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ocamlopt - Man Page

The OCaml native-code compiler

Examples (TL;DR)


ocamlopt [ options ] filename ...

ocamlopt.opt (same options)


The OCaml high-performance native-code compiler ocamlopt(1) compiles OCaml source files to native code object files and link these object files to produce standalone executables.

The ocamlopt(1) command has a command-line interface very close to that of ocamlc(1). It accepts the same 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 ocamlopt(1) compiler produces a compiled interface in the file x.cmi. The interface produced is identical to that produced by the bytecode compiler ocamlc(1).

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 ocamlopt(1) compiler produces two files: x.o, containing native object code, and x.cmx, containing extra information for linking and optimization of the clients of the unit. The compiled implementation should always be referred to under the name x.cmx (when given a .o file, ocamlopt(1) assumes that it contains code compiled from C, not from OCaml).

The implementation is checked against the interface file x.mli (if it exists) as described in the manual for ocamlc(1).

Arguments ending in .cmx are taken to be compiled object code.  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 native-code executable program. The order in which .cmx 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.cmx file must come before all .cmx files that refer to the unit x.

Arguments ending in .cmxa are taken to be libraries of object code. Such a library packs in two files lib.cmxa and lib.a a set of object files (.cmx/.o files). Libraries are build with ocamlopt -a (see the description of the -a option below). The object files contained in the library are linked as regular .cmx files (see above), in the order specified when the library 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.

Arguments ending in .o or .a are assumed to be C object files and libraries. They are linked with the program.

The output of the linking phase is a regular Unix executable file. It does not need ocamlrun(1) to run.

ocamlopt.opt is the same compiler as ocamlopt, but compiled with itself instead of with the bytecode compiler ocamlc(1). Thus, it behaves exactly like ocamlopt, but compiles faster. ocamlopt.opt is not available in all installations of OCaml.


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


Build a library (.cmxa/.a file) with the object files (.cmx/.o files) given on the command line, instead of linking them into an executable file. The name of the library must be set with the -o option.

If -cclib or -ccopt options are passed on the command line, these options are stored in the resulting .cmxa library.  Then, linking with this library automatically adds back the -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.


Show absolute filenames in error messages.


Do not try to show absolute filenames in error messages.


Deprecated since OCaml 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.


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.

-cc ccomp

Use ccomp as the C linker called to build the final executable and as the C compiler for compiling .c source files.

-cclib -llibname

Pass the -llibname option to the linker. This causes the given C library to be linked with the program.

-ccopt option

Pass the given option to the C compiler and linker. 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 environment variable "OCAML_COLOR" is considered if -color is not provided. Its values are auto/always/never as above.

If -color is not provided, "OCAML_COLOR" is not set and the environment variable "NO_COLOR" is set, then color output is disabled. Otherwise, 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.

-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.


Optimize the produced code for space rather than for time. This results in smaller but slightly slower programs. The default is to optimize for speed.


Print the version number of ocamlopt(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.

-depend ocamldep-args

Compute dependencies, as ocamldep would do.

-for-pack module-path

Generate an object file (.cmx and .o files) that can later be included as a sub-module (with the given access path) of a compilation unit constructed with -pack. For instance, ocamlopt -for-pack P -c A.ml will generate a.cmx and a.o files that can later be used with ocamlopt -pack -o P.cmx a.cmx.


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


Do not record debugging information (default).


Cause the compiler to print all defined names (with their inferred types or their definitions) when compiling an implementation (.ml file). No compiled files (.cmo and .cmi files) are produced. 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.

-cmi-file filename

Type-check the source implementation to be compiled against the specified interface file (by-passes the normal lookup for .mli and .cmi files).

-I directory

Add the given directory to the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmx), and libraries (.cmxa). 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. See also option -nostdlib.

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

Compile the file filename as an implementation file, even if its extension is not .ml.

-inline n

Set aggressiveness of inlining to n, where n is a positive integer. Specifying -inline 0 prevents all functions from being inlined, except those whose body is smaller than the call site. Thus, inlining causes no expansion in code size. The default aggressiveness, -inline 1, allows slightly larger functions to be inlined, resulting in a slight expansion in code size. Higher values for the -inline option cause larger and larger functions to become candidate for inlining, but can result in a serious increase in code size.


Enables the instruction scheduling pass in the compiler backend.

-intf filename

Compile the file filename as an interface file, even if its extension is not .mli.

-intf-suffix string

Recognize file names ending with string as interface files (instead of the default .mli).


Keep documentation strings in generated .cmi files.


Keep locations in generated .cmi files.


Labels are not ignored in types, labels may be used in applications, and labelled parameters can be given in any order.  This is the default.


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. 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.


Use linear scan register allocation.  Compiling with this allocator is faster than with the usual graph coloring allocator, sometimes quite drastically so for long functions and modules. On the other hand, the generated code can be a bit slower.


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.


Deactivates the applicative behaviour of functors. With this option, each functor application generates new types in its result and applying the same functor twice to the same argument yields two incompatible structures.


Do not compile assertion checks.  Note that the special form assert false is always compiled because it is typed specially. This flag has no effect when linking already-compiled files.


When linking .cmxa libraries, ignore -cclib and -ccopt options potentially contained in the libraries (if these options were given when building the libraries).  This can be useful if a library contains incorrect specifications of C libraries or C options; in this case, during linking, set -noautolink and pass the correct C libraries and options on the command line.


Allow the compiler to use some optimizations that are valid only for code that is never dynlinked.


Disables the instruction scheduling pass in the compiler backend.


Do not automatically add the standard library directory to the list of directories searched for compiled interface files (.cmi), compiled object code files (.cmx), and libraries (.cmxa). See also option -I.


Ignore non-optional labels in types. Labels cannot be used in applications, and parameter order becomes strict.

-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. If the -shared option is given, specify the name of plugin 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.


When compiling a .mli interface file, this has the same effect as the -opaque option of the bytecode compiler. When compiling a .ml implementation file, this produces a .cmx file without cross-module optimization information, which reduces recompilation on module change.

-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 an 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 compiled shared/dynamic library (.so extension). -output-complete-obj Same as -output-obj except the object file produced includes the runtime and autolink libraries.


Build an object file (.cmx and .o files) and its associated compiled interface (.cmi) that combines the .cmx object files given on the command line, making them appear as sub-modules of the output .cmx file.  The name of the output .cmx file must be given with the -o option.  For instance, ocamlopt -pack -o P.cmx A.cmx B.cmx C.cmx generates compiled files P.cmx, P.o and P.cmi describing a compilation unit having three sub-modules A, B and C, corresponding to the contents of the object files A.cmx, B.cmx and C.cmx.  These contents can be referenced as P.A, P.B and P.C in the remainder of the program.

The .cmx object files being combined must have been compiled with the appropriate -for-pack option.  In the example above, A.cmx, B.cmx and C.cmx must have been compiled with ocamlopt -for-pack P.

Multiple levels of packing can be achieved by combining -pack with -for-pack. See The OCaml user's manual, chapter "Native-code compilation" for more details.

-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.

-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.


Check information path during type-checking, to make sure that all types are derived in a principal way. All programs accepted in -principal mode are also accepted in default mode with equivalent types, but different binary signatures.


Do not check principality of type inference. This is the default.


Allow arbitrary recursive types during type-checking.  By default, only recursive types where the recursion goes through an object type are supported. Note that once you have created an interface using this flag, you must use it again for all dependencies.


Do no allow arbitrary recursive types during type-checking. This is the default.

-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.


Keep the assembly code produced during the compilation. The assembly code for the source file x.ml is saved in the file x.s.


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


Do not use type information to optimize pattern-matching. This allows to detect match failures even if a pattern-matching was wrongly assumed to be exhaustive. This only impacts GADT and polymorphic variant compilation.

-save-ir-after pass

Save intermediate representation after the given compilation pass. The currently supported passes are: scheduling.


Build a plugin (usually .cmxs) that can be dynamically loaded with the Dynlink module. The name of the plugin must be set with the -o option. A plugin can include a number of OCaml modules and libraries, and extra native objects (.o, .a files). Building native plugins is only supported for some operating system. Under some systems (currently, only Linux AMD 64), all the OCaml code linked in a plugin must have been compiled without the -nodynlink flag. Some constraints might also apply to the way the extra native objects have been compiled (under Linux AMD 64, they must contain only position-independent code).


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.

-stop-after pass

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


The left-hand part of a sequence must have type unit.


Left-hand part of a sequence need not have type unit. This is the default.


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.


Turn bound checking off for array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds. Additionally, turn off the check for zero divisor in integer division and modulus operations. With -unsafe, an integer division (or modulus) by zero can halt the program or continue with an unspecified result instead of raising a Division_by_zero exception.


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.


Print the version number of the compiler and the location of the standard library directory, then exit.


Print all external commands before they are executed, in particular invocations of the assembler, C compiler, and linker.

-version or -vnum

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. See ocamlc(1) for the syntax of warning-list.

-warn-error warning-list

Mark as fatal 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 (no warning 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

Process file as a file name, even if it starts with a dash (-) character.

-help or --help

Display a short usage summary and exit.

Options for the Flambda Middle-End

When the Flambda code generator has been enabled at configuration time, its behavior may be tuned up with the following additional options:


Perform more optimisation than usual. Compilation times may be lengthened.


Perform even more optimisation than usual, possibly including unrolling of recursive functions. Compilation times may be significantly lengthened.


Makes inlining decisions at the point of definition of a function rather than at the call site(s). This mirrors the behaviour of OCaml compilers not using Flambda. Compared to compilation using the new Flambda inlining heuristics (for example at -O2) it produces smaller .cmx files, shorter compilation times and code that probably runs rather slower.


Emit .inlining files (one per round of optimisation) showing all of the inliner's decisions.

Options for the Amd64 Architecture

The AMD64 code generator (64-bit versions of Intel Pentium and AMD Athlon) supports the following additional options:


Generate position-independent machine code.  This is the default.


Generate position-dependent machine code.

See Also

The OCaml user's manual, chapter "Native-code compilation".

Referenced By

guestfs-release-notes-1.26(1), nbdkit-ocaml-plugin(3), ocaml(1), ocamlc(1), ocamlcp(1), ocamldep(1), ocamldoc(1).

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