CKD-Offline

Edit datasets on 3390 volumes without IPLing the system that owns them: z/OS volumes for IBM ZD&T, or MVS 3.8j volumes on Hercules. It opens the volume's image file and gives you an ISPF-style DSLIST, member list, EDIT, BROWSE and a hex editor in a browser tab. Saves go straight into the image, including the VTOC, the VTOC index, PDS directories and ISPF statistics.

Typical uses: fix a PARMLIB or PROCLIB member, patch some JCL, edit a sequential file, zap a load module, then put the volume back into service.

Supported image files:

Format	Used by
CKD (uncompressed)	ZD&T and Hercules
CCKD, CCKD64 (compressed)	Hercules only

Requirements

• Python 3.10 or newer, on macOS, Linux or Windows.
• A web browser.
• Nothing else. CKD-Offline uses only the Python standard library. It does not need, include or call Hercules, ZD&T or any other emulator: it reads and writes the image file itself. (Hercules is used only by the developer test suite; see Tests.)

Install

pip install ckdoffline

To keep it in its own environment, pipx install ckdoffline or uv tool install ckdoffline work too. From a source checkout: pip install . (add -e for development).

Usage

1. Stop the emulator that uses the volume (ZD&T or Hercules), or detach the device.

2. Make a copy of the image to work on. Keep the original until you have checked the result.

3. Start CKD-Offline with the image:

   ckdoffline work.cckd
   

   Your default browser opens at http://localhost:3270/ with the volume's data set list. Without an image argument you start on the Open panel, where you can type a path or pick a file.

4. Edit: M on a PDS for its member list, E to edit, PF3 (or Esc) to save and return. See Keys.

5. Stop CKD-Offline with Ctrl-C in the terminal, then attach the image to the emulator again.

Making the copy:

System	Command
macOS (APFS: instant clone)	cp -c MYVOL.CCKD work.cckd
Linux	cp --reflink=auto MYVOL.CCKD work.cckd
Windows (PowerShell)	Copy-Item MYVOL.CCKD work.cckd

On Windows, if ckdoffline isn't found after installing (Python's Scripts folder isn't on PATH), run py -m ckdoffline work.cckd instead. Anywhere, python3 -m ckdoffline works too.

Options:

Option	Meaning
--readonly	open the image read-only
--no-backup	skip the .bak-* copy before the first write
--codepage 1047|037	EBCDIC code page (default 1047; also on the Open panel)
--userid ID	user ID written into ISPF statistics (default: your login name)
--port N	HTTP port (default 3270)
--no-browser	don't open a browser

The server listens on 127.0.0.1 only. It is for one person on one machine.

What you can do

• DSLIST: every dataset on the volume with DSORG, RECFM, LRECL, BLKSIZE, tracks and extents. Line commands E edit, B browse, M member list, I info (full format-1/3 DSCB decode), Z compress. Primary command L prefix.
• Member list: ISPF statistics, aliases. Line commands S, E, B, D delete, R rename. Primary commands S name (create a new member, as in ISPF) and L name.
• EDIT (PS and PDS members, RECFM F/FB/FBS/FBA, V/VB/VBA): line commands I, D, R, C/M with A/B, and DD, CC, MM, RR. Primary commands SAVE, END, CANCEL, FIND/RFIND, CHANGE/RCHANGE, RESET, CAPS ON|OFF, NUM ON|OFF, HEX ON|OFF, L n. Lines you don't touch are written back byte for byte, including bytes that have no character in the code page.
• BROWSE: read-only view of any PS or PO text, HEX ON available.
• Hex editor (RECFM U): overtype in place, for a member of a load library or a whole sequential (PS) RECFM U dataset (E on it in DSLIST). Block lengths never change; all changed blocks are written in one transaction.
• Compress a PDS in place, like IEBCOPY: members (and their aliases and note lists) are moved down, the directory TTRs are relocated, and the freed tracks stay allocated.
• Extend: when a save no longer fits, the dataset gets a new extent from its secondary allocation: format-1/3 DSCBs, format 4, and the VTOC index (or format 5 on a non-indexed VTOC) are updated. Up to 16 extents on the volume. Format 5 maintenance is tested on synthetic volumes only. If the free-space map disagrees with the extents, the volume still opens and in-place edits work; DSLIST shows why the free space is unknown and only extends are refused.

Every save sets DS1DSIND "changed since backup" and DS1REFDT to today.

Keys

Laid out so that a Mac keyboard without an F-key row works too. The legend is on every screen.

Key	ISPF	Action
Enter	Enter	process the command line and line commands
F3 or Esc	PF3	End (in EDIT: save and leave)
F5	PF5	Repeat find
F7 or PgUp	PF7	Scroll up
F8 or PgDn	PF8	Scroll down
F10 or Shift+←	PF10	Scroll left
F11 or Shift+→	PF11	Scroll right
F12 or Ctrl+Shift+X	PF12	Cancel (in EDIT: leave without saving)
Home	Home	cursor to the command line
Tab	Tab	next field
Ctrl+I or Insert	Insert	toggle insert mode

Safety model

• Stop the emulator first (ZD&T or Hercules), or detach the device. CKD-Offline can't rely on a file lock, and writing to an image the emulator has open will corrupt it. Don't run the emulator and CKD-Offline on the same image at once, and after the emulator has used the image, close and reopen it in CKD-Offline.
• External changes are detected. If anything else writes the image while it is open (the emulator, another CKD-Offline), the next save is refused with "image changed on disk since it was opened; reopen it", so nothing is written from stale copies. Reopen the image (Enter on the Open panel) and redo the change. An "opened" flag that appears after the open needs confirming again.
• One volume per server. Opening an image (or changing the code page) in one browser tab makes the other tabs' next request fail with "Volume changed in another tab": they go back to the Open panel instead of saving into the wrong volume.
• Opened-flag warning. If a CCKD/CCKD64 image has its "opened" flag set, an emulator is running with it, or crashed. You get a warning and choose to continue read-write (only if the emulator is really stopped) or reopen read-only. Plain CKD has no such flag, so the Open panel always reminds you.
• Backup. Before the first write of a session the image is copied to <image>.bak-YYYYMMDD-HHMMSS in the same folder. On macOS this is an APFS clone (instant, and no extra space until the files diverge); elsewhere it is a full copy, so leave room for one. An existing backup is never overwritten. --no-backup turns this off.
• Undo journal. Every save is one transaction. The tool works out every byte it will change, writes the original bytes to <image>.journal and fsyncs it, applies the change, fsyncs, and deletes the journal. A refused save (line too long, directory full, no space) writes nothing.
• Recovery. If the tool was killed mid-save (kill -9, power loss), the journal is left behind. The next read-write open rolls the image back to how it was before that save, and says so. A read-only open refuses and offers to reopen read-write to recover.
• Read-only mode (--readonly or the Open panel field) never writes the image.
• Refusals. It won't touch a VTOC marked as needing recovery (DIRF), an index it can't read, a dataset with no secondary allocation, or anything past the end of the volume.

Out of scope

• Creating, deleting or renaming datasets (that needs catalog entries, which aren't on the volume).
• Anything multi-volume.
• PDSE, HFS, VSAM, DA/IS and every DSORG other than PS and PO. These are listed, not opened.
• RECFM VBS is browse only. New RECFM U members can't be created (S name is refused on a load library); existing ones can be patched, renamed and deleted.
• Devices other than 3390, FBA, EAV volumes (format 8/9 DSCBs), Hercules shadow files and split plain-CKD images.
• Getting data in or out other than by typing: no upload/download, XMIT or IEBCOPY unload.
• More than one user, remote access, a real 3270 data stream.

Tests

The tests check every write with Hyperion's own utilities (dasdls, dasdcat, dasdseq, dasdload, dasdcopy, cckdcdsk) as an independent oracle. Using the tool doesn't need any of this. Only running the tests does.

The .oracle/ directory

Everything the oracle needs lives in .oracle/ in the repo root. It is git-ignored and never committed: it holds source checkouts, build output and test volumes, all of which you build locally.

Path	What
.oracle/src/hyperion	SDL Hyperion source (or set HYPERION_SRC to an existing checkout)
.oracle/ext/<pkg>	sources of the Hercules external packages
.oracle/extpkgs	those packages, built and installed
.oracle/hyperion	the build copy of Hyperion with the utilities
.oracle/fixtures	small synthetic 3390 test volumes, made on first test run

Setting it up

Tested on macOS on Apple Silicon. Needs a C toolchain, CMake and Homebrew's libtool (for ltdl.h). On Intel or Linux, drop -m aarch64 below and adjust the library path in tools/build_oracle.sh.

E=$PWD/.oracle
git clone https://github.com/SDL-Hercules-390/hyperion.git $E/src/hyperion
for p in crypto decNumber SoftFloat telnet; do
  git clone https://github.com/SDL-Hercules-390/$p.git $E/ext/$p
  mkdir -p $E/work/$p $E/extpkgs
  (cd $E/work/$p && $E/ext/$p/build -d $E/ext/$p -n $p -m aarch64 -a 64 -c Release -i $E/extpkgs)
done
tools/build_oracle.sh

The external packages are needed because Hyperion's repository ships them prebuilt for x86 only. tools/build_oracle.sh copies the Hyperion source into .oracle/hyperion and builds the utilities there; the source checkout itself is left untouched. The synthetic test volumes are built by tools/make_fixtures.sh on the first test run.

Tests against real volumes

tests/local/ is git-ignored and meant for tests against your own volumes, which can't be published. Put the image (a *.cckd/*.ckd file, also git-ignored) and a conftest.py providing a fixture that clones it into tmp_path there; the tests should only ever touch the clone. pytest picks the directory up automatically.

During development the tool was also run end to end against a real 3390 volume: member edits, adds, deletes and renames; a VB dataset grown by 50,000 records into four extents with a format-3 DSCB; a load module patch; PDS compress; and saves through the HTTP API. After each step the image passed the integrity checks and the free-space maps matched the extents.

Running

Needs pytest; the JS tests need Node:

pip install -e '.[dev]'
python3 -m pytest -q -m "not slow"   # fast
python3 -m pytest -q                 # full, including tests/local/ if present
node --test tests/js/                # front-end editing model

Notes

• Hyperion's DIRF constant is out of date. dasdblks.h names X'40' in DS4VTOCI as DS4VTOCI_DIRF. In current IBM documentation DS4DIRF is X'04', and X'40' is DS4DVTOC. This tool uses X'04', and refuses to write if either bit is set (see the comment in ckdoffline/vtoc/space.py).

Final check is yours

The tests prove Hyperion's DASD utilities read the result cleanly. The last word belongs to the system that uses the volume: attach an edited copy to ZD&T for z/OS, or to Hercules for MVS 3.8j, IPL or vary the volume online, and browse the changed members. If you like, run IEHLIST LISTVTOC FORMAT against it, and on z/OS also ICKDSF ANALYZE or BUILDIX.

Trademarks

IBM, z/OS, MVS, ZD&T, ISPF, DFSMS, and other IBM product names used here are trademarks of International Business Machines Corporation. They are used only to describe what this tool works with. CKD-Offline is an independent project, not affiliated with, sponsored by or endorsed by IBM. Hercules is an open-source project unrelated to IBM.

License

MIT. See LICENSE.

CKD-Offline is an independent implementation. SDL Hyperion (Hercules) was used as a reference for the image formats, and its utilities serve as a test oracle, but no Hercules code or binaries are included in this repository or in the package on PyPI. The test setup clones and builds Hyperion locally from its own upstream repositories (under Hyperion's own licence) into the git-ignored .oracle/ directory; nothing from it is redistributed.