isCOBOL Evolve : Language Reference : Procedure Division Statements : SYNCHRONIZED
SYNCHRONIZED
General Format
SYNCHRONIZED
 
  [ ON Sync-Var ]
 
    [Statement-1]
    [Statement-2]
    [Statement-n]
 
  [END-SYNCHRONIZED]
Syntax Rules
1. Statement-1 through statement-n represent either one or more imperative statements or a conditional statement optionally preceded by one or more imperative statements.
2. Sync-Var is any item defined in the Data Division.
General rules
1. SYNCHRONIZED statement allows to synchronize a block of statements when running in mutithread environment. When a thread is executing the statements of a synchronized block, other threads willing to execute the same block of statements block and wait for the first thread to finish.
2. When the ON clause is specified, Sync-Var becomes the lock, otherwise the block itself is the lock. Two blocks that are synchronized on the same Sync-Var can’t be executed at the same time.
 
Examples
The following code may set the item varx to a unexpected value because it’s not safe to access the same data items from different threads:
 working-storage section.
 77 t1     handle of thread.
 77 t2     handle of thread.
 77 t3     handle of thread.
 77 varx   pic XX.
 procedure division.
 main.
    move "AA" to varx
    perform thread p1 handle t1.
    perform thread p2 handle t2.
    perform thread p3 handle t3.
    ACCEPT OMITTED
    STOP RUN. 
 p1.      
     perform UNTIL 1 = 2
       MOVE "AA" TO varx
     end-perform.   
 p2.      
     perform UNTIL 1 = 2
       MOVE "BB" TO varx
     end-perform.   
 p3.      
     perform UNTIL 1 = 2
       IF varx = "AA" or "BB"
          continue
       ELSE
          display "Unexpected condition! varx=" varx
          STOP RUN
       END-IF
     end-perform. 
Synchronizing the access to varx as follows, resolves the issue:
 working-storage section.
 77 t1     handle of thread.
 77 t2     handle of thread.
 77 t3     handle of thread.
 77 varx   pic XX.
 procedure division.
 main.
    move "AA" to varx
    perform thread p1 handle t1.
    perform thread p2 handle t2.
    perform thread p3 handle t3.
    ACCEPT OMITTED
    STOP RUN. 
 p1.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       MOVE "AA" TO varx
       END-SYNCHRONIZED
     end-perform.   
 p2.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       MOVE "BB" TO varx
       END-SYNCHRONIZED
     end-perform.   
 p3.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       IF varx = "AA" or "BB"
          continue
       ELSE
          display "Unexpected condition! varx=" varx
          STOP RUN
       END-IF
       END-SYNCHRONIZED
     end-perform. 
The following is a similar case that reproduces a NullPointerException instead of setting items to a undefined value:
 working-storage section.
 77 t1     handle of thread.
 77 t2     handle of thread.
 77 t3     handle of thread.
 01 vars.
    03 var1   pic 9(3).
    03 var2   pic 9(5).
 procedure division.
 main.
    move 1 to var1
    move 2 to var2
    perform thread P1 handle t1.
    perform thread P2 handle t2.
    perform thread P3 handle t3.
    ACCEPT OMITTED
    STOP RUN. 
 p1.      
     perform UNTIL 1 = 2
       MOVE 1 TO var1
     end-perform.   
 p2.      
     perform UNTIL 1 = 2
       MOVE 2 TO var2
     end-perform.   
 p3.      
     perform UNTIL 1 = 2
       add 1 to var1
       add 1 to var2
     end-perform.
Also in this case, the SYNCHRONIZED statement allows to fix the issue:
 working-storage section.
 77 t1     handle of thread.
 77 t2     handle of thread.
 77 t3     handle of thread.
 01 vars.
    03 var1   pic 9(3).
    03 var2   pic 9(5).
 procedure division.
 main.
    move 1 to var1
    move 2 to var2
    perform thread P1 handle t1.
    perform thread P2 handle t2.
    perform thread P3 handle t3.
    ACCEPT OMITTED
    STOP RUN. 
 p1.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       MOVE 1 TO var1
       END-SYNCHRONIZED
     end-perform.   
 p2.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       MOVE 2 TO var2
       END-SYNCHRONIZED
     end-perform.   
 p3.      
     perform UNTIL 1 = 2
       SYNCHRONIZED
       add 1 to var1
       add 1 to var2
       END-SYNCHRONIZED
     end-perform.