5.3.7. poll and select

View from Userspace:

• process can check if one or more files can be read from or written to without blocking

• this allows to work with multiple file descriptors

• example: telnet: reads from socket and terminal, can't block on either!

• But why? We've got:
  • O_NONBLOCK, we can poll? -> yes, but this wastes CPU time!
  • threads -> yes, but threads are considered problematic for many reasons

View from Kernelspace:

• driver:
  1. call poll_wait on one or more waitqueues that inform of change of poll=/=select status (typically read and write queue)
  2. return a bitmask describing what operations can be performed without blocking.

• kernel: if a requested operation is possible the kernel returns, otherwise sets process to sleep on waitqueues and handles timeouts.

API: poll

header and poll operation

#include <linux/poll.h>
unsigned int poll(struct file *filp, poll_table *wait);

step 1: add waitqueues

void poll_wait(struct file*, wait_queue_head_t *, poll_table *);

step 2: return appropriate status (more in asm/poll.h)

/* readable */
return (POLLIN | POLLRDNORM);

/* writeable */
return (POLLOUT | POLLWRNORM);