How to setup select function on channels?

Question

linux

Hello.
Help, please, with the call to select - I have two files in the directory created with mkfifo - in1 and in2. And I write to them using echo
echo "hi"> in1
, I want to be able to simultaneously write something to these files and read from them.
I do
echo "hey"> in1
and I see

Descriptor f1 = 3
Descriptor f2 = 4
temp = 56 
temp = 53 
temp = 73 
send f1 = 3
Descriptor f1 = 5
Descriptor f2 = 6
temp = 56 
temp = 53 
temp = 73 
send f1 = 5

And I do not understand - why the information is duplicated ?!
Why it is written twice send f1 = 3 and send f1 = 5 - I can’t understand - I wrote everything down once.
Here is the code

void read_and_calc(int fd) {
  char buf[100];

  int bytes = read(fd, buf, 100);
  int temp = 0;
  int i;
  for(i = 0; i < strlen(buf) - 1; i++) {
    int temp = buf[i] - '0';
    sum += temp;
    printf("temp = %d \n", temp);
  }
  buf[bytes] = 0;
}

while(1) {
  int f1 = open("./in1", O_NONBLOCK | O_RDONLY);
  int f2 = open("./in2", O_NONBLOCK | O_RDONLY);

  if(!(f1 && f2)) {
    printf("%d\n", sum);
    return 0;
  }

  FD_ZERO(&read_set);
  FD_SET(f1, &read_set);
  FD_SET(f2, &read_set);

  int pipe_num = select(f2 + 1, &read_set, NULL, NULL, NULL);
  if(pipe_num > 0){

    if (FD_ISSET (f1, &read_set)){
      read_and_calc(f1);
      printf("send f1 = %d\n", f1);
    }

    else if (FD_ISSET (f2, &read_set)){
      read_and_calc(f2);
      printf("send f2 = %d\n", f2);
    }
  }
}

thanks to all.

Answer 1

It's not like that, you just don't understand what's going on. Firstly, files are opened in an endless loop, which is most likely not meant, they must be opened once. Secondly
, it will not work, because if it fails, open() returns -1. Thirdly, you use strlen for the contents of a buffer that was received by a non-string function and most likely does not contain '\0' (buf[bytes] = 0 should have been written before strlen).