Backwards migration problem with load modules? – try using DLLs

I had a problem where I compiled a program on z/OS 2.4. When it ran on z/OS 2.3 it abended. This was because I had included some z/OS code with my load module, which was not compatible with back level versions of z/OS.
I didn’t have a z/OS 2.3 to run on …. what could I do? A quick search showed me that using DLL’s was the right answer. You may be using this today without knowing it, for example your C program does not contain the whole printf executable, it just contains a link to it.

What did I do?

My old JCL had

//COMPILE EXEC PROC=EDCCB,….
//BIND.SYSLMOD DD DISP=SHR,DSN=&LOADLIB.
//BIND.SYSLIB DD DISP=SHR,DSN=&LIBPRFX..SCEELKED
//BIND.OBJLIB DD DISP=SHR,DSN=COLIN.OBJLIB
//BIND.GSK DD DISP=SHR,DSN=SYS1.SIEALNKE
//BIND.CSS DD DISP=SHR,DSN=SYS1.CSSLIB
//BIND.SYSIN DD *
INCLUDE GSK(GSKCMS31)
INCLUDE GSK(GSKSSL)
INCLUDE CSS(IRRSDL00)
NAME AMSCHECK(R)

Where the INCLUDE GSK(..) copied in the (z/OS 2.4 specific ) code needed to execute.

My new JCL had

//BIND.SYSIN DD *
SETOPT PARM(CASE=MIXED)
include /usr/lpp/gskssl/lib/GSKSSL.x
include /usr/lpp/gskssl/lib/GSKCMS31.x

INCLUDE CSS(IRRSDL00)
NAME AMSCHECK(R)

How does it work?

The GSKSSL.x has

include /usr/lpp/gskssl/lib/GSKSSL.x
IMPORT CODE,’GSKSSL’,’gsk_attribute_get_buffer’
IMPORT CODE,’GSKSSL’,’gsk_attribute_get_cert_info’

This says gsk_attribute_get_buffer is in module GSKSSL, use dynamic resolve at execution time.

When the program is executed, the loader looks for entries which have pending dynamic resolve. In this case it sees gsk_attribute_get_buffer, It loads the module GSKSSL from the executing system, looks inside it, locates the gsk_attribute_get_buffer code and resolves the address.

On my z/OS 2.4 system it loaded the 2.4 version of GSKSSL, on the z/OS 2.3 system it loaded the 2.3 version of GSKSSL. As long as the parameters to the calls are consistent it will work.

Does this work for my own code?

Yes. You have to build it a certain way. Look for side-deck in the C user guide and C Programming guide.

Are there any other benefits?

Yes – your load modules are smaller, because the imported code is obtained at run time, and not included in your load module. Also, as Charles M. pointed out, another benefit is that you are no longer violating IBM’s license by shipping z/OS licensed code as part of your product.

What can I ship?

Charles Mills pointed me to this documented list of what you can do. See here under Redistribution of Code Routines. Which starts

This document includes information about certain callable service stub and linkage-assist (stub) routines and other routines that are contained in specific data sets and file system directories that are intended to be bound, link-edited, or otherwise included with code and run on z/OS systems. With your authorized use of z/OS, you can include these routines into your modules and distribute your
modules with the included routines for the purposes of developing, using, marketing, and distributing programs that conform to the documented programming interfaces for z/OS, provided that each routine is included in its entirety, including any IBM copyright statements.

It then goes on and lists which libraries you can use.

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