ocamlc - The Objective Caml bytecode compiler
ocamlc [ options ] filename ...
ocamlc.opt [ options ] filename ...
The Objective Caml bytecode compiler ocamlc(1) compiles Caml 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:
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
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).
Arguments ending in .so are assumed to be C shared libraries (DLLs).
During linking, they are searched for external C functions referenced
from the Caml 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 Objective Caml 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
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 Objective Caml.
The following command-line options are recognized by ocamlc(1).
-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 must be set with the -o
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.
-annot Dump detailed information about the compilation (types,
bindings, tail-calls, etc). The information for file src.ml is
put into file src.annot. In case of a type error, dump all the
information inferred by the type-checker before the error. The
src.annot file can be used with the emacs commands given in
emacs/caml-types.el to display types and other annotations
-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
Use ccomp as the C linker when linking in "custom runtime" mode
(see the -custom option) and as the C compiler for compiling .c
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 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
Print the version number of ocamlc(1) and a detailed summary of
its configuration, then exit.
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 Caml code with user-defined C
Never use the strip(1) command on executables produced by
ocamlc -custom, this would remove the bytecode part of the
Arrange for the C shared library dlllibname.so to be loaded
dynamically by the run-time system ocamlrun(1) at program start-
Adds the directory dir to the run-time search path for shared C
libraries. At link-time, shared libraries are searched in the
standard search path (the one corresponding to the -I option).
The -dllpath option simply stores dir in the produced executable
file, where ocamlrun(1) can find it and use it.
-g 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 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.
Add the given 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 -l xxx. 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.
If the given directory starts with +, it is taken relative to
the standard library directory. For instance, -I +labltk adds
the subdirectory labltk of the standard library to the search
Compile the file filename as an implementation file, even if its
extension is not .ml.
Compile the file filename as an interface file, even if its
extension is not .mli.
Recognize file names ending with string as interface files
(instead of the default .mli).
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 (option -a), setting the -linkall option
forces all subsequent links of programs involving that library
to link all the modules contained in the library.
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.
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 .cma libraries, ignore -custom, -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.
Ignore non-optional labels in types. Labels cannot be used in
applications, and parameter order becomes strict.
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.
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. The name of the output
object file is camlprog.o by default; it can 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
-pack Build a bytecode object file (.cmo file) and its associated
compiled interface (.cmi) that combines the object files given
on the command line, making them appear as sub-modules of the
output .cmo file. The name of the output .cmo file must be
given with the -o option. For instance,
ocamlc -pack -o p.cmo a.cmo b.cmo c.cmo generates compiled files
p.cmo and p.cmi describing a compilation unit having three sub-
modules A, B and C, corresponding to the contents of the object
files a.cmo, b.cmo and c.cmo. These contents can be referenced
as P.A, P.B and P.C in the remainder of the program.
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.
Check information path during type-checking, to make sure that
all types are derived in a principal way. When using labelled
arguments and/or polymorphic methods, this flag is required to
ensure future versions of the compiler will be able to infer
types correctly, even if internal algorithms change. All
programs accepted in -principal mode are also accepted in the
default mode with equivalent types, but different binary
signatures, and this may slow down type checking; yet it is a
good idea to use it once before publishing source code.
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
Compile or link multithreaded programs, in combination with the
system "threads" library described in
The Objective Caml users manual.
Turn bound checking off for array and string accesses (the
v.(i)ands.[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.
Generate a bytecode executable file that can be executed on the
custom runtime system runtime-name, built earlier with
ocamlc -make-runtime runtime-name.
-v 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 C compiler and linker in -custom
mode. Useful to debug C library problems.
Print the version number of the compiler in short form (e.g.
"3.11.0"), then exit.
Compile or link multithreaded programs, in combination with the
VM-level threads library described in
The Objective Caml users manual.
Enable or disable warnings according to the argument
warning-list. The argument is a set of letters. If a letter is
uppercase, it enables the corresponding warnings; lowercase
disables the warnings. The correspondence is the following:
A all warnings
C start of comments that look like mistakes
D use of deprecated features
E fragile pattern matchings (matchings that will remain
complete even if additional constructors are added to one of the
variant types matched)
F partially applied functions (expressions whose result has
function type and is ignored)
L omission of labels in applications
M overriding of methods
P missing cases in pattern matchings (i.e. partial matchings)
S expressions in the left-hand side of a sequence that don’t
have type unit (and that are not functions, see F above)
U redundant cases in pattern matching (unused cases)
V overriding of instance variables
Y unused variables that are bound with let or as, and don’t
start with an underscore (_) character
Z all other cases of unused variables that don’t start with an
underscore (_) character
X warnings that don’t fit in the above categories (except A)
The default setting is -w Aelz, enabling all warnings except
fragile pattern matchings, omitted labels, and innocuous unused
variables. Note that warnings F and S are not always triggered,
depending on the internals of the type checker.
Turn the warnings indicated in the argument warning-list into
errors. 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: an uppercase character turns the
corresponding warning into an error, a lowercase character
leaves it as a warning. The default setting is -warn-error a
(none of the warnings is treated as an error).
-where Print the location of the standard library, then exit.
- file Process file as a file name, even if it starts with a dash (-)
-help or --help
Display a short usage summary and exit.
ocamlopt(1), ocamlrun(1), ocaml(1).
The Objective Caml users manual, chapter "Batch compilation".