pseudo device driver (e.g. getting device driver-like behaviour from a userspace program

the pseudo device driver enters an infinite loop, creates a fifo for mkfifo /dev/pwm0; chmod a+w /dev/pwm0; to serial port action:

opens serial port once to listen on multiple device files (/dev/pwm0 and /dev/pwm1)
different threads would not work because there is a chance the device or resource would be busy
select can be set to timeout, nonblocking, etc..
select can select which one to read
select can tell status without actually reading the data out

see Advanced Programming in the UNIX Environment, Addison-Wesley, 1992, ISBN 0-201-56317-7, by Richard Stevens.

to understand this code: man select

NAME
       select,  FD_CLR,  FD_ISSET,  FD_SET, FD_ZERO - synchronous
       I/O multiplexing
...
       int  select(int  n,  fd_set  *readfds,  fd_set  *writefds,
       fd_set *exceptfds, struct timeval *timeout);

       FD_CLR(int fd, fd_set *set);
       FD_ISSET(int fd, fd_set *set);
       FD_SET(int fd, fd_set *set);
       FD_ZERO(fd_set *set);

DESCRIPTION
       select  waits  for  a number of file descriptors to change status.
       Three independent sets of descriptors are watched.   Those
       listed  in  readfds  will  be watched to see if characters
       become available for reading (more precisely, to see if  a
       read  will not block - in particular, a file descriptor is
       also ready on end-of-file),  those  in  writefds  will  be
       watched  to  see  if  a write will not block, and those in
       exceptfds will be watched for exceptions.   On  exit,  the
       sets  are  modified in place to indicate which descriptors
       actually changed status.

Four macros are provided to manipulate the sets:

FD_ZERO will clear a set;
FD_SET and FD_CLR add or remove a given descriptor from a set;
FD_ISSET tests to see if a descriptor is part of the set. this is useful after select returns.

fd_set rfds; // globally defined structure for file descriptor select
int fd0;
int fd1;

int openfifo() {
  fd0 = open("/dev/pwm0", O_RDWR | O_SYNC); // open fifo for reading
  fd1 = open("/dev/pwm1", O_RDWR | O_SYNC);
  if( fd0 == -1 ) { perror("open");exit(1); }
  if( fd1 == -1 ) { perror("open");exit(1); }
...
int readfromfifo(int *whichservo) { // returns whichservo and value of servo
...
  FD_ZERO(&rfds);
  FD_SET(fd0, &rfds);
  FD_SET(fd1, &rfds);
  select( max(fd0, fd1)+1, &rfds, NULL, NULL, NULL);
...
  if (FD_ISSET(fd0, &rfds)) {
    read( fd0, &buffer, 1);
    *whichservo=0;
    printf("servo %d got some data\n", *whichservo);
}
  if (FD_ISSET(fd1, &rfds)) {
    read( fd1, &buffer, 1);
    *whichservo=1;
    printf("servo %d got some data\n", *whichservo);
  }
...
  // connect to the ssc
  if(ssc_open(port,baudrate)) perror("failed to open serial port");
  openfifo(); // global variable istead of return
  while (1) {
    x_in = readfromfifo(&servo); // servo number and x_in returned from function
...
    ssc_move( servo, x_in  );