Waiting for Something (Screenplay)#

read() Not Implemented#

  • Use strace to show what cat does

  • ⟶ ENOSYS, likely (read() not implemented)

Implementing read() Naively#

  • read(), nonblocking version (see File Operations on Character Devices)

  • implement same functionality as MY_GET_ONE_EVENT

  • size check of buffer ⟶ -EINVAL (just like read() on a timerfd

  • Get one event, and copy that to the userspace buffer supplied by read.

  • Return 0 if nothing’s there

Discussion: What If Nothing’s There?#

  • 0 is usually end of file ⟶ cat terminates

  • EAGAIN is normally returned in such cases

  • but file descriptor has to be marked non-blocking, by convention

  • Write userspace test program test-read-events.c

    • Reads until EOF or error

  • ⟶ return -EAGAIN if nothing’s there

    • Check how test program behaves

Implementing O_NONBLOCK#

  • Check blocking-status of file descriptor

    filp->f_flags & O_NONBLOCK

  • If non-blocking, enter the code that we have now

  • Else ⟶ wait, hm

  • Add parameter to test-read-events.c: blocking/nonblocking

    • Test the wait