CMI File

What are CMI files and how to open them

Are you having problems opening a CMI 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 CMI files.

What is a CMI file?

A .CMI file is an OCaml Bytecode (compiled Interface) file.

OCaml bytecode compiled interface files represent the public interface (types, values, modules) of an OCaml compilation unit in a binary form that the compiler and related tools use. They are generated during compilation either from a source interface file (.mli) or by inferring the interface from an implementation file (.ml). These files let the compiler know what a module exposes to others so that dependent modules can be typechecked and linked without needing the full implementation.

When building a project, the OCaml compilers (ocamlc for bytecode, ocamlopt for native code) produce interface files so other units can import them. A compiled interface file captures only the exported signatures, not the implementation bodies. During linking or cross-module optimization, the compiler and linker consult these interface files to ensure consistency, compatibility, and correct dependency resolution.

The interface files are used when the compiler processes interfaces (.mli) or implementations (.ml). If an .mli file exists, compiling it produces the compiled interface. If not, when compiling the implementation, the compiler produces a compiled interface inferred from that implementation. The interface file carries a unit name derived from the source filename (capitalized), and OCaml tools search for the compiled interface by that name in the module search path. Renaming a compiled interface without preserving its base name leads to errors.

These files are written in a binary format with an ASCII header string Caml1999I at offset zero.

How to open CMI files

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

While we have not verified the apps ourselves yet, our users have suggested six different CMI openers which you will find listed below.

Last updated: August 31, 2026

Various apps that use files with this extension

These apps are known to open certain types of CMI files. Remember, different programs may use CMI files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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