CMI File – What is .cmi file and how to open it?

CMI File Format

A CMI file, or Compiled OCaml Interface, is a binary file format that stores the compiled interface for OCaml, a functional programming language. It contains information about the types, values, and functions defined in an OCaml module, allowing programs to link with and use the module without requiring the original source code. CMI files are generated by the OCaml compiler and are essential for building and linking OCaml programs.

Benefits of Using CMI Files

CMI files provide several benefits for OCaml development. Firstly, they speed up compilation and linking times by caching the interface information for a module. This is particularly useful for large projects with numerous modules, as it eliminates the need to recompile the entire project each time a single module is modified. Secondly, CMI files enhance code modularity by allowing modules to be developed and compiled independently. This promotes code reuse and enables developers to easily combine modules from different sources into a single program.

Understanding CMI Files

CMI files are Compiled OCaml Interface files used in the OCaml programming language. They contain precompiled information about OCaml modules, such as type definitions, function signatures, and global values. CMI files are generated by the OCaml compiler (ocamlc) when compiling OCaml source code (.ml files).

By storing precompiled information in CMI files, OCaml can avoid the need to recompile the same module multiple times. This can significantly speed up the compilation process, especially for large projects with many modules. Additionally, CMI files can be shared between different OCaml programs, allowing for code reuse and modular development.

Opening CMI Files

CMI files are not directly executable and cannot be opened like regular text files. Instead, they are used as inputs to the OCaml linker (ocamlc -linkpkg) to create executable programs (.exe or .native) or libraries (.cma). The linker reads the CMI files and combines the precompiled information into a single executable or library file.

To open a CMI file, you can use the OCaml linker command with the -linkpkg option followed by the path to the CMI file. For example:


ocamlc -linkpkg my_module.cmi


This command will create an executable file named “my_module” that contains the compiled code from the CMI file. You can then run the executable file to execute the OCaml code.

CMI File Format

The CMI file format stores compiled OCaml interfaces. OCaml is a functional programming language developed by Inria. An interface is a contract between two modules that specifies the functions and values that the modules provide to each other. The CMI file format is a binary format that stores the following information about an interface:

• The name of the interface
• The names of the functions and values provided by the interface
• The types of the arguments and results of the functions and values

CMI files are used by the OCaml compiler to check that the modules that use the interface conform to the contract specified by the interface. The compiler uses the information in the CMI file to generate code that calls the functions and values provided by the interface.

Use Cases of CMI Files

CMI files are used in a variety of situations, including:

• When compiling OCaml modules that use an interface
• When linking OCaml modules that use an interface
• When generating documentation for an interface

CMI files are an important part of the OCaml programming language. They help to ensure that modules that use an interface conform to the contract specified by the interface. This helps to prevent errors and makes it easier to develop and maintain OCaml programs.

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