程序 1 在 Linux 管道上写入的数据有时会被程序 2 读取两次

Question

I'm trying to establish an inter process communication on Linux using named pipes. At first glance my code seems to work, but I noticed that sometimes data written to the pipe by the "main" process is read twice by the "child" process (in fact it's two separate programs).

Program 1 (write only, starts first) is a legacy program. The below code is implemented in a onclick-function for a button on a GUI. When I press the button the data string shall be sent via pipe to another program. Code:

main.h

const char * FSCpipe = "/tmp/fscpipe";
int counter = 1;

main()

::mkfifo(FSCpipe, 0666);

OnClick-Function

QString data = "179.45";
int fd;
char msg[10];

fd = ::open(FSCpipe, O_WRONLY | O_NONBLOCK);
for(int i = 0; i < data.length(); i++) {
    msg[i] = data.at(i).toLatin1();
}
msg[data.length()] = '\0';
::write(fd, msg, strlen(msg)+1);
::close(fd);
qDebug() << QString("Counter: %1").arg(counter);
counter++;

Program 2 (only read, starts second) is started in a new terminal. Code:

int main(int argc, char **argv)
{    
   int fd;
   const char * FSCpipe = "/tmp/fscpipe";
   char data[10];
   size_t nbytes;
   nbytes = sizeof(data);
   while(1) {
      fd = open(FSCpipe, O_RDONLY);
      ssize_t bytesread = read(fd, data, nbytes);
      printf("received: %s\n", data);
      close(fd);
      printf("Counter: %d ", counter);
      printf("chars: %d\n", bytesread);
      counter++;
   }
   return(0);
}

Sometimes I noticed a strange behaviour, feeling like there is more data read then written. So I implemented independent counter in both programs. When I press the button in program 1, the data is send via the pipe and the counter increments. When program 2 receives and outputs the data in the terminal, its counter is also incremented. After some clicks on the button in program 1, I can see a higher counter value in program 2, indicating that it reads more often then program 1 writes. Why is that ?

Apart from that, the data string should contain special characters encoded in UTF-8, like this:

QString data = "197,45 \xc2\xb5m";

How should I convert the string to send it correctly via pipe?

Thank you for helping me!

Answer 1

I found some helpful information here , stating:

"If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0)"

I took out fd = ::open(...); and ::close(fd); from my onclick-function in program 1 and put them into the mainwindow-constructor and the mainwindow-close-function respectively. So the write end is always opened and program 2 works as intended.

Thank you all for your help.