Multithreading: Reentrant, quasi-reentrant, and threadsafe programs

Multithreading is a technique where a single copy of an application program can be processed by several transactions concurrently. For example, one transaction can begin to execute an application program. When an EXEC CICS command is reached, causing a CICS® WAIT and call to the dispatcher, another transaction can then execute the same copy of the application program.

Compare this technique with single-threading, which is the execution of a program to completion: processing of the program by one transaction is completed before another transaction can use it.

Multithreading requires that all CICS application programs are quasi-reentrant; that is, they must be serially reusable between entry and exit points. CICS application programs that use the EXEC CICS interface obey this rule automatically. For COBOL, C, and C++ programs, reentrancy is ensured by obtaining a fresh copy of working storage each time the program is invoked. You should always use the RENT option on the compile or pre-link utility, even for C and C++ programs that do not have writable statics and that are naturally reentrant. Temporary variables and DFHEIPTR fields inserted by the CICS translator are usually defined as writable static variables and require the RENT option. For these programs to stay reentrant, variable data should not appear as static storage in PL/I, or as a DC in the program CSECT in assembler language.

As well as requiring that your application programs are compiled and link-edited as reentrant, CICS also identifies programs as being either quasi-reentrant or threadsafe; these attributes are set on the PROGRAM resource definition. The following table shows you the CONCURRENCY and API settings that are available on the program, and the type of TCB that the application program is run on.