Is there a non-blocking function which returns the current rx queue length of a serial port in Windows, using C?
All examples I've seen simply call ReadFile which blocks until the specified timeout, so I was wondering if it's possible to check if there is anything in the buffer before reading?
E.g. I can simply do this for each character:
void ReadCharacter(char *theCharacter)
{
DWORD numBytesRead = 0;
while (numBytesRead == 0)
{
ReadFile(comPorthandle,
theCharacter,
sizeof(char),
&numBytesRead,
NULL);
}
}
But is it possible to have something like
int numBytesRx = GetNumBytesRx(&portHandle);
if (numBytesRx > 0)
Read(&portHandle, targetBuffer, numBytesRead);
To perform async IO with COM port via ReadFile consider using last parameter of function, LPOVERLAPPED and OVERLAPPED structure. OVERLAPPED is common practice for async IO in Windows.
Here you can find examples
ReadFile will always return whatever is already in the rx buffer. So if you set no timeouts, you'll get the contents instantly.
Though please note that no professional application puts ReadFile in a busy-wait loop. Not only will this needlessly use up CPU, it will also block the thread where the loop exists.
So you should put ReadFile inside a thread of its own. This is common practice with all I/O functions. This will solve the blocking problem, but you'll still have the problem with high CPU use.
As an alternative you can use what Windows calls "asynchronous I/O" (*), by using the ReadFileEx function. It lets you specify a callback function, which will get triggered whenever you actually receive some data.
Now if you combine "asynchronous I/O" with threading, you get a non-blocking communication which consumes no CPU when there is no data to process. Your I/O thread could either wait for I/O with SleepEx, or it could WaitFor an event that you set manually from inside the callback.
(*) "Asynchronous I/O" is a nonsense Windows term, since technically, all serial port communication is always asynchronous. Would you send data synchronously with no pause in between, there would just be no way for a slow desktop PC to keep up.
Related
So I have just discovered that libuv is a fairly small library as far as C libraries go (compare to FFmpeg). I have spent the past 6 hours reading through the source code to get a feel for the event loop at a deeper level. But still not seeing where the "nonblockingness" is implemented. Where some event interrupt signal or whatnot is being invoked in the codebase.
I have been using Node.js for over 8 years so I am familar with how to use an async non-blocking event loop, but I never actually looked into the implementation.
My question is twofold:
Where exactly is the "looping" occuring within libuv?
What are the key steps in each iteration of the loop that make it non-blocking and async.
So we start with a hello world example. All that is required is this:
#include <stdio.h>
#include <stdlib.h>
#include <uv.h>
int main() {
uv_loop_t *loop = malloc(sizeof(uv_loop_t));
uv_loop_init(loop); // initialize datastructures.
uv_run(loop, UV_RUN_DEFAULT); // infinite loop as long as queue is full?
uv_loop_close(loop);
free(loop);
return 0;
}
The key function which I have been exploring is uv_run. The uv_loop_init function essentially initializes data structures, so not too much fancness there I don't think. But the real magic seems to happen with uv_run, somewhere. A high level set of code snippets from the libuv repo is in this gist, showing what the uv_run function calls.
Essentially it seems to boil down to this:
while (NOT_STOPPED) {
uv__update_time(loop)
uv__run_timers(loop)
uv__run_pending(loop)
uv__run_idle(loop)
uv__run_prepare(loop)
uv__io_poll(loop, timeout)
uv__run_check(loop)
uv__run_closing_handles(loop)
// ... cleanup
}
Those functions are in the gist.
uv__run_timers: runs timer callbacks? loops with for (;;) {.
uv__run_pending: runs regular callbacks? loops through queue with while (!QUEUE_EMPTY(&pq)) {.
uv__run_idle: no source code
uv__run_prepare: no source code
uv__io_poll: does io polling? (can't quite tell what this means tho). Has 2 loops: while (!QUEUE_EMPTY(&loop->watcher_queue)) {, and for (;;) {,
And then we're done. And the program exists, because there is no "work" to be done.
So I think I have answered the first part of my question after all this digging, and the looping is specifically in these 3 functions:
uv__run_timers
uv__run_pending
uv__io_poll
But not having implemented anything with kqueue or multithreading and having dealt relatively little with file descriptors, I am not quite following the code. This will probably help out others along the path to learning this too.
So the second part of the question is what are the key steps in these 3 functions that implement the nonblockingness? Assuming this is where all the looping exists.
Not being a C expert, does for (;;) { "block" the event loop? Or can that run indefinitely and somehow other parts of the code are jumped to from OS system events or something like that?
So uv__io_poll calls poll(...) in that endless loop. I don't think is non-blocking, is that correct? That seems to be all it mainly does.
Looking into kqueue.c there is also a uv__io_poll, so I assume the poll implementation is a fallback and kqueue on Mac is used, which is non-blocking?
So is that it? Is it just looping in uv__io_poll and each iteration you can add to the queue, and as long as there's stuff in the queue it will run? I still don't see how it's non-blocking and async.
Can one outline similar to this how it is async and non-blocking, and which parts of the code to take a look at? Basically, I would like to see where the "free processor idleness" exists in libuv. Where is the processor ever free in the call to our initial uv_run? If it is free, how does it get reinvoked, like an event handler? (Like a browser event handler from the mouse, an interrupt). I feel like I'm looking for an interrupt but not seeing one.
I ask this because I want to implement an MVP event loop in C, but just don't understand how nonblockingness actually is implemented. Where the rubber meets the road.
I think that trying to understand libuv is getting in your way of understanding how reactors (event loops) are implemented in C, and it is this that you need to understand, as opposed to the exact implementation details behind libuv.
(Note that when I say "in C", what I really means is "at or near the system call interface, where userland meets the kernel".)
All of the different backends (select, poll, epoll, etc) are, more-or-less, variations on the same theme. They block the current process or thread until there is work to be done, like servicing a timer, reading from a socket, writing to a socket, or handling a socket error.
When the current process is blocked, it literally is not getting any CPU cycles assigned to it by the OS scheduler.
Part of the issue behind understanding this stuff IMO is the poor terminology: async, sync in JS-land, which don't really describe what these things are. Really, in C, we're talking about non-blocking vs blocking I/O.
When we read from a blocking file descriptor, the process (or thread) is blocked -- prevented from running -- until the kernel has something for it to read; when we write to a blocking file descriptor, the process is blocked until the kernel accepts the entire buffer.
In non-blocking I/O, it's exactly the same, except the kernel won't stop the process from running when there is nothing to do: instead, when you read or write, it tells you how much you read or wrote (or if there was an error).
The select system call (and friends) prevent the C developer from having to try and read from a non-blocking file descriptor over and over again -- select() is, in effect, a blocking system call that unblocks when any of the descriptors or timers you are watching are ready. This lets the developer build a loop around select, servicing any events it reports, like an expired timeout or a file descriptor that can be read. This is the event loop.
So, at its very core, what happens at the C-end of a JS event loop is roughly this algorithm:
while(true) {
select(open fds, timeout);
did_the_timeout_expire(run_js_timers());
for (each error fd)
run_js_error_handler(fdJSObjects[fd]);
for (each read-ready fd)
emit_data_events(fdJSObjects[fd], read_as_much_as_I_can(fd));
for (each write-ready fd) {
if (!pendingData(fd))
break;
write_as_much_as_I_can(fd);
pendingData = whatever_was_leftover_that_couldnt_write;
}
}
FWIW - I have actually written an event loop for v8 based around select(): it really is this simple.
It's important also to remember that JS always runs to completion. So, when you call a JS function (via the v8 api) from C, your C program doesn't do anything until the JS code returns.
NodeJS uses some optimizations like handling pending writes in a separate pthreads, but these all happen in "C space" and you shouldn't think/worry about them when trying to understand this pattern, because they're not relevant.
You might also be fooled into the thinking that JS isn't run to completion when dealing with things like async functions -- but it absolutely is, 100% of the time -- if you're not up to speed on this, do some reading with respect to the event loop and the micro task queue. Async functions are basically a syntax trick, and their "completion" involves returning a Promise.
I just took a dive into libuv's source code, and found at first that it seems like it does a lot of setup, and not much actual event handling.
Nonetheless, a look into src/unix/kqueue.c reveals some of the inner mechanics of event handling:
int uv__io_check_fd(uv_loop_t* loop, int fd) {
struct kevent ev;
int rc;
rc = 0;
EV_SET(&ev, fd, EVFILT_READ, EV_ADD, 0, 0, 0);
if (kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL))
rc = UV__ERR(errno);
EV_SET(&ev, fd, EVFILT_READ, EV_DELETE, 0, 0, 0);
if (rc == 0)
if (kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL))
abort();
return rc;
}
The file descriptor polling is done here, "setting" the event with EV_SET (similar to how you use FD_SET before checking with select()), and the handling is done via the kevent handler.
This is specific to the kqueue style events (mainly used on BSD-likes a la MacOS), and there are many other implementations for different Unices, but they all use the same function name to do nonblocking IO checks. See here for another implementation using epoll.
To answer your questions:
1) Where exactly is the "looping" occuring within libuv?
The QUEUE data structure is used for storing and processing events. This queue is filled by the platform- and IO- specific event types you register to listen for. Internally, it uses a clever linked-list using only an array of two void * pointers (see here):
typedef void *QUEUE[2];
I'm not going to get into the details of this list, all you need to know is it implements a queue-like structure for adding and popping elements.
Once you have file descriptors in the queue that are generating data, the asynchronous I/O code mentioned earlier will pick it up. The backend_fd within the uv_loop_t structure is the generator of data for each type of I/O.
2) What are the key steps in each iteration of the loop that make it non-blocking and async?
libuv is essentially a wrapper (with a nice API) around the real workhorses here, namely kqueue, epoll, select, etc. To answer this question completely, you'd need a fair bit of background in kernel-level file descriptor implementation, and I'm not sure if that's what you want based on the question.
The short answer is that the underlying operating systems all have built-in facilities for non-blocking (and therefore async) I/O. How each system works is a little outside the scope of this answer, I think, but I'll leave some reading for the curious:
https://www.quora.com/Network-Programming-How-is-select-implemented?share=1
The first thing to keep in mind is that work must be added to libuv's queues using its API; one cannot just load up libuv, start its main loop, and then code up some I/O and get async I/O.
The queues maintained by libuv are managed by looping. The infinite loop in uv__run_timers isn't actually infinite; notice that the first check verifies that a soonest-expiring timer exists (presumably, if the list is empty, this is NULL), and if not, breaks the loop and the function returns. The next check breaks the loop if the current (soonest-expiring) timer hasn't expired. If neither of those conditions breaks the loop, the code continues: it restarts the timer, calls its timeout handler, and then loops again to check more timers. Most times when this code runs, it's going to break the loop and exit, allowing the other loops to run.
What makes all this non-blocking is the caller/user following the guidelines and API of libuv: adding your work to queues, and allowing libuv to perform its work on those queues. Processing-intensive work may block these loops and other work from running, so it's important to break your work into chunks.
btw, uv__run_idle, uv__run_check, uv__run_prepare 's source code is defined on src/unix/loop-watcher.c
I have to send a command over serial and receive back an answer based on the command and do something based on the message received. I was told that I have to use callbacks as this is an asynchronous operation.
I have a 2 threads, one that can send messages and one that receives the messages.
Example:
//Thread 1
sendMessage("Initialize");
//Thread 2
while(1)
{
checkForMessages();
}
How can I write a function that is initialized for a specific message and handles the message recieved.
Example:
CommHandle(Command,MsgReceived)
{
if(command)
{
if(MsgReceived == ok)
...
if(MsgReceived == error)
...
}
}
I was told that I have to use callbacks as this is an asynchronous operation.
Not necessarily. There is something in Windows called "asynchronous I/O", this is to be regarded as an internal Windows term, which is synonymous with "overlapped I/O" (explanation here). When you are using overlapped I/O, you will get a callback when the transmission is finished. This is nice, since it reduces CPU load, but it is not really necessary if your program has nothing better to do while waiting. So it depends on the nature of your application.
But no matter the nature of your application, you should indeed handle all serial communication through threads, so that you won't cause the main GUI thread to freeze up in embarrassing ways.
Having one rx and one tx thread gives you a dilemma though: they are using the same port handle and they cannot freely access it, because that wouldn't be thread-safe. The solution is to either make one single super-thread handling all transmissions, or to protect the port handle through a mutex.
I'm not sure which method that is best, I have no recommendation. I have only used the "super-thread" one myself: one obvious advantage was that I could centralize WaitFor instructions like "kill thread", "port is open", "port is closed" at one place. But at the same time the code turned out rather complex.
How can I write a function that is initialized for a specific message and handles the message recieved.
Let your thread(s) shovel their received data into some buffers. A tx buffer and a rx buffer. Depending on your serial protocol and performance, you might have to use double buffers: one that is used for the current transmission and one that contains the most recently received data.
Then from main, pick up the data from the buffers. They need to be thread-safe. Once you have gotten that far, simply write a parser like you would with any form of data and take actions from there
I need to write a Terminal to communicate with a COM - port and I need to be able to send commands from the COM-Port, as well as from the Console at the same time.
(I want to get access to a computer via two sensor nodes, that are communicating with each other wirelessly, so I still need a way to send something from the node to the Computer)
Now, I have implemented a Non Overlapped Serial Communication, but I am not sure, how to implement the "Send and Receive at the same time"-Part and I only have around 4 days to solve the problem. There is not really that much information out there, so I would welcome any pointers on how to implement that fastest or easiest way.
Overlapped I/O-Communication is not exactly very time friendly as far as I can see.
Is it possible to do this with multi-threading (only overlapped)?
I am guessing in that case I would have to read the buffer every few ms and make an own thread for the input?
Whether to use overlapped I/O or not isn't really the issue: overlapping just frees up some time for your program. I have written many programs like this and the conclusion is always to use a thread to handle all COM routines. Whether this thread calls overlapped or synchronous methods is less relevant, as long as the thread lies idle doing WaitForMultipleObjects().
The way I have written my most recent COM terminal is this (pseudo):
thread()
{
while not kill the thread event
{
WaitForMultipleObjects (open port, close port, kill the thread event)
if (open port)
{
send();
receive();
wait_for_send_and_receive();
}
}
}
send()
{
take COM_port mutex
if(there is something to send)
{
copy send_data to local_data, protect this with mutex
WriteFileEx(COM_port,
local_data,
size,
some_overlapped_struct_stuff);
handle errors
}
release COM_port mutex
}
receive()
{
take COM_port mutex
ReadFileEx(COM_port, ...);
handle errors
release COM_port mutex
}
wait_for_send_and_receive()
{
WaitForMultipleObjects (open port,
close port,
kill the thread event,
send done event from send callback routine (overlapped I/O),
receive done event from receive callback routine (overlapped I/O)
);
}
Naturally this is quite an over-simplification since you need various functions for open/close COM port, data shuffling etc. Several mutices are likely needed.
I'd share the real, working production code if it wasn't corporate property :( 4 days seems a bit optimistic, judging from my project log, it took me several months to develop a working COM port terminal to production quality level. The COM port driver alone is around 1k loc, with a lot of Win API calls all over.
i have the following code:
{
send(dstSocket, rcvBuffer, recvMsgSize, 0);
sndMsgSize = recv(dstSocket, sndBuffer, RCVBUFSIZE, 0);
send(rcvSocket, sndBuffer, sndMsgSize, 0);
recvMsgSize = recv(rcvSocket, rcvBuffer, RCVBUFSIZE, 0);
}
which eventually should become part of a generic TCP-Proxy. Now as it stands, it doesn't work quite correctly, since the recv() waits for input so the data only gets transmitted in chunks, depending where it currently is.
What i read up on it is that i need something like "non-blocking sockets" and a mechanism to monitor them. This mechanism as i found out is either select, poll or epoll in Linux. Could anyone give me a confirmation that i am on the right track here? Or could this excercise also be done with blocking sockets?
Regards
You are on the right track.
"select" and "poll" are system calls where you can pass in one or more sockets and block (for a specific amount of time) until data has been received (or ready for sending) on one of those sockets.
"non-blocking sockets" is a setting you can apply to a socket (or a recv call flag) such that if you try to call recv, but no data is available, the call will return immediately. Similar semantics exist for "send". You can use non-blocking sockets with or without the select/poll method described above. It's usually not a bad idea to use non-blocking operations just in case you get signaled for data that isn't there.
"epoll" is a highly scalable version of select and poll. A "select" set is actually limited to something like 64-256 sockets for monitoring at a time and it takes a perf hit as the number of monitored sockets goes up. "epoll" can scale up to thousands of simultaneous network connections.
Yes you are in the right track. Use non-blocking socket passing their relative file descriptors to select (see FD_SET()).
This way select will monitor for events (read/write) on them.
When select returns you can check on which fd has occurred an event (look at FD_ISSET()) and handle it.
You can also set a timeout on select, and it will return after that period even if not events have been occurred.
Yes you'll have to use one of those mechanisms. poll is portable and IMO the most easy one to use. You don't have to turn off blocking in this case, provided you use a small enough value for RCVBUFSIZE (around 2k-10k should be appropriate). Non-blocking sockets are a bit more complicated to handle, since if you get EAGAIN on send, you can't just loop to try again (well you can, but you shouldn't since it uses CPU unnecessarily).
But I would recommend to use a wrapper such as libevent. In this case a struct bufferevent would work particularly well. It will make a callback when new data is available, and you just queue it up for sending on the other socket.
Tried to find an bufferevent example but seems to be a bit short on them. The documentation is here anyway: http://monkey.org/~provos/libevent/doxygen-2.0.1/index.html
I'm writing a program in linux to interface, through serial, with a piece of hardware. The device sends packets of approximately 30-40 bytes at about 10Hz. This software module will interface with others and communicate via IPC so it must perform a specific IPC sleep to allow it to receive messages that it's subscribed to when it isn't doing anything useful.
Currently my code looks something like:
while(1){
IPC_sleep(some_time);
read_serial();
process_serial_data();
}
The problem with this is that sometimes the read will be performed while only a fraction of the next packet is available at the serial port, which means that it isn't all read until next time around the loop. For the specific application it is preferable that the data is read as soon as it's available, and that the program doesn't block while reading.
What's the best solution to this problem?
The best solution is not to sleep ! What I mean is a good solution is probably to mix
the IPC event and the serial event. select is a good tool to do this. Then you have to find and IPC mechanism that is select compatible.
socket based IPC is select() able
pipe based IPC is select() able
posix message queue are also selectable
And then your loop looks like this
while(1) {
select(serial_fd | ipc_fd); //of course this is pseudo code
if(FD_ISSET(fd_set, serial_fd)) {
parse_serial(serial_fd, serial_context);
if(complete_serial_message)
process_serial_data(serial_context)
}
if(FD_ISSET(ipc_fd)) {
do_ipc();
}
}
read_serial is replaced with parse_serial, because if you spend all your time waiting for complete serial packet, then all the benefit of the select is lost. But from your question, it seems you are already doing that, since you mention getting serial data in two different loop.
With the proposed architecture you have good reactivity on both the IPC and the serial side. You read serial data as soon as they are available, but without stopping to process IPC.
Of course it assumes you can change the IPC mechanism. If you can't, perhaps you can make a "bridge process" that interface on one side with whatever IPC you are stuck with, and on the other side uses a select()able IPC to communicate with your serial code.
Store away what you got so far of the message in a buffer of some sort.
If you don't want to block while waiting for new data, use something like select() on the serial port to check that more data is available. If not, you can continue doing some processing or whatever needs to be done instead of blocking until there is data to fetch.
When the rest of the data arrives, add to the buffer and check if there is enough to comprise a complete message. If there is, process it and remove it from the buffer.
You must cache enough of a message to know whether or not it is a complete message or if you will have a complete valid message.
If it is not valid or won't be in an acceptable timeframe, then you toss it. Otherwise, you keep it and process it.
This is typically called implementing a parser for the device's protocol.
This is the algorithm (blocking) that is needed:
while(! complete_packet(p) && time_taken < timeout)
{
p += reading_device.read(); //only blocks for t << 1sec.
time_taken.update();
}
//now you have a complete packet or a timeout.
You can intersperse a callback if you like, or inject relevant portions in your processing loops.