Class: Monitor

• Inheritance
• Description
• Related information
• Class protocol
  • instance creation
• Instance protocol
  • enter & leave
  • initialization
  • queries
• Examples

Inheritance:

   Object
   |
   +--Monitor

Package:

stx:libbasic2

Category:

Kernel-Processes

Version:

rev: 1.23 date: 2017/08/09 12:13:35

user: cg

file: Monitor.st directory: libbasic2

module: stx stc-classLibrary: libbasic2

Author:

Claus Gittinger

Description:

Monitors (as used in Java) provide a functionality much like RecursionLocks, 
but are not block based.
Therefore, monitors are not unwind-save (i.e. a return or unwind while a
monitor is locked, will lead to a deadlock situation).
You have to care for unwind protection yourself.

Notice:
    This is an unused demo class - there is no WARRANTY.
    It is not used by the system itself.
    Smalltalkers should use Semaphores and RecursionLocks, which
    are unwind-save.

Related information:

    RecursionLock
    Semaphore
    Delay
    SharedQueue
    Block

Class protocol:

 new

Instance protocol:

 critical: aBlock

a critical section. Executes a block as a critical section, secured by the receiver.

 enter

enter the monitor

 exit

exit the monitor

 fakeEnter: aProcess count: additionalCount

(fake-)enter the monitor, without blocking.
Raises an error, if the monitor is not free and owned by another process

 initialize

(comment from inherited method)
just to ignore initialize to objects which do not need it

 count

 isFree

return true, if the monitor is free
(i.e. noone waits and count is zero)

 owningProcess

return the monitors owner; or nil, if it's free

Examples:

        |mon p1 p2 p3|

        mon := Monitor new.

        p1 := [
             10 timesRepeat:[
                 Delay waitForSeconds:0.3.
                 mon enter.
                 'p1 got it' printNL.
                 Delay waitForSeconds:0.3.
                 'p1 leaves' printNL.
                 mon exit
             ]
        ] fork.

        p2 := [
             20 timesRepeat:[
                 Delay waitForSeconds:0.2.
                 mon enter.
                 'p2 got it' printNL.
                 Delay waitForSeconds:0.2.
                 'p2 leaves' printNL.
                 mon exit
             ]
        ] fork.

        p3 := [
             30 timesRepeat:[
                 Delay waitForSeconds:0.1.
                 mon enter.
                 'p3 got it' printNL.
                 Delay waitForSeconds:0.1.
                 'p3 leaves' printNL.
                 mon exit
             ]
        ] fork.