Ok ... after several days of working in the garden a new day with updates on
PLC4C :-)
So today I managed to finish and commit some changes that perform a full
roundtrip of connection, read, disconnect using the "simulated" driver.
Right now that only parses the address strings and returns a random int for
every item .. this is not yet on-par with the java version of the simulated
driver, but more a preview of the API we are planning to use for PLC4C.
All asynchronous operations:
- Connect
- Read
- Disconnect
Are implemented using something we call "system-tasks" which I described in my
last summary.
All seems to be working nicely and I really like the structure of the code ...
I know it will need quite a bit of cleaning up and I would be super grateful,
if some C professionals could review what I have created. Constructive feedback
is essential here for the continued work on this.
Chris
Am 29.04.20, 19:57 schrieb "Christofer Dutz" <christofer.d...@c-ware.de>:
Aaaand another update :-)
So today I continued adding code-flesh to the empty API body.
I implemented two basic data-structures: List and Queue (Noticing at the
end that my queue isn't even needed).
Starting from today the functionality for: connecting, reading, writing,
etc. is implemented by callback functions generating so-called system_tasks.
These are data-structures consisting of 4 properties:
- a state-machine-function callback
- an int representing the state-machine state the task is currently in
- a pointer to a context data-structure (The state-machine controls what's
in there)
- a completed-flag that tells the system if a task is finished
So now if a driver for example is asked to connect, it generates a
system-task and that's added to the task-list.
Then as soon as the system_loop function is called, this function goes
through the list of queued system-tasks and for each calls the state-machine
function if comes with and passes in the task as an argument.
In the end the system loop checks If after executing the
state-machine-function the task is marked as "completed" ... if it is it
removes this task form the queue and continues with the next task in the list.
The cool thing is that this way we can even ensure the system_loop function
doesn't hog too much processing time ... so we could give the loop a maximum
execution time and as soon as that's exceeded the function returns and the
following tasks will be executed the next time the system_loop function is
called. In this case I would probably change the task-list into a ring
data-structure.
Right now my example hello world program if correctly loading the
"simulated" driver and a "dummy" transport and correctly connecting using the
above mechanisms.
Guess tomorrow I'll be writing string-parsers again to continue working on
implementing the read/write operations on the simulated-driver.
So far the update from today,
Chris
Am 28.04.20, 19:38 schrieb "Christofer Dutz" <christofer.d...@c-ware.de>:
Hi folks,
even if this wasn't discussed as much as the other things we discussed,
I still think it's important.
So I was a strong advocate of the "promised land" ... I wanted to use
promises and register callbacks for async execution.
Theoretically this I cool ... however I had to notice it's cool as long
as you have someone cleaning up for you.
In most of the languages I encountered heavy usage of this pattern
there are garbage collection mechanisms in place and then this is a great
feature.
In C however this is not the case. Here you have to manually free
previously allocated memory and if you don't do that, you'll run out of it
pretty soon.
My main issue was that with promises it is difficulty to explicitly
clean them up as you have no means to see which part of the program is still
keeping a reference to it. If you clean that up and the other code access it,
the failure depends on your OS but in all cases it's pretty bad.
So I decided to undo the promises, I just introduced a few days
earlier.
To keep the code half-clean I decided to instead use some manual
state-machine like code.
Not much more to report besides the fact that I started implementing
the actual logic. Up to now all methods sort of just returned empty or default
objects. Now at least you can register drivers at the plc4c system and create
connection structures, that correctly parse plc4x connection strings and lookup
matching drivers in the registry.
Guess in the following days I'll have to work on how I can actually
define protocols in a way that I can have them processed in the central "loop"
method without blocking the OS.
Feel free to discuss here or on slack.
I'll continue to post summaries to what's been going on.
Chris
Am 23.04.20, 18:34 schrieb "Christofer Dutz"
<christofer.d...@c-ware.de>:
Hi,
today we worked quite a bit on making the directory structure more
C-folks-friendly.
So what we've now done, it we left the general directory structure
unchanged, but eliminated the maven-like structure.
So now every module has a "CMakeList.txt" file and a "src",
"include" and "test" directory.
"src" is pretty much what src/main/c and src/main/resources would
have been, just mixed.
Same applies for "test".
The directory structure now matches that of a lot of other projects
and it would even allow adding a second set of build configurations to allow
building PLC4C with MyNewts "newt" build tool (But that's for the future ... no
worries)
Today I also added a "simulated" driver which should act similar
like the one in PLC4J ... in the end it should respect the PLC4C API and be
used as some dummy driver which mainly helps with writing hello_world
applications as well as testing new integration modules.
Also did I extend the "system" with a function to manually create
and register drivers.
Especially in limited environments like embedded systems, I think
the typical: "I just add all drivers and load what I need" doesn't work. Same
as you can't just copy a driver to the device, a dynamic search and discovery
mechanism would be overkill. We will definitely add similar loading mechanisms
like in Java, but for now I'll work with manually registering drivers to keep
it simple, small and efficient.
So far the update for today.
Chris
Am 20.04.20, 16:41 schrieb "Christofer Dutz"
<christofer.d...@c-ware.de>:
Hi all,
currently we’re in a mode of synchronous exchange on creating
the PLC4C API on Apache slack “the-asf.slack.com” (Please send an email if you
need an invite).
We’re doing that as specially in the beginning this simplifies
and speeds up things drastically.
But as we’re at Apache and here “If it didn’t happen on the
list, it didn’t happen” I’m trying to keep the list in sync with the outcome
and give you folks the chance to participate by writing up summaries.
So we have the great situation that Otto and Björn have quite
some experience in C and C++ and are being a huge help with getting this
started.
So a few weeks ago I have already setup a build that’s
integrated into our normal Maven build. However you need to enable to profiles
in order to do that:
* with-sandbox
* with-c
You need to run the maven build at least once as our
plc4x-maven-plugin is a maven plugin and we need to have it generate some code
first (or the build will fail)
The actual build however is done with CMake. This allows us to
use any CMake-capable IDE to develop PLC4C (I’m using CLion, but VisualStudio
seems to also work fine).
Today I created a branch feature/c-api as I think I’ll
sometimes be committing unbildable stuff in order to allow collaboration (I
commit something and others fix it ;-) ).
So please follow this branch.
Regarding the directory structure, we decided to stick to a
structure that is sort of aligned with the one we have in PLC4X as this makes
it easier to understand the structure (at least from a PLC4J point of view).
The structure should also not be too strange for a c developer. What C
developers might consider “an abomination” is that I started off using a
directory structure for modules which is maven-inspired.
The reason is that I think this allows to cleanly separate prod
from test code and code from resources (In this case code from header files) …
so a C module currently has the general structure:
CMakeLists.txt
src
- main
- c
- include
- test
- c
- include
The CMakeLists.txt file is sort of the equivalent to our
pom.xml … it tells CMake how to build the module. This is also where you define
“dependencies” … however in C you rather define the locations of the
header-files you intend on using … the actual using of code will happen by the
linker when compiling.
So now come the parts where I was super happy to have some help
with:
* In the API module we have a set of header files which
define the core functions and types
* However these types don’t really expose any information
on the internals of PLC4X. So even if you have a connection structure, you
can’t access any properties of this directly. If you want to access any
properties, you need to use the corresponding function. So if in the
“connection” domain you have a plc4c_connection structure, in order to access
the connection_string property of that, you need to call the
plc4c_connection_get_connection_string function and pass in your connection
object.
* As a currently implicit naming convention we started
using a prefix “plc4c_” for everything.
* We also split up the API functions into domains like
“system”, “connection”, … all the functions which then results in names like
“plc4c_system_create”. As we’re expecting the domains to grow, this way we hope
to keep the sizes of the code blocks manageable.
* While the interface a user would use is defined in the
API module, the implementation is then done in the SPI module (pretty much like
in PLC4J)
* Initially we started playing around with callbacks, much
like we’re doing in Java and as C does allow function pointers, that seemed
like a good idea. But people actually coding in C mentioned that this form of
coding feels very strange for full-blooded C developers. As we want this API to
feel native for C developers, we’ll go down this path.
* As C doesn’t support a try-catch-finally style error
handling, almost every function returns a return_code enum which can be
translated into error messages by using some helper functions. So it is
important to check if an operation returned OK and to do some error handling,
if this is not OK.
* The API will completely not use synchronous operations.
So we won’t provide a blocking “connect” function that returns as soon as the
connection is established, as some of the systems we’re going to support are
single threaded and don’t have any mutlitasking or scheduling. Therefore we’ll
have a “plc4c_system_loop()” function which is cyclically called in order to do
something.
So far the update … I hope I didn’t skip anything …
More will follow :-)
Chris
