OCaml has a similar compilation model to C or C++. (Indeed, the OCaml compiler will let you link the two together.) However, it’s complicated by the fact that it can target both bytecode and native code. As a result, there are a lot of possible file extensions, and it’s not obvious what they mean.

There are a lot of pages on the internet that specify some of the extensions that I’ve encountered in practice, but no single page documents all of them. I’m documenting here what I’ve found about each of them.

Update 2022-03-18: The online book Real World OCaml now has a summary of file extensions.

Bytecode vs native

OCaml can be compiled to bytecode or native code. See Real World OCaml, Chapter 23 for more detail.

Bytecode:

  • Portable, as long as the runtime exists on the target system.
  • Probably faster to compile.
  • Slower to execute.
  • Can be used in conjunction with ocamldebug (e.g. to support time-traveling debugging).

Native code:

  • Non-portable.
  • Probably slower to compile.
  • Faster to execute.
  • Can’t be used with ocamldebug.
    • Note that a traditional debugger like gdb will still work.
    • You can choose to compile OCaml to native code with debugging symbols using the -g flag to ocamlopt. This is required to preserve stack traces.

File extensions

Source files:

  • .ml: Implementation file.
  • .mli: Interface file.

Compiled module interface:

  • .cmi (both bytecode and native).
  • This appears to be used to describe the signature of a module, but not contain its implementation. The analog in C or C++ might be a precompiled header.

Compiled module object code:

  • Bytecode: .cmo
  • Native: .cmx
  • Analogous to .o files in C or C++.

Compiled module archive:

  • Bytecode: .cma
  • Native: .cmxa
  • Analogous to .a files in C or C++.

Dynamically-linkable library:

  • Native: .cmxs.
  • No obvious counterpart for bytecode.
  • Can be used with the Dynlink library.

  • Analogous to .so or .dll files in C or C++.

Documentation file:

  • Documentation for source file: .cmt.
  • Documentation for interface file: .cmti.
  • These extensions seem to be specific to tooling, and are not used to generate code.
  • It’s unclear which tools make use of these. My guesses:
    • Yes: odoc, codoc.
    • No: ocamldoc.

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