LddPollAndSelect < Training2

%SECTION0{name=LddPollAndSelect}% =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 [[Training2.UnixIPCToAvoid][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)
      1 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*
<verbatim>
#include <linux/poll.h>
unsigned int poll(struct file *filp, poll_table *wait);
</verbatim>

*step 1: add waitqueues*
<verbatim>
void poll_wait(struct file*, wait_queue_head_t *, poll_table *);
</verbatim>

*step 2: return appropriate status (more in [[http://lxr.linux.no/linux/include/asm-generic/poll.h][asm/poll.h]])*

<verbatim>
/* readable */
return (POLLIN | POLLRDNORM);

/* writeable */
return (POLLOUT | POLLWRNORM);
</verbatim>
5.3.6. Non-Blocking IO		1. Denx Training Topics		5.3.8. Memory mapping
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