Whats the difference between select() and poll()?

  From Richard Stevens (rstevens@noao.edu):

  The basic difference is that select()'s fd_set is a bit mask and
  therefore has some fixed size.  It would be possible for the kernel to
  not limit this size when the kernel is compiled, allowing the
  application to define FD_SETSIZE to whatever it wants (as the comments
  in the system header imply today) but it takes more work.  4.4BSD's
  kernel and the Solaris library function both have this limit.  But I
  see that BSD/OS 2.1 has now been coded to avoid this limit, so it's
  doable, just a small matter of programming. :-)  Someone should file a
  Solaris bug report on this, and see if it ever gets fixed.

  With poll(), however, the user must allocate an array of pollfd
  structures, and pass the number of entries in this array, so there's
  no fundamental limit.  As Casper notes, fewer systems have poll() than
  select, so the latter is more portable.  Also, with original
  implementations (SVR3) you could not set the descriptor to -1 to tell
  the kernel to ignore an entry in the pollfd structure, which made it
  hard to remove entries from the array; SVR4 gets around this.
  Personally, I always use select() and rarely poll(), because I port my
  code to BSD environments too.  Someone could write an implementation
  of poll() that uses select(), for these environments, but I've never
  seen one. Both select() and poll() are being standardized by POSIX
  1003.1g.



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