SocketCAN continuous reading and writing

Question

I'm writing a program that continuously sends "Hello" to a CAN-bus and reads data from the bus via SocketCAN. I want those two steps to be independent. That means even if there is no data on the bus, the program will still send "Hello". But with usual CAN reading it is impossible, because this function stops program run and waits for data.

Is there a way how to make it non-blocking or waiting for data only for a few milliseconds?

Answer 1

You can use following constellation (this is not the complete solution, but just algorithm):

while(1) {
    FD_ZERO(&rdfs);
    FD_SET(s, &rdfs);

    tv.tv_sec = 0;
    tv.tv_usec = 10000; // microseconds

    rc = select(s + 1, &rdfs, NULL, NULL, &tv);

    // rc == 0 - timeout
    if (!rc) {
         // write your CAN frame
    }

    if (FD_ISSET(s, &rdfs)) {
         // read CAN frames
    }
}

See man select for more information and how to handle return value.

Answer 2

Another way I found - threads. Just make CAN reading work in thread and it won't stop main cycle. For Linux systems it looks like this:

 #include <pthread.h>

    void *thread(int cansock) {
       struct can_frame rxmsg;      
       while (1) {   
            read(cansock, &rxmsg, sizeof(rxmsg));
            printf("message received\n");                     
        }
    }

    int main(){
// initialize CAN socket and message to send
    pthread_t pth;
    pthread_create(&pth, NULL, thread, cansock);
    while(1){
      write(cansock, &txmsg, sizeof(txmsg));
      printf("message sent\n");
      }
    return 0;
    }

Answer 3

Be careful with this solution:

void *thread(int cansock) {
       struct can_frame rxmsg;      
       while (1) {   
            read(cansock, &rxmsg, sizeof(rxmsg));
            printf("message received\n");                     
        }
    }

because if read() starts returning an error without block, and this is out in the field and there's nobody to look at the results of printf(), you're going into a busy loop.

Answer 4

I solved it by add the following code on initialization

timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 10000;
setsockopt(skt_, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));

and add the loop read in another thread.