Module reusability

Reusability is a generic term describing the degree to which a module can be shared, reused or replaced during execution. It incorporates the following attributes:

Nonreusable. The module is designed for single use only and must be refreshed before it can be reused.
Serially reusable. The module is designed to be reused and therefore must contain the necessary logic to reset control variables and data areas at entry or exit. A second task cannot enter the module until the first task has finished.
Reenterable (reentrant). The module is designed for concurrent execution by multiple tasks. If a reenterable module modifies its own data areas or other shared resources in any way, appropriate serialization must be in place to prevent interference between using tasks.
Refreshable. All or part of the module can be replaced at any time, without notice, by the operating system. Therefore, refreshable modules must not modify themselves in any way.

Unlike AMODE, reusability is an attribute of the entire module, not any particular entry point. It should be chosen based on the operational characteristics of the module and not on the reusability status of individual control sections or data classes.

The linkage editor processed the serially reusable (REUS), reenterable (RENT) and refreshable (REFR) attributes as separate and independent options. The binder, however, treats them as a single, multivalued attribute with an implied hierarchical relationship: “refreshable” implies “reenterable” and “reenterable” implies “serially reusable”. This might result in some confusion for prior linkage editor users who are accustomed to specifying inconsistent combinations of these attributes, such as “REFR,NORENT”. In such situations the binder selects the strongest reusability attribute among those specified. In addition, unlike the linkage editor, the binder honors any override of reusability specified in the PARM statement.

In order to eliminate such conflicts, specify only a single attribute from the set. Use the keyword(value) form, such as REUS(RENT), rather than keyword-only specifications, such as NORENT or REFR.