What is the LCK file format?
.lck is a resource lock file - a concurrency marker that an application creates automatically to signal that a file or database is currently in use. The .lck extension is typically appended to the name of the locked file; for example, locking document.doc results in the creation of a companion file named document.doc.lck whose mere existence on disk acts as the lock signal. .lck files are most often completely empty, but some programs store a small amount of additional data inside, such as the username, host name, or process ID (PID) of the session that holds the lock. .lck files should not be opened or edited manually; they are created and deleted automatically by the owning application.
Each locked file is protected from being written to by a second process or user. The application that holds the lock keeps the .lck file active for the duration of the session to prevent the original file from being deleted or corrupted. This mechanism is widely used by database engines, spreadsheet applications, and word processors handling formats such as .DOC files. Locking also applies when multiple users on a network access the same shared file simultaneously; in that case, the .lck file may track which user holds the active session or which portion of the resource is locked.
Different applications implement the locking mechanism differently. Some related lock-file conventions use extensions such as .lock, .LDB, or .LACCDB. Because .lck is a generic cross-platform convention with no single owner or specification - used on Windows, macOS, Linux, and Unix alike - its internal content (if any) is entirely application-specific.
Security & safety
RISK: LOWA .lck is a passive marker file - usually empty, no executable code - so it is harmless to have and safe to read. The only real caution is operational, not security: do NOT delete a .lck while the file it protects is genuinely open by someone, because that defeats the protection and can let two users write at once and corrupt the data. Only remove a .lck when you've confirmed the underlying file is closed everywhere (a 'stale' lock left after a crash). Deleting a live lock, not the file itself, is the risk.
Format details
in a nutshellPrograms that open LCK files
Windows2 apps
Owning application (close to release)
Built-in Don't open the .lck - close the program/file that created it on every machine; the app deletes the .lck automatically. If a stale .lck remains and nobody has the file open, delete the .lck manually.
Notepad (inspect only)
Built-in If you must see what's inside, open with Notepad - it's empty or a short owner/PID record. Don't edit it while the file is in use. 
macOS1 app
Owning application (close to release)
Built-in Quit the app that created the lock; it removes the .lck on exit. Delete a stale .lck only after confirming the file isn't open anywhere. 
Linux1 app
Owning application / file manager
Built-in Stop the process holding the lock (the app, or check with 'lsof file.lck'); remove the leftover .lck only once you've confirmed nothing is using the file. Technical details
deep spec| Format type | Existence-based concurrency lock marker; the file's presence on disk is the lock signal, not its contents |
| Typical file size | 0 bytes (most common); occasionally a few bytes to a few hundred bytes when owner data is stored |
| File signature (magic bytes) | None - no header or magic bytes; the file carries no identifying signature |
| Encoding | Empty (most cases); plain text or small application-specific binary when non-empty |
| Content when non-empty | Short owner record containing the locking session's username, host name, or process ID (PID) |
| Naming convention | Companion file appended with .lck - e.g., `report.xls` is accompanied by `report.xls.lck` |
| Locking scope | Prevents concurrent write access to the associated file; some implementations still permit read-only access by other processes |
| Lifecycle | Created when a file is opened for exclusive use; deleted automatically when the session ends or the application closes cleanly |
| Stale lock risk | If the owning process crashes, an orphaned `.lck` file may remain on disk, blocking access until removed manually |
| Multiple-user environments | Coordinates shared-file access over a network; may record which user or workstation holds the active lock |
| MIME type | application/octet-stream (default); text/plain when the file holds plain-text owner data |
| Associated operating systems | Windows, macOS, Linux, Unix - a cross-platform convention with no OS-specific dependency |
| User data | Contains no document content or user data; serves exclusively as a control file for the owning application |
| Standardization | No formal specification; content format and behavior are defined individually by each application that uses it |
| Common application types | Database engines, spreadsheet and word-processor applications, version control systems, integrated development environments |
| Related lock-file extensions | .lock, .ldb, .laccdb, .tmp, ~lock - alternative naming conventions used by other applications for the same purpose |
| Released | Long-standing OS/application convention (decades old); no single introduction date |
| Latest version | N/A - not a versioned format |
LCK conversions
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