The control program uses a copy of the load module already in the requesting task's load list or the job pack area if the copy can be used. Whether the copy can be used or not depends on the reusability and current status of the load module, that is, the load module attributes, as designated using linkage editor control statements, and whether the load module has already been used or is in use. The status information is available to the control program only when you specify the load module entry name on an EXEC statement, or when you use ATTACH, ATTACHX, LINK, LINKX, XCTL, or XCTLX macros to transfer control to the load module. The control program protects you from obtaining an unusable copy of a load module if you always "formally" request a copy using these macros (or the EXEC statement). If you pass control in any other manner (for instance, a branch or a CALL macro), the control program, because it is not informed, cannot protect your copy. If your program is in AR mode, and the SYSSTATE ASCENV=AR macro has been issued, use the ATTACHX, LINKX, and XCTLX macros instead of ATTACH, LINK, and XCTL. The macros whose names end with "X" generate code and addresses that are appropriate for AR mode.
All reenterable modules (modules designated as reenterable using the linkage editor) from any library are completely reusable. Only one copy is normally placed in the link pack area or brought into your job pack area, and you get immediate control of the load module. However, there might be circumstances beyond your control that can cause an additional copy to be placed into your job pack area. The control program might do this, for example, to preserve system integrity.
If the module is serially reusable, only one copy is ever placed in the job pack area; this copy is always used for a LOAD macro. If the copy is in use, however, and the request is made using a LINK, LINKX, ATTACH, ATTACHX, XCTL, or XCTLX macro, the task requiring the load module is placed in a wait condition until the copy is available. You should not issue a LINK or LINKX macro for a serially reusable load module currently in use for the same task; the task will be abnormally terminated. (This could occur if an exit routine issued a LINK or LINKX macro for a load module in use by the main program.)
If the load module is not reusable, a LOAD macro will always bring in a new copy of the load module; an existing copy is used only if you issued a LINK, LINKX, ATTACH, ATTACHX XCTL or XCTLX macro and the copy has not been used previously. Remember, the control program can determine if a load module has been used or is in use only if all of your requests are made using LINK, LINKX, ATTACH, ATTACHX, XCTL or XCTLX macros.