ocamlc - The OCaml bytecode compiler
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 exe-
cutable 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 corre-
sponding 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
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 descrip-
tion 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
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 exe-
cuted 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 argu-
ments 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 man-
ages 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).
-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 option.
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 automat-
ically adds back the -custom, -cclib and -ccopt options as if they had been pro-
vided 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 interactively.
Has been deprecated. Please use -annot instead.
-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 use-
ful to compile modules separately.
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.
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
-ccopt Pass the given option to the C compiler and linker, when linking in "custom run-
time" mode (see the -custom option). For instance, -ccopt -Ldir causes the C linker
to search for C libraries in directory dir.
Print the version number of ocamlc(1) and a detailed summary of its configuration,
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, ocaml-
run(1). In the custom runtime mode, the linker produces an output file that con-
tains 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.
Arrange for the C shared library dlllibname.so to be loaded dynamically by the run-
time system ocamlrun(1) at program start-up time.
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 cor-
responding to the -I option). The -dllpath option simply stores dir in the pro-
duced 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 back-
traces 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 declara-
tions 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 path.
Compile the file filename as an implementation file, even if its extension is not
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
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
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 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).
-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 com-
piled. If there are no compilation errors, the intermediate file is deleted after-
wards. 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)
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 meth-
ods, 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 depen-
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 sup-
ported and gives a debug version of the runtime.
The left-hand part of a sequence must have type unit.
Compile or link multithreaded programs, in combination with the system "threads"
library described in The OCaml user's manual.
Turn bound checking off for array and string accesses (the v.(i)ands.[i] con-
structs). Programs compiled with -unsafe are therefore slightly faster, but unsafe:
anything can happen if the program accesses an array or string outside of its
Generate a bytecode executable file that can be executed on the custom runtime sys-
tem 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.
-vnum or -version
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 OCaml user's manual.
Enable, disable, or mark as errors the warnings specified by the argument warn-
Each warning can be enabled or disabled, and each warning can be marked (as error)
or unmarked. If a warning is disabled, it isn't displayed and doesn't affect com-
pilation in any way (even if it is marked). If a warning is enabled, it is dis-
played normally by the compiler whenever the source code triggers it. If it is
enabled and marked, the compiler will stop with an error after displaying the warn-
ings if the source code triggers it.
The warning-list argument is a sequence of warning specifiers, with no separators
between them. A warning specifier is one of the following:
+num Enable warning number num.
-num Disable warning number num.
@num Enable and mark 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 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 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 syntax.
4 Fragile pattern matching: matching that will remain complete even if addi-
tional constructors are added to one of the variant types matched.
5 Partially applied function: expression whose result has function type and is
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 Camlp4 warning.
23 Useless record with clause.
24 Bad module name: the source file name is not a valid OCaml module name.
25 Pattern-matching with all clauses guarded.
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 (_) pat-
29 A non-escaped end-of-line was found in a string constant. This may cause
portability problems between Unix and Windows.
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
K 32, 33, 34, 35, 36, 37
U 11, 12
X 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30
The default setting is -w +a-4-6-9-27-29-32..39. Note that warnings 5 and 10 are
not always triggered, depending on the internals of the type checker.
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) turns
the corresponding warnings into errors, a - sign (or a lowercase letter) turns them
back into warnings, and a @ sign both enables and marks 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.
The default setting is -warn-error -a (none of the warnings is treated as an
-where Print the location of the standard library, then exit.
- 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.
ocamlopt(1), ocamlrun(1), ocaml(1).
The OCaml user's manual, chapter "Batch compilation".