Using a Monitor

Monitors provide a solution to particular access problems not handled easily by mutexes. For example, you might want many threads to be able to simultaneously read data, but only one thread to be able write data. While the thread writes data, you might want to block read access by other threads.

A monitor is used by a critical section to declare what type of data access the critical section will be performing on the protected data; that is, reading, writing, or browsing.

The monitor synchronization facility can be extended to provide a critical section for browsing; this can be very useful in real world applications. A browser reads data and, depending on conditions you have set, might or might not write to protected data items. While a browser is active, any number of critical sections that simply read the data are allowed, while any other critical sections that browse or write are not allowed. If the browser thread determines that it needs to write to the protected data, it requests a conversion of the browse lock to a write lock. The conversion process waits until all critical sections that read the data are finished, and then bars any other critical sections that read or write data from accessing the data. The browser proceeds to write with exclusive access to the protected data items (whose state is guaranteed to be the same as it was when the browser was just reading the protected data).

Example

The following example code shows a monitor that controls the access of multiple threads that count items in, or add items to, a table. The code:

  • Enables multiple threads to access data for counting
  • Disables any threads that add items to the table when a thread that counts items is active
  • Enables one thread that adds items to a table; while this thread is active, the code disables access to the table by any thread that counts items 
 Working-Storage Section.
 78  table-length                   value 20.
 01  table-monitor              usage monitor-pointer.
 01  table-current              pic x(4) comp-x value 0.
 01  table-1.
     05  table-item-1           pic x(10) occurs table-length.

 Local-Storage Section.
 01  table-count                pic x(4) comp-x.
 01  table-i                    pic x(4) comp-x.
  . . .
*> Initialization code, executed while in single
*> threaded mode
     move 0 to table-current
     open table-monitor

* Add an item to table-1, this is a writer critical section
     set table-monitor to writing
     if table-current < table-length
         add 1 to table-current
         move 'filled in' to table-item-1(table-current)
     end-if
     set table-monitor to not writing

* Count items in table-1, this is a reader critical section
     set table-monitor to reading
     move 0 to table-count
     perform varying table-i from 1 by 1 
             until table-i > table-current
         if table-item-1 (table-i) = 'filled in'
             add 1 to