Question

I would like to have the following scheme:

one mutex struct.

a writer can enter the critical section only if no other thread is there.

a reader can enter the critical section only if the other threads currently there are also readers.

I am not sure how to implement this with critical_sections (as I don’t want the readers to be mutually
exclusive)

Any ideas?

Thanks!

Answer 1

You may use semaphores.

• One mutex to exlusively lock if a
  writer is inside the critical
  section.
• And a semaphore for readers with the maximum amount of readers.

You can check on most semaphore structures how much permits are available and when … the maximum amount of permits equals the available amount of permits you know that there is no reader inside the critical section.

Some pseudo code

Writer

# Take Writer-Mutex(blocking)
    # Check until the maximum amount of Semaphore Permits == Max Amount
           #loop
    #
    # critical section
# Release Mutex

Reader

 # Take Writer-Mutex(blocking)
 # Take Semaphore Permit
 # Release Writer-Mutex
 # Critical Section
 # Release Semaphore Permit