On Mon, 24 Aug 2015 17:40:33 -0700, you wrote: >Hey Harvey, and Walter > >Just kind of an update. Last night after our discussion I found a really >good resource / discussion of what fork() is and the different ways it can >be used. So with this information in mind along with our discussion >yesterday it seems that what I want to do can indeed be done without using >POSIX shared memory( I had little doubt ) - *and* seemingly more simple.
That sounds good > >I'd still have to use a Semaphore - I think to keep the web server callback >from stalling my canbus routines. But I think that seems fairly reasonable. > That also sounds quite reasonable to do. As your programs get more complicated, you'll have to figure out how to interlock/protect/manage resources. I have a project that manages a graphics engine (software), I2C slave (ditto), heartbeat/errortask, I2C error reporting task, and the like; and uses a FIFO, semaphores, queues and the like to protect resources and manage memory. Probably a bit too complex, but it kinda grew that way. >Still I may just implement semaphores into my current code to check it out, >but not sure when. Been a semi rough day, and I'm whooped . . . I know the feeling. Good luck and don't hesitate to ask for more help, or at least advice, for whatever I can do. Linux I can't really talk about, the fundamentals I think I can. Ask either on the list or private email if you want. Harvey > >On Sun, Aug 23, 2015 at 9:44 PM, William Hermans <[email protected]> wrote: > >> OK have a good one, thanks for the discussion. >> >> On Sun, Aug 23, 2015 at 9:11 PM, Harvey White <[email protected]> >> wrote: >> >>> On Sun, 23 Aug 2015 20:18:26 -0700 (PDT), you wrote: >>> >>> > >>> >> >>> >> *Well, you're certainly right that the callback is messing* >>> >> * things up. If I assume the same callback, then the callback is* >>> >> * certainly changing data. If you can set the right breakpoint, you >>> can* >>> >> * tag the situation *if* the breakpoint also knows that the process is* >>> >> * reading from the CAN bus.* >>> >> >>> >> * Had you considered disabling that callback function until the read* >>> >> * from the CANbus is finished? Would it be practical? That's where >>> the* >>> >> * semaphore might help a lot.* >>> >> >>> >> * what variables could be common between the two routines?* >>> >> >>> >> * Harvey* >>> >> >>> > >>> >Well this is where previous experience fails me. I've pretty much avoided >>> >code related to threading in software. In the past. I do know of fork() >>> and >>> >roughly what it is capable of, and I know about threads, but not to >>> >implement them in C on Linux. Or what can be done with them. Lets talk >>> code >>> >a minute. >>> >>> OK, as well as I can follow it. >>> >>> > >>> >*IPC - Server - Reads from canbus* >>> >int main(){ >>> > struct can_frame frame; >>> > int sock = InitializeCAN("vcan0"); >>> > >>> > statistics_t *stats = NULL; >>> > >>> > const long shm_size = sysconf(_SC_PAGESIZE); >>> > >>> > int shm_fd = shm_open("acme", O_CREAT | O_RDWR, FILE_PERMS); >>> >>> **NOTE: the problem may be "acme", since we know that acme products >>> are not effective against roadrunners..... >>> >>> > if(shm_fd == -1) >>> > HandleError(strerror(errno)); >>> > >>> > const int retval = ftruncate(shm_fd, shm_size); >>> > if(retval == -1) >>> > HandleError(strerror(errno)); >>> > >>> > shared_memory = InitializeShm(shm_size * sizeof(char), shm_fd); >>> > close(shm_fd); >>> > >>> > while(1){ >>> > frame = ReadFrame(sock); >>> > if(frame.can_dlc == FRAME_DLC) >>> > stats = ProcessFastpacket(frame); >>> >>> right at this point, you have no protection against access and no >>> interlocking. >>> >>> I'll have to give you pseudocode, because I don't know how to do this >>> in Linux. >>> >>> In the init routine, before you set up either main as a >>> process (I assume you do this). Declare a semaphore: >>> >>> semaphore_handle shared_access; // create semaphore >>> handle accessible to both processes. >>> semaphore_create (shared_access); // create >>> semaphore >>> >>> >>> then modify this next section to: >>> >>> if(stats != NULL){ >>> if (semaphore_take(shared_access), <wait forever>) >>> { >>> WriteToShm(shared_memory, stats); >>> semaphore_give (shared_access); >>> } >>> stats = NULL; >>> printf("%s", ReadFromShm(shared_memory)); >>> } >>> task_delay(n); >>> >>> NOTE: Process A hangs until it can "get" the semaphore; if Process B >>> has it, B can keep it only long enough to send the packet >>> > >>> > if(stats != NULL){ >>> > WriteToShm(shared_memory, stats); >>> > stats = NULL; >>> > printf("%s", ReadFromShm(shared_memory)); >>> > } >>> > } >>> >}/* main() */ >>> > >>> > >>> > >>> >*IPC - Client / webserver* >>> > >>> >int main(void) { >>> > struct mg_server *server = mg_create_server(NULL, ev_handler); >>> > >>> > mg_set_option(server, "listening_port", "8000"); >>> > mg_set_option(server, "document_root", "./web"); >>> > >>> > printf("Started on port %s\n", mg_get_option(server, >>> >"listening_port")); >>> > >>> > // POSIX IPC - shared memory >>> > const long shm_size = sysconf(_SC_PAGESIZE); >>> > int shm_fd = shm_open("file", O_CREAT | O_RDWR, FILE_PERMS); >>> > if(shm_fd == -1) >>> > HandleError(strerror(errno)); >>> > >>> > const int retval = ftruncate(shm_fd, shm_size); >>> > if(retval == -1) >>> > HandleError(strerror(errno)); >>> > >>> > shared_memory = InitializeShm(shm_size * sizeof(char), shm_fd); >>> > >>> > close(shm_fd); >>> > >>> > char id = 0x00; >>> > for (;;) { >>> > mg_poll_server(server, 10); >>> > >>> then do the same here >>> >>> if (semaphore_take(shared_access), <wait forever>) >>> { >>> if(shared_memory->sdata.data[19] != id){ >>> push_message(server,shared_memory->sdata.data); >>> id = >>> shared_memory->sdata.data[19]; >>> } >>> semaphore_give (shared_access); >>> } >>> task_delay (n clock ticks); >>> >>> semaphore_take gets the semaphore if and only if it's available. It >>> does so in a thread safe manner. the <wait_forever> is whatever value >>> the system uses to tell the process to hang. You don't want the >>> process to wait and then just go. >>> >>> Because each example here releases the semaphore (semaphore_give) if >>> and only if it could get it, and since giving and taking the semaphore >>> is thread safe, the two threads should be fine. >>> >>> So your "consumer" thread can't check for valid data until there's >>> something there. When it first starts up, it has to get bad (null) >>> data and throw that away, since you can't guarantee that one thread >>> starts before the other (unless you block the thread using a suspend, >>> but that's not really the best thing to do), so you have to consider >>> that you have two parallel and independent threads. >>> >>> The consumer thread can access shared memory only when it's not been >>> actively written to. It has to figure out if data is good and what to >>> do with it. However, once written, that data will remain uncorrupted >>> until the consumer has read and processed it (because the consumer has >>> the semaphore and doesn't give it up until then). >>> >>> The producer thread checks to see if the data is there to send, >>> accesses shared memory by getting the semaphore (when the consumer is >>> not reading it), and then writes that shared memory. It then releases >>> the semaphore, goes idle (because the task switcher has to have a time >>> to start up the other task unless you have multiple cores), and then >>> checks for data, and waits to see when it can write that data. >>> >>> The typical task clock is either 1 ms or 10 ms, and the clock tick is >>> that (1 ms or 10 ms per tick). You play with the values for best >>> throughput on the n delays. >>> >>> >>> > if(shared_memory->sdata.data[19] != id){ >>> > push_message(server, shared_memory->sdata.data); >>> > id = shared_memory->sdata.data[19]; >>> > } >>> > } >>> > >>> > mg_destroy_server(&server); >>> > return 0; >>> >} >>> > >>> >In the context of whats interesting where threading is concerned. The >>> loops >>> >in each executable here might be useful. If somehow each, or even just >>> the >>> >for loop in the IPC client could somehow use objects in memory from the >>> IPC >>> >server. >>> >>> That was the shared memory, right? >>> >>> >That is let us suppose for a minute IPC was removed entirely, then >>> >somehow I could turn off the callback in the IPC client. This is what I'm >>> >having a problem imagining. How could this be done ? >>> >>> You may possibly be able to schedule *when* the callback happens. >>> >>> What causes the callback, sending a CAN message? >>> >>> > In the context of >>> >libmongoose I'm not sure. In the context of threading or using fork() I'm >>> >also not sure. >>> >>> Fork creates a separate process which can be controlled or killed as >>> needed, running as a sub-process (IIRC). >>> >>> you're dealing with creating two processes (really two programs) and >>> interprocess communication. >>> >>> >But if I could somehow through using threading context to >>> >disable the callback I think that would be ideal. That way I could simply >>> >disable that whole thread for a fraction of a second, and then resume it >>> >once a fastpacket is constructed. >>> >>> >>> >>> Well, synchronizing the two tasks with semaphores says that if the >>> callback happens and you can turn off that callback, then the data is >>> ok as long as you can schedule the callback. No idea when that >>> happens. >>> >>> So you maybe able to >>> 1) produce data >>> 2) keep from overwriting it >>> 3) enable the consumer to read data >>> 4) have it send data (and I assume the callback happens here) >>> 5) data is clobbered in the shared area, but we don't care since it's >>> sent already >>> 6) give the semaphore back allowing new data to be written >>> 7) that data can't be clobbered by the callback (assuming) until after >>> it's read and in the send process >>> >>> May solve the problem... >>> >>> >>> > >>> >Anyway, a little information that might be needed. socketCAN reads data >>> in >>> >8 byte lengths for each frame..fastpackets are several frames in length, >>> >and with the only current one I'm tracking being 11 frames long. Or 88 >>> >total bytes, not discounting the initial char from each frame which is a >>> >sequence number. If there is a way, and I'm sure there is, I am all for >>> >changing from an IPC model to a threaded model. But I still have some >>> >doubts. Such as will it be fast enough to track multiple fastpackets a >>> >second ? Past that how complex will it be ? >>> >>> Won't be all that complex, I think >>> the processes are written as two parts >>> one is a system call to set up a process >>> the other is the process itself which looks like >>> >>> void processA(void* arguments if any) >>> { >>> // declarations and inits the first time through >>> while (1) >>> { >>> basic process loop; >>> } >>> } >>> >>> not complicated at all, how to create the process ought to be well >>> documented >>> >>> you just need to make sure that the two processes have access to >>> shared memory >>> >>> assuming 1000 us available per process, a context switching time of 50 >>> us (may be shorter, but it's a number) >>> >>> You have 950 us to send a complete message without it having a delay >>> you have that same 950 us to detect and build a message. >>> >>> that gives you 500 message cycles/second >>> >>> taking twice as long gives you 250 message cycles/second and about >>> 1950 us to compose and send a message, that's with a 2 ms clock tick. >>> All that clock tick does is control task switching. The processor >>> clock controls the speed of operations otherwise. >>> >>> > >>> >I have given multiple approaches consideration, just having a hard time >>> >imaging how to work this out using a threading model. >>> >>> perhaps this might help >>> >>> Harvey >>> >>> (off to bed, have to be in training for 8 am classes in a week). >>> >>> -- >>> For more options, visit http://beagleboard.org/discuss >>> --- >>> You received this message because you are subscribed to the Google Groups >>> "BeagleBoard" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. 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