What is a .dll file?
Dynamic Link Library · Disk image · Microsoft · 1980s
A DLL is a shared code library that Windows programs load to reuse common functions. You rarely open one directly, but you meet DLLs when software reports a missing or corrupted library file.
What is a .dll file used for?
A DLL holds code and data that many programs can share, which saves memory and avoids duplicating the same functions in every app.
You do not run a DLL on its own. Programs load the ones they need at runtime, which is why an error about a missing DLL means an app cannot find a library it depends on.
Because many programs share the same DLLs, version conflicts are possible, the old headache nicknamed 'DLL hell'. Modern Windows eases this with side-by-side versioning and per-app copies, but it is why dropping in a DLL from the wrong source can break an app that expected a different version.
DLLs use the same executable structure as EXE files, so they can also carry malicious code if they come from an untrusted place.
What opens a .dll file?
The .dll format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- Loaded by programs, not opened directly
- Dependency viewers for inspection
macOS
- Not applicable
Linux
- Not applicable (uses .so instead)
Web
- Not applicable
Open a .dll file step by step
- Confirm the file really is a .dll. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with Loaded by programs, not opened directly or Dependency viewers for inspection.
- On Mac, use Not applicable.
- On Linux, try Not applicable (uses .so instead).
- Still stuck? Convert the .dll to Not convertible, which opens more widely.
Why can't I open my .dll file?
Almost always because no installed app understands DLL. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .dll at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .dll file
You can convert a .dll file to Not convertible using a conversion tool or the export menu of an app that opens it.
What is the .dll file signature (magic bytes)?
A .dll file starts with the hex bytes 4D 5A, which read as MZ in ASCII. Programs identify DLL by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for DLL. For a full breakdown, see the PE (EXE / DLL) file signature page.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 4D 5A | 0 | MZ | 'MZ' DOS header (same as EXE) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .dll file?
A DLL is a shared code library that Windows programs load to reuse common functions. You rarely open one directly, but you meet DLLs when software reports a missing or corrupted library file.
A program says a DLL is missing. What do I do?
Reinstall or repair the program that needs it, or install the matching runtime (such as the Visual C++ Redistributable). Do not download loose DLLs from random sites.
Is it safe to download a DLL file?
No. Third-party DLL download sites are a well-known malware vector, and a mismatched version usually causes more errors than it fixes. Get the file only by reinstalling the software or installing its official runtime.
Related formats
Sources
Details on this page were checked against authoritative references. How sources are ranked and what is deliberately left out is set out in the methodology.