Explicit and Implicit Transfers of Control

The flow of control mechanism transfers control from statement to statement in the sequence in which they were written unless an explicit transfer of control overrides this sequence or there is no next executable statement to which control may be passed. The transfer of control from statement to statement occurs without the writing of an explicit Procedure Division statement, and, therefore, is an implicit transfer of control.

COBOL provides both an explicit and an implicit means of altering the implicit control transfer mechanism.

In addition to the implicit transfer of control between consecutive statements, implicit transfer of control occurs when the normal flow is altered without the execution of a procedure branching statement. COBOL provides the following types of implicit control flow alterations which override the statement-to-statement transfers of control:

1. If a paragraph is being executed under control of another COBOL statement (for example, PERFORM, USE, SORT and MERGE) and the paragraph is the last paragraph in the range of the controling statement, then an implied transfer of control occurs from the last statement in the paragraph to the control mechanism of the last executed controling statement. Further, if a paragraph is being executed under the control of a PERFORM statement which causes iterative execution and that paragraph is the first paragraph in the range of that PERFORM statement, an implicit transfer of control occurs between the control mechanism associated with that PERFORM statement and the first statement in that paragraph for each iterative execution of the paragraph.
2. When a SORT or MERGE statement is executed, an implicit transfer of control occurs to any associated input or output procedures.
3. When any COBOL statement is executed that results in the execution of a declarative section, an implicit transfer of control to the declarative section occurs.
   Note: Another implicit transfer of control occurs after execution of the declarative section, as described in item 1. above.
4.  In any file operation (including OPEN and CLOSE), if a file does not have a FILE STATUS data item declared for it and the file is not explicitly covered by a USE statement, then it is covered by an implicit USE statement. The implied USE procedure is equivalent to:

   USE AFTER ERROR PROCEDURE ON file-name. 
   IF status-key-1 >= 3
     DISPLAY error-message UPON CONSOLE 
     STOP RUN.

   See your COBOL system documentation for descriptions of error messages.

An explicit transfer of control consists of an alteration of the implicit control transfer mechanism by the execution of a procedure branching, ending or conditional statement. An explicit transfer of control between statements can be caused only by the execution of a procedure branching or conditional statement. An explicit transfer of control between sentences can be caused only by the execution of the NEXT SENTENCE phrase of an IF or SEARCH statement.

The execution of the procedure branching statement ALTER does not in itself constitute an explicit transfer of control, but affects the explicit transfer of control that occurs when the associated GO TO statement is executed. The procedure branching statement EXIT PROGRAM causes an explicit transfer of control only when the statement is executed in a called program.

The term "next executable sentence" is used to refer to the next COBOL sentence to which control is either implicitly transferred according to the rules above, or explicitly transferred as a result of execution of the NEXT SENTENCE phrase. The next executable sentence is the first sentence following the separator period that terminates the current sentence. There is no next executable sentence when there is no next executable statement as described below.

The term "next executable statement" is used to refer to the next COBOL statement to which control is transferred according to the rules above and the rules associated with each language element in the Procedure Division.

There is no next executable statement following:

• The last statement in a declarative section when the paragraph in which it appears is not being executed under the control of some other COBOL statement
• The last statement in a source element when the paragraph in which it appears is not being executed under the control of some other COBOL statement in that source element
• The last statement in a declarative section, when the statement is in the range of an active PERFORM statement executed in a different section, and this last statement of the declarative section is not also the last statement of the procedure that is the exit of the active PERFORM statement
• A STOP RUN statement

   , GOBACK statement

   , EXIT METHOD

   , EXIT FUNCTION

  or EXIT PROGRAM statement that transfers control outside the COBOL source element

• The end marker.