APPUP File

What are APPUP files and how to open them

Are you having problems opening an APPUP file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your APPUP files.

What is an APPUP file?

An .APPUP file is an Erlang Application Upgrade file.

Erlang Application Upgrade files define how one version of an Erlang application is upgraded or downgraded at runtime. They are plain text files written as a single Erlang term. Created by the Erlang/OTP system (specifically its release tools), they are used during the release process, especially by systools, to generate "relup" files that guide live upgrades and downgrades. Their main purpose is ensuring that a running Erlang system can transition between application versions without stopping the whole system.

Such files are placed in the ebin directory of an application and named with the application's name followed by .appup, matching the vsn string defined in the application's .app file. The file syntax is one tuple term of the form {Vsn, UpList, DownList} where Vsn is a string naming the current version; UpList is a list of {OldVsn, Instructions} pairs defining how to upgrade from earlier versions; DownList is similarly structured for downgrades. Version identifiers in those lists may be exact strings or binaries acting as regular expressions to match multiple old version patterns. Instructions inside the lists are release upgrade instructions: actions like loading modules, adding or deleting modules, restarting applications, or performing more advanced operations including state transformations using callback functions.

Technically, the internal signature begins with ASCII "{" at offset 0, followed by a dot (".") at offset 3, matching the usual syntax of an Erlang tuple with version string followed by lists.

How to open APPUP files

Important: Different programs may use files with the APPUP file extension for different purposes, so unless you are sure which format your APPUP file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 18, 2026

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