       decrement the semaphore before continuing; causing the
+       producer to block. Another (more often) problem is that the
+       consumers would then be synchronized; if, for instance, one of
+       five consumers somehow locks up, then the semaphore is never
+       released -causing most or all consumers to wait indefinitely. 
+
+
+       Using a condition variable here allows the consumers to be
+       asynchronously notified when the producer has placed new data
+       into the ring buffer. When each consumer catches up with the
+       producer it is told to cease execution until it is notified by
+       the signaling of the condition variable mechanism. Once the
+       producer has put new data into a slot, it sends a signal to the
+       threads or process that are waiting; notifying each of them
+       that new data has arrived.
+
+       Example of a condition variable program:
+	       An example of the use of condition variables is
+	       provided here
+	       (www.enteract.com/~parasyte/html/projects/Cond_var/cond_var.tgz).
+	       Please note that the example provided here is a
+	       multi-platform solution that gives linux and non-linux
+	       solutions. 
+

Condition_Variable_FAQ.LCK

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+checkout (redmank)

INSTALL

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+quick:
+	READ THE README FILE!!!!
+
+	type "./configure"
+	type "make"
+	type "cd src"
+
+	run one producer and upto 32 consumer programs -parasyte
+
+
+
+
+Basic Installation
+==================
+
+   These are generic installation instructions.
+
+   The `configure' shell script attempts to guess correct values for
+various system-dependent variables used during compilation.  It uses
+those values to create a `Makefile' in each directory of the package.
+It may also create one or more `.h' files containing system-dependent
+definitions.  Finally, it creates a shell script `config.status' that
+you can run in the future to recreate the current configuration, a file
+`config.cache' that saves the results of its tests to speed up
+reconfiguring, and a file `config.log' containing compiler output
+(useful mainly for debugging `configure').
+
+   If you need to do unusual things to compile the package, please try
+to figure out how `configure' could check whether to do them, and mail
+diffs or instructions to the address given in the `README' so they can
+be considered for the next release.  If at some point `config.cache'
+contains results you don't want to keep, you may remove or edit it.
+
+   The file `configure.in' is used to create `configure' by a program
+called `autoconf'.  You only need `configure.in' if you want to change
+it or regenerate `configure' using a newer version of `autoconf'.
+
+The simplest way to compile this package is:
+
+  1. `cd' to the directory containing the package's source code and type
+     `./configure' to configure the package for your system.  If you're
+     using `csh' on an old version of System V, you might need to type
+     `sh ./configure' instead to prevent `csh' from trying to execute
+     `configure' itself.
+
+     Running `configure' takes awhile.  While running, it prints some
+     messages telling which features it is checking for.
+
+  2. Type `make' to compile the package.
+
+  3. Optionally, type `make check' to run any self-tests that come with
+     the package.
+
+  4. Type `make install' to install the programs and any data files and
+     documentation.
+
+  5. You can remove the program binaries and object files from the
+     source code directory by typing `make clean'.  To also remove the
+     files that `configure' created (so you can compile the package for
+     a different kind of computer), type `make distclean'.  There is
+     also a `make maintainer-clean' target, but that is intended mainly
+     for the package's developers.  If you use it, you may have to get
+     all sorts of other programs in order to regenerate files that came
+     with the distribution.
+
+Compilers and Options
+=====================
+
+   Some systems require unusual options for compilation or linking that
+the `configure' script does not know about.  You can give `configure'
+initial values for variables by setting them in the environment.  Using
+a Bourne-compatible shell, you can do that on the command line like
+this:
+     CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
+
+Or on systems that have the `env' program, you can do it like this:
+     env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
+
+Compiling For Multiple Architectures
+====================================
+
+   You can compile the package for more than one kind of computer at the
+same time, by placing the object files for each architecture in their
+own directory.  To do this, you must use a version of `make' that
+supports the `VPATH' variable, such as GNU `make'.  `cd' to the
+directory where you want the object files and executables to go and run
+the `configure' script.  `configure' automatically checks for the
+source code in the directory that `configure' is in and in `..'.
+
+   If you have to use a `make' that does not supports the `VPATH'
+variable, you have to compile the package for one architecture at a time
+in the source code directory.  After you have installed the package for
+one architecture, use `make distclean' before reconfiguring for another
+architecture.
+
+Installation Names
+==================
+
+   By default, `make install' will install the package's files in
+`/usr/local/bin', `/usr/local/man', etc.  You can specify an
+installation prefix other than `/usr/local' by giving `configure' the
+option `--prefix=PATH'.
+
+   You can specify separate installation prefixes for
+architecture-specific files and architecture-independent files.  If you
+give `configure' the option `--exec-prefix=PATH', the package will use
+PATH as the prefix for installing programs and libraries.
+Documentation and other data files will still use the regular prefix.
+
+   In addition, if you use an unusual directory layout you can give
+options like `--bindir=PATH' to specify different values for particular
+kinds of files.  Run `configure --help' for a list of the directories
+you can set and what kinds of files go in them.
+
+   If the package supports it, you can cause programs to be installed
+with an extra prefix or suffix on their names by giving `configure' the
+option `--program-prefix=PREFIX' or `--program-suffix=SUFFIX'.
+
+Optional Features
+=================
+
+   Some packages pay attention to `--enable-FEATURE' options to
+`configure', where FEATURE indicates an optional part of the package.
+They may also pay attention to `--with-PACKAGE' options, where PACKAGE
+is something like `gnu-as' or `x' (for the X Window System).  The
+`README' should mention any `--enable-' and `--with-' options that the
+package recognizes.
+
+   For packages that use the X Window System, `configure' can usually
+find the X include and library files automatically, but if it doesn't,
+you can use the `configure' options `--x-includes=DIR' and
+`--x-libraries=DIR' to specify their locations.
+
+Specifying the System Type
+==========================
+
+   There may be some features `configure' can not figure out
+automatically, but needs to determine by the type of host the package
+will run on.  Usually `configure' can figure that out, but if it prints
+a message saying it can not guess the host type, give it the
+`--host=TYPE' option.  TYPE can either be a short name for the system
+type, such as `sun4', or a canonical name with three fields:
+     CPU-COMPANY-SYSTEM
+
+See the file `config.sub' for the possible values of each field.  If
+`config.sub' isn't included in this package, then this package doesn't
+need to know the host type.
+
+   If you are building compiler tools for cross-compiling, you can also
+use the `--target=TYPE' option to select the type of system they will
+produce code for and the `--build=TYPE' option to select the type of
+system on which you are compiling the package.
+
+Sharing Defaults
+================
+
+   If you want to set default values for `configure' scripts to share,
+you can create a site shell script called `config.site' that gives
+default values for variables like `CC', `cache_file', and `prefix'.
+`configure' looks for `PREFIX/share/config.site' if it exists, then
+`PREFIX/etc/config.site' if it exists.  Or, you can set the
+`CONFIG_SITE' environment variable to the location of the site script.
+A warning: not all `configure' scripts look for a site script.
+
+Operation Controls
+==================
+
+   `configure' recognizes the following options to control how it
+operates.
+
+`--cache-file=FILE'
+     Use and save the results of the tests in FILE instead of
+     `./config.cache'.  Set FILE to `/dev/null' to disable caching, for
+     debugging `configure'.
+
+`--help'
+     Print a summary of the options to `configure', and exit.
+
+`--quiet'
+`--silent'
+`-q'
+     Do not print messages saying which checks are being made.  To
+     suppress all normal output, redirect it to `/dev/null' (any error
+     messages will still be shown).
+
+`--srcdir=DIR'
+     Look for the package's source code in directory DIR.  Usually
+     `configure' can determine that directory automatically.
+
+`--version'
+     Print the version of Autoconf used to generate the `configure'
+     script, and exit.
+
+`configure' also accepts some other, not widely useful, options.

Makefile.am

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+SUBDIRS = src include
+
+EXTRA_DIST = Condition_Variable_FAQ