ocamlc - Man Page

The OCaml bytecode compiler

Examples (TL;DR)


ocamlc [ options ] filename ...

ocamlc.opt [ options ] filename ...


The OCaml bytecode compiler ocamlc(1) compiles OCaml source files to bytecode object files and links these object files to produce standalone bytecode executable files. These executable files are then run by the bytecode interpreter ocamlrun(1).

The ocamlc(1) command has a command-line interface similar to the one of most C compilers. It accepts several types of arguments and processes them sequentially, after all options have been processed:

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(1) 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(1) 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 OCaml 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).

Arguments ending in .so are assumed to be C shared libraries (DLLs).  During linking, they are searched for external C functions referenced from the OCaml code, and their names are written in the generated bytecode executable. The run-time system ocamlrun(1) then loads them dynamically at program start-up time.

The output of the linking phase is a file containing compiled bytecode that can be executed by the OCaml bytecode interpreter: the command ocamlrun(1). If caml.out is the name of the file produced by the linking phase, the command ocamlrun caml.out arg1  arg2 ... argn executes the compiled code contained in caml.out, passing it as arguments the character strings arg1 to argn. (See ocamlrun(1) for more details.)

On most systems, the file produced by the linking phase can be run directly, as in: ./caml.out arg1  arg2 ... argn. The produced file has the executable bit set, and it manages to launch the bytecode interpreter by itself.

ocamlc.opt is the same compiler as ocamlc, but compiled with the native-code compiler ocamlopt(1). Thus, it behaves exactly like ocamlc, but compiles faster. ocamlc.opt may not be available in all installations of OCaml.


The following command-line options are recognized by ocamlc(1).



If -custom-cclib or -ccopt options are passed on the command line, these options are stored in the resulting .cma library.  Then, linking with this library automatically adds back the -custom-cclib and -ccopt options as if they had been provided on the command line, unless the -noautolink option is given. Additionally, a substring $CAMLORIGIN inside a  -ccopt options will be replaced by the full path to the .cma library, excluding the filename. -absname Show absolute filenames in error messages.


Deprecated since 4.11. Please use -bin-annot instead.


Dump detailed information about the compilation (types, bindings, tail-calls, etc) in binary format. The information for file src.ml is put into file src.cmt. In case of a type error, dump all the information inferred by the type-checker before the error. The annotation files produced by -bin-annot contain more information and are much more compact than the files produced by -annot.



-cc ccomp

Use ccomp as the C linker when linking in "custom runtime" mode (see the -custom option) and as the C compiler for compiling .c source files.

-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.

-color mode

Enable or disable colors in compiler messages (especially warnings and errors). The following modes are supported:

auto use heuristics to enable colors only if the output supports them (an ANSI-compatible tty terminal);

always enable colors unconditionally;

never disable color output.

The default setting is auto, and the current heuristic checks that the "TERM" environment variable exists and is not empty or "dumb", and that isatty(stderr) holds.

The environment variable "OCAML_COLOR" is considered if -color is not provided. Its values are auto/always/never as above.

-error-style mode

Control the way error messages and warnings are printed. The following modes are supported:

short only print the error and its location;

contextual like "short", but also display the source code snippet corresponding to the location of the error.

The default setting is contextual.

The environment variable "OCAML_ERROR_STYLE" is considered if -error-style is not provided. Its values are short/contextual as above.


Check that the generated bytecode executable can run on 32-bit platforms and signal an error if it cannot. This is useful when compiling bytecode on a 64-bit machine.


Print the version number of ocamlc(1) and a detailed summary of its configuration, then exit.


Print the value of a specific configuration variable from the -config output, then exit. If the variable does not exist, the exit code is non-zero.


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(1). 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(1) command is not installed. Moreover, the "custom runtime" mode enables linking OCaml code with user-defined C functions.

Never use the strip(1) command on executables produced by ocamlc -custom, this would remove the bytecode part of the executable.

Security warning: never set the "setuid" or "setgid" bits on executables produced by ocamlc -custom, this would make them vulnerable to attacks.

-depend ocamldep-args

Compute dependencies, as ocamldep would do.

-dllib -llibname

Arrange for the C shared library dlllibname.so to be loaded dynamically by the run-time system ocamlrun(1) at program start-up time.

-dllpath dir


-for-pack module-path

Generate an object file (.cmo file) that can later be included as a sub-module (with the given access path) of a compilation unit constructed with -pack. For instance, ocamlc -for-pack P -c A.ml will generate a.cmo that can later be used with ocamlc -pack -o P.cmo a.cmo. Note: you can still pack a module that was compiled without -for-pack but in this case exceptions will be printed with the wrong names.


Add debugging information while compiling and linking. This option is required in order to be able to debug the program with ocamldebug(1) and to produce stack backtraces when the program terminates on an uncaught exception.



-I directory


If the given directory starts with +, it is taken relative to the standard library directory. For instance, -I +compiler-libs adds the subdirectory compiler-libs of the standard library to the search path.

-impl filename


-intf filename


-intf-suffix string



Keep documentation strings in generated .cmi files.


Keep locations in generated .cmi files.




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 (option -a), setting the -linkall option forces all subsequent links of programs involving that library to link all the modules contained in the library. When compiling a module (option -c), setting the -linkall option ensures that this module will always be linked if it is put in a library and this library is linked.


Build a custom runtime system (in the file specified by option -o) incorporating the C object files and libraries given on the command line.  This custom runtime system can be used later to execute bytecode executables produced with the option ocamlc -use-runtime runtime-name.


Set number of rows of context used during pattern matching compilation. Lower values cause faster compilation, but less optimized code. The default value is 32.


Do not record dependencies for module aliases.










Do not automatically add the standard library directory to the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmo), libraries (.cma), and C libraries specified with -cclib -lxxx . See also option -I.

-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 -pack option is given, specify the name of the packed object file produced.  If the -output-obj option is given, specify the name of the output file produced. This can also be used when compiling an interface or implementation file, without linking, in which case it sets the name of the cmi or cmo file, and also sets the module name to the file name up to the first dot.


Interface file compiled with this option are marked so that other compilation units depending on it will not rely on any implementation details of the compiled implementation. The native compiler will not access the .cmx file of this unit -- nor warn if it is absent. This can improve speed of compilation, for both initial and incremental builds, at the expense of performance of the generated code.

-open module

Opens the given module before processing the interface or implementation files. If several -open options are given, they are processed in order, just as if the statements open! module1;; ... open! moduleN;; were added at the top of each file.


Cause the linker to produce a C object file instead of a bytecode executable file. This is useful to wrap OCaml code as a C library, callable from any C program. The name of the output object file must be set with the -o option. This option can also be used to produce a C source file (.c extension) or a compiled shared/dynamic library (.so extension).



-pp command


-ppx command

After parsing, pipe the abstract syntax tree through the preprocessor command. The module Ast_mapper(3) implements the external interface of a preprocessor.





-runtime-variant suffix

Add suffix to the name of the runtime library that will be used by the program. If OCaml was configured with option -with-debug-runtime, then the d suffix is supported and gives a debug version of the runtime.

-stop-after pass

Stop compilation after the given compilation pass. The currently supported passes are: parsing, typing.


Enforce the separation between types string and bytes, thereby making strings read-only. This is the default.


When a type is visible under several module-paths, use the shortest one when printing the type's name in inferred interfaces and error and warning messages.




When a type is unboxable (i.e. a record with a single argument or a concrete datatype with a single constructor of one argument) it will be unboxed unless annotated with [@@ocaml.boxed].


When a type is unboxable  it will be boxed unless annotated with [@@ocaml.unboxed]. This is the default.




Identify the types string and bytes, thereby making strings writable. This is intended for compatibility with old source code and should not be used with new software.

-use-runtime runtime-name






-vnum or -version

Print the version number of the compiler in short form (e.g. "3.11.0"), then exit.

-w warning-list

Enable, disable, or mark as fatal the warnings specified by the argument warning-list.

Each warning can be enabled or disabled, and each warning can be fatalor non-fatal. If a warning is disabled, it isn't displayed and doesn't affect compilation in any way (even if it is fatal).  If a warning is enabled, it is displayed normally by the compiler whenever the source code triggers it.  If it is enabled and fatal, the compiler will also stop with an error after displaying it.


+num   Enable warning number num.

-num   Disable warning number num.

@num   Enable and mark as fatal warning number num.

+num1..num2   Enable all warnings between num1 and num2 (inclusive).

-num1..num2   Disable all warnings between num1 and num2 (inclusive).

@num1..num2   Enable and mark as fatal all warnings between num1 and num2 (inclusive).

+letter   Enable the set of warnings corresponding to letter. The letter may be uppercase or lowercase.

-letter   Disable the set of warnings corresponding to letter. The letter may be uppercase or lowercase.

@letter   Enable and mark as fatal the set of warnings corresponding to letter. The letter may be uppercase or lowercase.

uppercase-letter   Enable the set of warnings corresponding to uppercase-letter.

lowercase-letter   Disable the set of warnings corresponding to lowercase-letter.

The warning numbers are as follows.

1    Suspicious-looking start-of-comment mark.

2    Suspicious-looking end-of-comment mark.

3    Deprecated feature.


5    Partially applied function: expression whose result has function type and is ignored.

6    Label omitted in function application.

7    Method overridden without using the "method!" keyword.

8    Partial match: missing cases in pattern-matching.

9    Missing fields in a record pattern.

10   Expression on the left-hand side of a sequence that doesn't have type unit (and that is not a function, see warning number 5).

11   Redundant case in a pattern matching (unused match case).

12   Redundant sub-pattern in a pattern-matching.

13   Override of an instance variable.

14   Illegal backslash escape in a string constant.

15   Private method made public implicitly.

16   Unerasable optional argument.

17   Undeclared virtual method.

18   Non-principal type.

19   Type without principality.

20   Unused function argument.

21   Non-returning statement.

22   Preprocessor warning.

23   Useless record with clause.

24   Bad module name: the source file name is not a valid OCaml module name.

25   Deprecated: now part of warning 8.

26   Suspicious unused variable: unused variable that is bound with let or as, and doesn't start with an underscore (_) character.

27   Innocuous unused variable: unused variable that is not bound with let nor as, and doesn't start with an underscore (_) character.

28   A pattern contains a constant constructor applied to the underscore (_) pattern.

29   A non-escaped end-of-line was found in a string constant.  This may cause portability problems between Unix and Windows.

30   Two labels or constructors of the same name are defined in two mutually recursive types.

31   A module is linked twice in the same executable.

32   Unused value declaration.

33   Unused open statement.

34   Unused type declaration.

35   Unused for-loop index.

36   Unused ancestor variable.

37   Unused constructor.

38   Unused extension constructor.

39   Unused rec flag.

40   Constructor or label name used out of scope.

41   Ambiguous constructor or label name.

42   Disambiguated constructor or label name.

43   Nonoptional label applied as optional.

44   Open statement shadows an already defined identifier.

45   Open statement shadows an already defined label or constructor.

46   Error in environment variable.

47   Illegal attribute payload.

48   Implicit elimination of optional arguments.

49   Missing cmi file when looking up module alias.

50   Unexpected documentation comment.

59   Assignment on non-mutable value.

60   Unused module declaration.

61   Unannotated unboxable type in primitive declaration.

62   Type constraint on GADT type declaration.

63   Erroneous printed signature.

64   -unsafe used with a preprocessor returning a syntax tree.

65   Type declaration defining a new '()' constructor.

66   Unused open! statement.

67   Unused functor parameter.

The letters stand for the following sets of warnings.  Any letter not mentioned here corresponds to the empty set.

A  all warnings

C  1, 2

D  3

E  4

F  5

K  32, 33, 34, 35, 36, 37, 38, 39

L  6

M  7

P  8

R  9

S  10

U  11, 12

V  13


Y  26

Z  27

The default setting is -w +a-4-6-7-9-27-29-30-32..42-44-45-48-50-60-66. Note that warnings 5 and 10 are not always triggered, depending on the internals of the type checker.

-warn-error warning-list

Mark as errors the warnings specified in the argument warning-list. The compiler will stop with an error when one of these warnings is emitted.  The warning-list has the same meaning as for the -w option: a + sign (or an uppercase letter) marks the corresponding warnings as fatal, a - sign (or a lowercase letter) turns them back into non-fatal warnings, and a @ sign both enables and marks as fatal the corresponding warnings.

Note: it is not recommended to use the -warn-error option in production code, because it will almost certainly prevent compiling your program with later versions of OCaml when they add new warnings or modify existing warnings.

The default setting is -warn-error -a+31 (only warning 31 is fatal).


Show the description of all available warning numbers.


Print the location of the standard library, then exit.


Include the runtime system in the generated program. This is the default.


The compiler does not include the runtime system (nor a reference to it) in the generated program; it must be supplied separately.

- file


-help or --help

Display a short usage summary and exit.

See Also

The OCaml user's manual, chapter "Batch compilation".

Referenced By

camlp5(1), coqdep(1), guestfs-release-notes-1.26(1), libnbd-release-notes-1.4(1), ocaml(1), ocamlcp(1), ocamldebug(1), ocamldep(1), ocamldoc(1), ocamlmktop(1), ocamlopt(1), ocamlrun(1).

The man page ocamlc.opt(1) is an alias of ocamlc(1).