Batch compilation (ocamlc)

This chapter describes the Objective Caml batch compiler ocamlc, which compiles Caml source files to bytecode object files and link these object files to produce standalone bytecode executable files. These executable files are then run by the bytecode interpreter ocamlrun.

Overview of the compiler

The ocamlc command has a command-line interface similar to the one of most C compilers. It accepts several types of arguments:

Arguments ending in .mli are taken to be source files for compilation unit interfaces. Interfaces specify the names exported by compilation units: they declare value names with their types, define public data types, declare abstract data types, and so on. From the file x.mli, the ocamlc compiler produces a compiled interface in the file x.cmi.
Arguments ending in .ml are taken to be source files for compilation unit implementations. Implementations provide definitions for the names exported by the unit, and also contain expressions to be evaluated for their side-effects. From the file x.ml, the ocamlc compiler produces compiled object bytecode in the file x.cmo.
If the interface file x.mli exists, the implementation x.ml is checked against the corresponding compiled interface x.cmi, which is assumed to exist. If no interface x.mli is provided, the compilation of x.ml produces a compiled interface file x.cmi in addition to the compiled object code file x.cmo. The file x.cmi produced corresponds to an interface that exports everything that is defined in the implementation x.ml.
Arguments ending in .cmo are taken to be compiled object bytecode. These files are linked together, along with the object files obtained by compiling .ml arguments (if any), and the Caml Light standard library, to produce a standalone executable program. The order in which .cmo and .ml arguments are presented on the command line is relevant: compilation units are initialized in that order at run-time, and it is a link-time error to use a component of a unit before having initialized it. Hence, a given x.cmo file must come before all .cmo files that refer to the unit x.
Arguments ending in .cma are taken to be libraries of object bytecode. A library of object bytecode packs in a single file a set of object bytecode files (.cmo files). Libraries are built with ocamlc -a (see the description of the -a option below). The object files contained in the library are linked as regular .cmo files (see above), in the order specified when the .cma file was built. The only difference is that if an object file contained in a library is not referenced anywhere in the program, then it is not linked in.
Arguments ending in .c are passed to the C compiler, which generates a .o object file. This object file is linked with the program if the -custom flag is set (see the description of -custom below).
Arguments ending in .o or .a are assumed to be C object files and libraries. They are passed to the C linker when linking in -custom mode (see the description of -custom below).

The output of the linking phase is a file containing compiled bytecode that can be executed by the Objective Caml bytecode interpreter: the command named ocamlrun. If caml.out is the name of the file produced by the linking phase, the command

        ocamlrun caml.out arg₁ arg₂ ... arg_n

executes the compiled code contained in caml.out, passing it as arguments the character strings arg₁ to arg_n. (See chapter 9 for more details.)

On most Unix systems, the file produced by the linking phase can be run directly, as in:

        ./caml.out arg₁ arg₂ ... arg_n

The produced file has the executable bit set, and it manages to launch the bytecode interpreter by itself.

Options

The following command-line options are recognized by ocamlc.

-a: Build a library (.cma file) with the object files (.cmo files) given on the command line, instead of linking them into an executable file. The name of the library can be set with the -o option. The default name is library.cma.
-c: Compile only. Suppress the linking phase of the compilation. Source code files are turned into compiled files, but no executable file is produced. This option is useful to compile modules separately.
-cclib -llibname: Pass the -llibname option to the C linker when linking in ``custom runtime'' mode (see the -custom option). This causes the given C library to be linked with the program.
-ccopt option: Pass the given option to the C compiler and linker, when linking in ``custom runtime'' mode (see the -custom option). For instance, -ccopt -Ldir causes the C linker to search for C libraries in directory dir.
-custom: Link in ``custom runtime'' mode. In the default linking mode, the linker produces bytecode that is intended to be executed with the shared runtime system, ocamlrun. In the custom runtime mode, the linker produces an output file that contains both the runtime system and the bytecode for the program. The resulting file is larger, but it can be executed directly, even if the ocamlrun command is not installed. Moreover, the ``custom runtime'' mode enables linking Caml code with user-defined C functions, as described in chapter 15.
-g: Add debugging information while compiling and linking. This option is required in order to be able to debug the program with ocamldebug (see chapter 13).
-i: Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (.ml file). This can be useful to check the types inferred by the compiler. Also, since the output follows the syntax of interfaces, it can help in writing an explicit interface (.mli file) for a file: just redirect the standard output of the compiler to a .mli file, and edit that file to remove all declarations of unexported names.
-I directory: Add the given directory to the list of directories searched for compiled interface files (.cmi) and compiled object code files (.cmo). By default, the current directory is searched first, then the standard library directory. Directories added with -I are searched after the current directory, in the order in which they were given on the command line, but before the standard library directory.
-impl filename: Compile the file filename as an implementation file, even if its extension is not .ml.
-intf filename: Compile the file filename as an interface file, even if its extension is not .mli.
-linkall: Force all modules contained in libraries to be linked in. If this flag is not given, unreferenced modules are not linked in. When building a library (-a flag), setting the -linkall flag forces all subsequent links of programs involving that library to link all the modules contained in the library.
-noassert: Turn assertion checking off: assertions are not compiled. This flag has no effect when linking already compiled files.
-o exec-file: Specify the name of the output file produced by the linker. The default output name is a.out, in keeping with the Unix tradition. If the -a option is given, specify the name of the library produced. If the -output-obj option is given, specify the name of the output file produced.
-output-obj: Cause the linker to produce a C object file instead of a bytecode executable file. This is useful to wrap Caml code as a C library, callable from any C program. See chapter 15, section 15.6. The name of the output object file is camlprog.o by default; it can be set with the -o option.
-pp command: Cause the compiler to call the given command as a preprocessor for each source file. The output of command is redirected to an intermediate file, which is compiled. If there are no compilation errors, the intermediate file is deleted afterwards. The name of this file is built from the basename of the source file with the extension .ppi for an interface (.mli) file and .ppo for an implementation (.ml) file.
-thread: Compile or link multithreaded programs, in combination with the threads library described in chapter 21. What this option actually does is select a special, thread-safe version of the standard library.
-unsafe: Turn bound checking off on array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds.
-v: Print the version number of the compiler.

Modules and the file system

This short section is intended to clarify the relationship between the names of the modules corresponding to compilation units and the names of the files that contain their compiled interface and compiled implementation.

The compiler always derives the module name by taking the capitalized base name of the source file (.ml or .mli file). That is, it strips the leading directory name, if any, as well as the .ml or .mli suffix; then, it set the first letter to uppercase, in order to comply with the requirement that module names must be capitalized. For instance, compiling the file mylib/misc.ml provides an implementation for the module named Misc. Other compilation units may refer to components defined in mylib/misc.ml under the names Misc.name; they can also do open Misc, then use unqualified names name.

The .cmi and .cmo files produced by the compiler have the same base name as the source file. Hence, the compiled files always have their base name equal (modulo capitalization of the first letter) to the name of the module they describe (for .cmi files) or implement (for .cmo files).

When the compiler encounters a reference to a free module identifier Mod, it looks in the search path for a file mod.cmi (note lowercasing of first letter) and loads the compiled interface contained in that file. As a consequence, renaming .cmi files is not advised: the name of a .cmi file must always correspond to the name of the compilation unit it implements. It is admissible to move them to another directory, if their base name is preserved, and the correct -I options are given to the compiler. The compiler will flag an error if it loads a .cmi file that has been renamed.

Compiled bytecode files (.cmo files), on the other hand, can be freely renamed once created. That's because the linker never attempts to find by itself the .cmo file that implements a module with a given name: it relies instead on the user providing the list of .cmo files by hand.

Common errors

This section describes and explains the most frequently encountered error messages.

Cannot find file filename

The named file could not be found in the current directory, nor in the directories of the search path. The filename is either a compiled interface file (.cmi file), or a compiled bytecode file (.cmo file). If filename has the format mod.cmi, this means you are trying to compile a file that references identifiers from module mod, but you have not yet compiled an interface for module mod. Fix: compile mod.mli or mod.ml first, to create the compiled interface mod.cmi.

If filename has the format mod.cmo, this means you are trying to link a bytecode object file that does not exist yet. Fix: compile mod.ml first.

If your program spans several directories, this error can also appear because you haven't specified the directories to look into. Fix: add the correct -I options to the command line.

Corrupted compiled interface filename

The compiler produces this error when it tries to read a compiled interface file (.cmi file) that has the wrong structure. This means something went wrong when this .cmi file was written: the disk was full, the compiler was interrupted in the middle of the file creation, and so on. This error can also appear if a .cmi file is modified after its creation by the compiler. Fix: remove the corrupted .cmi file, and rebuild it.

This expression has type t₁, but is used with type t₂

This is by far the most common type error in programs. Type t₁ is the type inferred for the expression (the part of the program that is displayed in the error message), by looking at the expression itself. Type t₂ is the type expected by the context of the expression; it is deduced by looking at how the value of this expression is used in the rest of the program. If the two types t₁ and t₂ are not compatible, then the error above is produced.

In some cases, it is hard to understand why the two types t₁ and t₂ are incompatible. For instance, the compiler can report that ``expression of type foo cannot be used with type foo'', and it really seems that the two types foo are compatible. This is not always true. Two type constructors can have the same name, but actually represent different types. This can happen if a type constructor is redefined. Example:

        type foo = A | B
        let f = function A -> 0 | B -> 1
        type foo = C | D
        f C

This result in the error message ``expression C of type foo cannot be used with type foo''.

The type of this expression, t, contains type variables that cannot be generalized

Type variables ('a, 'b, ...) in a type t can be in either of two states: generalized (which means that the type t is valid for all possible instantiations of the variables) and not generalized (which means that the type t is valid only for one instantiation of the variables). In a let binding let name = expr, the type-checker normally generalizes as many type variables as possible in the type of expr. However, this leads to unsoundness (a well-typed program can crash) in conjunction with polymorphic mutable data structures. To avoid this, generalization is performed at let bindings only if the bound expression expr belongs to the class of ``syntactic values'', which includes constants, identifiers, functions, tuples of syntactic values, etc. In all other cases (for instance, expr is a function application), a polymorphic mutable could have been created and generalization is therefore turned off.

Non-generalized type variables in a type cause no difficulties inside a given structure or compilation unit (the contents of a .ml file, or an interactive session), but they cannot be allowed inside signatures nor in compiled interfaces (.cmi file), because they could be used inconsistently later. Therefore, the compiler flags an error when a structure or compilation unit defines a value name whose type contains non-generalized type variables. There are two ways to fix this error:

Add a type constraint or a .mli file to give a monomorphic type (without type variables) to name. For instance, instead of writing

    let sort_int_list = Sort.list (<)
    (* inferred type 'a list -> 'a list, with 'a not generalized *)

write

    let sort_int_list = (Sort.list (<) : int list -> int list);;

If you really need name to have a polymorphic type, turn its defining expression into a function by adding an extra parameter. For instance, instead of writing

    let map_length = List.map Array.length
    (* inferred type 'a array list -> int list, with 'a not generalized *)

write

    let map_length lv = List.map Array.length lv

Reference to undefined global mod

This error appears when trying to link an incomplete or incorrectly ordered set of files. Either you have forgotten to provide an implementation for the compilation unit named mod on the command line (typically, the file named mod.cmo, or a library containing that file). Fix: add the missing .ml or .cmo file to the command line. Or, you have provided an implementation for the module named mod, but it comes too late on the command line: the implementation of mod must come before all bytecode object files that reference mod. Fix: change the order of .ml and .cmo files on the command line.

Of course, you will always encounter this error if you have mutually recursive functions across modules. That is, function Mod1.f calls function Mod2.g, and function Mod2.g calls function Mod1.f. In this case, no matter what permutations you perform on the command line, the program will be rejected at link-time. Fixes:

Put f and g in the same module.

Parameterize one function by the other. That is, instead of having

mod1.ml:    let f x = ... Mod2.g ...
mod2.ml:    let g y = ... Mod1.f ...

define

mod1.ml:    let f g x = ... g ...
mod2.ml:    let rec g y = ... Mod1.f g ...

and link mod1.cmo before mod2.cmo.

Use a reference to hold one of the two functions, as in :

mod1.ml:    let forward_g =
                ref((fun x -> failwith "forward_g") : <type>)
            let f x = ... !forward_g ...
mod2.ml:    let g y = ... Mod1.f ...
            let _ = Mod1.forward_g := g

This will not work if g is a polymorphic function, however.

The external function f is not available

This error appears when trying to link code that calls external functions written in C in ``default runtime'' mode. As explained in chapter 15, such code must be linked in ``custom runtime'' mode. Fix: add the -custom option, as well as the C libraries and C object files that implement the required external functions.