There are two methods of passing a connection as a parameter to a procedure: using a formal parameter based on a particular connection block, or using a parameter of type CONNECTION. The following table compares these two methods:
|
Formal Parameter Based on Connection Block |
Formal Parameter of Type CONNECTION |
|
|---|---|---|
|
Language Support |
ALGOL and NEWP |
ALGOL and NEWP |
|
Actual Parameter |
Must be based on a connection library type specified by the formal parameter. |
Can be a connection of any connection library. |
|
Connection Objects |
In ALGOL, provides access to procedures declared in the connection block. In NEWP, provides access to any object declared in the connection block. |
Does not provide access to connection objects. |
|
Library Attributes |
Provides access only to STATE attribute. |
Provides access only to STATE attribute. |
|
Parameters to Library Procedures |
Cannot be used as a formal parameter to an imported or exported procedure. |
Can be used as a formal parameter to an imported or exported procedure. |
Connection Parameters Example
The following NEWP program illustrates both of these methods of passing connections as parameters:
100 BEGIN 110 120 TYPE CB1_TYPE = CONNECTION BLOCK 130 BEGIN 140 PROCEDURE PROC1; 150 IMPORTED; 160 END; 170 180 TYPE CB2_TYPE = CONNECTION BLOCK 190 BEGIN 200 PROCEDURE PROC2(CLE); 210 CONNECTION CLE; 220 IMPORTED; 230 END; 240 250 CB1_TYPE LIBRARY LIB1(LIBACCESS = BYTITLE, 260 CONNECTIONS = 9, 270 TITLE = “OBJECT/LIB1.”); 280 290 CB2_TYPE LIBRARY LIB2(LIBACCESS = BYTITLE, 300 CONNECTIONS = 3, 310 TITLE = “OBJECT/LIB2.”); 320 330 PROCEDURE PROC3(CB); 340 CB1_TYPE CB; 350 BEGIN 360 INTEGER I; 370 I := LIBRARY(CB).STATE; 380 IF I = VALUE(LINKED) THEN 390 CB.PROC1; 400 END; 410 420 PROCEDURE PROC4(CN); 430 CONNECTION CN; 440 BEGIN 450 INTEGER I; 460 I := CN.STATE; 470 END; 480 510 LINKLIBRARY(LIB1[0]); 520 LINKLIBRARY(LIB2[0]); 530 PROC3(LIB1[0]); 540 PROC4(LIB1[0]); 550 PROC4(LIB2[0]); 560 LIB2[0].PROC2(LIB1[0]); 570 580 END.
Explanation of Example: Connection Block Parameter
In this example, procedure PROC3 (at line 330) declares a parameter CB that is based on connection block CB1_TYPE. As a result, statements in PROC3 are able to read the STATE attribute of the connection, and to initiate procedure PROC1 (which is declared in CB1_TYPE). However, the statement at line 530 that invokes PROC3 must pass a connection of a library (LIB1) that is based on CB1_TYPE. This statement could not have passed a connection of LIB2, because LIB2 is based on a different connection block type.
Explanation of Example: CONNECTION Parameter
Procedure PROC4 (at line 420) declares a parameter of type CONNECTION. As a result, this procedure cannot include any statements invoking procedures declared by the connection. On the other hand, PROC4 can receive connections from libraries based on different connection blocks. Thus, the statements at lines 540 and 550 pass connections of LIB1 and LIB2, which are based on two different connection block types.
