Chapter 22 Interfacing C with OCaml

This chapter describes how user-defined primitives, written in C, can be linked with OCaml code and called from OCaml functions, and how these C functions can call back to OCaml code.

1 Overview and compilation information

1.1 Declaring primitives

definition::= ...
external-declaration::=string-literal [ string-literal [ string-literal ] ]

User primitives are declared in an implementation file or structend module expression using the external keyword:

        external name : type = C-function-name

This defines the value name name as a function with type type that executes by calling the given C function. For instance, here is how the seek_in primitive is declared in the standard library module Stdlib:

        external seek_in : in_channel -> int -> unit = "caml_ml_seek_in"

Primitives with several arguments are always curried. The C function does not necessarily have the same name as the ML function.

External functions thus defined can be specified in interface files or sigend signatures either as regular values

        val name : type

thus hiding their implementation as C functions, or explicitly as “manifest” external functions

        external name : type = C-function-name

The latter is slightly more efficient, as it allows clients of the module to call directly the C function instead of going through the corresponding OCaml function. On the other hand, it should not be used in library modules if they have side-effects at toplevel, as this direct call interferes with the linker’s algorithm for removing unused modules from libraries at link-time.

The arity (number of arguments) of a primitive is automatically determined from its OCaml type in the external declaration, by counting the number of function arrows in the type. For instance, seek_in above has arity 2, and the caml_ml_seek_in C function is called with two arguments. Similarly,

    external seek_in_pair: in_channel * int -> unit = "caml_ml_seek_in_pair"

has arity 1, and the caml_ml_seek_in_pair C function receives one argument (which is a pair of OCaml values).

Type abbreviations are not expanded when determining the arity of a primitive. For instance,

        type int_endo = int -> int
        external f : int_endo -> int_endo = "f"
        external g : (int -> int) -> (int -> int) = "f"

f has arity 1, but g has arity 2. This allows a primitive to return a functional value (as in the f example above): just remember to name the functional return type in a type abbreviation.

The language accepts external declarations with one or two flag strings in addition to the C function’s name. These flags are reserved for the implementation of the standard library.

1.2 Implementing primitives

User primitives with arity n ≤ 5 are implemented by C functions that take n arguments of type value, and return a result of type value. The type value is the type of the representations for OCaml values. It encodes objects of several base types (integers, floating-point numbers, strings, ‍…) as well as OCaml data structures. The type value and the associated conversion functions and macros are described in detail below. For instance, here is the declaration for the C function implementing the In_channel.input primitive, which takes 4 arguments:

CAMLprim value input(value channel, value buffer, value offset, value length)

When the primitive function is applied in an OCaml program, the C function is called with the values of the expressions to which the primitive is applied as arguments. The value returned by the function is passed back to the OCaml program as the result of the function application.

User primitives with arity greater than 5 should be implemented by two C functions. The first function, to be used in conjunction with the bytecode compiler ocamlc, receives two arguments: a pointer to an array of OCaml values (the values for the arguments), and an integer which is the number of arguments provided. The other function, to be used in conjunction with the native-code compiler ocamlopt, takes its arguments directly. For instance, here are the two C functions for the 7-argument primitive Nat.add_nat:

CAMLprim value add_nat_native(value nat1, value ofs1, value len1,
                              value nat2, value ofs2, value len2,
                              value carry_in)
CAMLprim value add_nat_bytecode(value * argv, int argn)
  return add_nat_native(argv[0], argv[1], argv[2], argv[3],
                        argv[4], argv[5], argv[6]);

The names of the two C functions must be given in the primitive declaration, as follows:

        external name : type =
                 bytecode-C-function-name native-code-C-function-name

For instance, in the case of add_nat, the declaration is:

        external add_nat: nat -> int -> int -> nat -> int -> int -> int -> int
                        = "add_nat_bytecode" "add_nat_native"

Implementing a user primitive is actually two separate tasks: on the one hand, decoding the arguments to extract C values from the given OCaml values, and encoding the return value as an OCaml value; on the other hand, actually computing the result from the arguments. Except for very simple primitives, it is often preferable to have two distinct C functions to implement these two tasks. The first function actually implements the primitive, taking native C values as arguments and returning a native C value. The second function, often called the “stub code”, is a simple wrapper around the first function that converts its arguments from OCaml values to C values, calls the first function, and converts the returned C value to an OCaml value. For instance, here is the stub code for the Int64.float_of_bits primitive:

CAMLprim value caml_int64_float_of_bits(value vi)
  return caml_copy_double(caml_int64_float_of_bits_unboxed(Int64_val(vi)));

(Here, caml_copy_double and Int64_val are conversion functions and macros for the type value, that will be described later. The CAMLprim macro expands to the required compiler directives to ensure that the function is exported and accessible from OCaml.) The hard work is performed by the function caml_int64_float_of_bits_unboxed, which is declared as:

double caml_int64_float_of_bits_unboxed(int64_t i)

To write C code that operates on OCaml values, the following include files are provided:

Include fileProvides
caml/mlvalues.hdefinition of the value type, and conversion macros
caml/alloc.hallocation functions (to create structured OCaml objects)
caml/memory.hmiscellaneous memory-related functions and macros (for GC interface, in-place modification of structures, etc).
caml/fail.hfunctions for raising exceptions (see section ‍22.4.5)
caml/callback.hcallback from C to OCaml (see section ‍22.7).
caml/custom.hoperations on custom blocks (see section ‍22.9).
caml/intext.hoperations for writing user-defined serialization and deserialization functions for custom blocks (see section ‍22.9).
caml/threads.hoperations for interfacing in the presence of multiple threads (see section ‍22.12).

These files reside in the caml/ subdirectory of the OCaml standard library directory, which is returned by the command ocamlc -where (usually /usr/local/lib/ocaml or /usr/lib/ocaml).

The OCaml runtime system comprises three main parts: the bytecode interpreter, the memory manager, and a set of C functions that implement the primitive operations. Some bytecode instructions are provided to call these C functions, designated by their offset in a table of functions (the table of primitives).

In the default mode, the OCaml linker produces bytecode for the standard runtime system, with a standard set of primitives. References to primitives that are not in this standard set result in the “unavailable C primitive” error. (Unless dynamic loading of C libraries is supported – see section ‍22.1.4 below.)

In the “custom runtime” mode, the OCaml linker scans the object files and determines the set of required primitives. Then, it builds a suitable runtime system, by calling the native code linker with:

This builds a runtime system with the required primitives. The OCaml linker generates bytecode for this custom runtime system. The bytecode is appended to the end of the custom runtime system, so that it will be automatically executed when the output file (custom runtime + bytecode) is launched.

To link in “custom runtime” mode, execute the ocamlc command with:

If you are using the native-code compiler ocamlopt, the -custom flag is not needed, as the final linking phase of ocamlopt always builds a standalone executable. To build a mixed OCaml/C executable, execute the ocamlopt command with:

Starting with Objective Caml 3.00, it is possible to record the -custom option as well as the names of C libraries in an OCaml library file .cma or .cmxa. For instance, consider an OCaml library mylib.cma, built from the OCaml object files a.cmo and b.cmo, which reference C code in libmylib.a. If the library is built as follows:

        ocamlc -a -o mylib.cma -custom a.cmo b.cmo -cclib -lmylib

users of the library can simply link with mylib.cma:

        ocamlc -o myprog mylib.cma ...

and the system will automatically add the -custom and -cclib -lmylib options, achieving the same effect as

        ocamlc -o myprog -custom a.cmo b.cmo ... -cclib -lmylib

The alternative is of course to build the library without extra options:

        ocamlc -a -o mylib.cma a.cmo b.cmo

and then ask users to provide the -custom and -cclib -lmylib options themselves at link-time:

        ocamlc -o myprog -custom mylib.cma ... -cclib -lmylib

The former alternative is more convenient for the final users of the library, however.

Starting with Objective Caml 3.03, an alternative to static linking of C code using the -custom code is provided. In this mode, the OCaml linker generates a pure bytecode executable (no embedded custom runtime system) that simply records the names of dynamically-loaded libraries containing the C code. The standard OCaml runtime system ocamlrun then loads dynamically these libraries, and resolves references to the required primitives, before executing the bytecode.

This facility is currently available on all platforms supported by OCaml except Cygwin 64 bits.

To dynamically link C code with OCaml code, the C code must first be compiled into a shared library (under Unix) or DLL (under Windows). This involves 1- compiling the C files with appropriate C compiler flags for producing position-independent code (when required by the operating system), and 2- building a shared library from the resulting object files. The resulting shared library or DLL file must be installed in a place where ocamlrun can find it later at program start-up time (see section ‍15.3). Finally (step 3), execute the ocamlc command with

Do not set the -custom flag, otherwise you’re back to static linking as described in section ‍22.1.3. The ocamlmklib tool (see section ‍22.14) automates steps 2 and 3.

As in the case of static linking, it is possible (and recommended) to record the names of C libraries in an OCaml .cma library archive. Consider again an OCaml library mylib.cma, built from the OCaml object files a.cmo and b.cmo, which reference C code in If the library is built as follows:

        ocamlc -a -o mylib.cma a.cmo b.cmo -dllib -lmylib

users of the library can simply link with mylib.cma:

        ocamlc -o myprog mylib.cma ...

and the system will automatically add the -dllib -lmylib option, achieving the same effect as

        ocamlc -o myprog a.cmo b.cmo ... -dllib -lmylib

Using this mechanism, users of the library mylib.cma do not need to know that it references C code, nor whether this C code must be statically linked (using -custom) or dynamically linked.

1.5 Choosing between static linking and dynamic linking

After having described two different ways of linking C code with OCaml code, we now review the pros and cons of each, to help developers of mixed OCaml/C libraries decide.

The main advantage of dynamic linking is that it preserves the platform-independence of bytecode executables. That is, the bytecode executable contains no machine code, and can therefore be compiled on platform A and executed on other platforms B, C, …, as long as the required shared libraries are available on all these platforms. In contrast, executables generated by ocamlc -custom run only on the platform on which they were created, because they embark a custom-tailored runtime system specific to that platform. In addition, dynamic linking results in smaller executables.

Another advantage of dynamic linking is that the final users of the library do not need to have a C compiler, C linker, and C runtime libraries installed on their machines. This is no big deal under Unix and Cygwin, but many Windows users are reluctant to install Microsoft Visual C just to be able to do ocamlc -custom.

There are two drawbacks to dynamic linking. The first is that the resulting executable is not stand-alone: it requires the shared libraries, as well as ocamlrun, to be installed on the machine executing the code. If you wish to distribute a stand-alone executable, it is better to link it statically, using ocamlc -custom -ccopt -static or ocamlopt -ccopt -static. Dynamic linking also raises the “DLL hell” problem: some care must be taken to ensure that the right versions of the shared libraries are found at start-up time.

The second drawback of dynamic linking is that it complicates the construction of the library. The C compiler and linker flags to compile to position-independent code and build a shared library vary wildly between different Unix systems. Also, dynamic linking is not supported on all Unix systems, requiring a fall-back case to static linking in the Makefile for the library. The ocamlmklib command (see section ‍22.14) tries to hide some of these system dependencies.

In conclusion: dynamic linking is highly recommended under the native Windows port, because there are no portability problems and it is much more convenient for the end users. Under Unix, dynamic linking should be considered for mature, frequently used libraries because it enhances platform-independence of bytecode executables. For new or rarely-used libraries, static linking is much simpler to set up in a portable way.

1.6 Building standalone custom runtime systems

It is sometimes inconvenient to build a custom runtime system each time OCaml code is linked with C libraries, like ocamlc -custom does. For one thing, the building of the runtime system is slow on some systems (that have bad linkers or slow remote file systems); for another thing, the platform-independence of bytecode files is lost, forcing to perform one ocamlc -custom link per platform of interest.

An alternative to ocamlc -custom is to build separately a custom runtime system integrating the desired C libraries, then generate “pure” bytecode executables (not containing their own runtime system) that can run on this custom runtime. This is achieved by the -make-runtime and -use-runtime flags to ocamlc. For example, to build a custom runtime system integrating the C parts of the “Unix” and “Threads” libraries, do:

        ocamlc -make-runtime -o /home/me/ocamlunixrun unix.cma threads.cma

To generate a bytecode executable that runs on this runtime system, do:

        ocamlc -use-runtime /home/me/ocamlunixrun -o myprog \
                unix.cma threads.cma your .cmo and .cma files

The bytecode executable myprog can then be launched as usual: myprog args or /home/me/ocamlunixrun myprog args.

Notice that the bytecode libraries unix.cma and threads.cma must be given twice: when building the runtime system (so that ocamlc knows which C primitives are required) and also when building the bytecode executable (so that the bytecode from unix.cma and threads.cma is actually linked in).

2 The value type

All OCaml objects are represented by the C type value, defined in the include file caml/mlvalues.h, along with macros to manipulate values of that type. An object of type value is either:

2.1 Integer values

Integer values encode 63-bit signed integers (31-bit on 32-bit architectures). They are unboxed (unallocated).

2.2 Blocks

Blocks in the heap are garbage-collected, and therefore have strict structure constraints. Each block includes a header containing the size of the block (in words), and the tag of the block. The tag governs how the contents of the blocks are structured. A tag lower than No_scan_tag indicates a structured block, containing well-formed values, which is recursively traversed by the garbage collector. A tag greater than or equal to No_scan_tag indicates a raw block, whose contents are not scanned by the garbage collector. For the benefit of ad-hoc polymorphic primitives such as equality and structured input-output, structured and raw blocks are further classified according to their tags as follows:

TagContents of the block
0 to No_scan_tag−1A structured block (an array of OCaml objects). Each field is a value.
Closure_tagA closure representing a functional value. The first word is a pointer to a piece of code, the remaining words are value containing the environment.
String_tagA character string or a byte sequence.
Double_tagA double-precision floating-point number.
Double_array_tagAn array or record of double-precision floating-point numbers.
Abstract_tagA block representing an abstract datatype.
Custom_tagA block representing an abstract datatype with user-defined finalization, comparison, hashing, serialization and deserialization functions attached.

2.3 Pointers outside the heap

In earlier versions of OCaml, it was possible to use word-aligned pointers to addresses outside the heap as OCaml values, just by casting the pointer to type value. This usage is no longer supported since OCaml 5.0.

A correct way to manipulate pointers to out-of-heap blocks from OCaml is to store those pointers in OCaml blocks with tag Abstract_tag or Custom_tag, then use the blocks as the OCaml values.

Here is an example of encapsulation of out-of-heap pointers of C type ty * inside Abstract_tag blocks. Section ‍22.6 gives a more complete example using Custom_tag blocks.

/* Create an OCaml value encapsulating the pointer p */
static value val_of_typtr(ty * p)
  value v = caml_alloc(1, Abstract_tag);
  *((ty **) Data_abstract_val(v)) = p;
  return v;

/* Extract the pointer encapsulated in the given OCaml value */
static ty * typtr_of_val(value v)
  return *((ty **) Data_abstract_val(v));

Alternatively, out-of-heap pointers can be treated as “native” integers, that is, boxed 32-bit integers on a 32-bit platform and boxed 64-bit integers on a 64-bit platform.

/* Create an OCaml value encapsulating the pointer p */
static value val_of_typtr(ty * p)
  return caml_copy_nativeint((intnat) p);

/* Extract the pointer encapsulated in the given OCaml value */
static ty * typtr_of_val(value v)
  return (ty *) Nativeint_val(v);

For pointers that are at least 2-aligned (the low bit is guaranteed to be zero), we have yet another valid representation as an OCaml tagged integer.

/* Create an OCaml value encapsulating the pointer p */
static value val_of_typtr(ty * p)
  assert (((uintptr_t) p & 1) == 0);  /* check correct alignment */
  return (value) p | 1;

/* Extract the pointer encapsulated in the given OCaml value */
static ty * typtr_of_val(value v)
  return (ty *) (v & ~1);

3 Representation of OCaml data types

This section describes how OCaml data types are encoded in the value type.

3.1 Atomic types

OCaml typeEncoding
intUnboxed integer values.
charUnboxed integer values (ASCII code).
floatBlocks with tag Double_tag.
bytesBlocks with tag String_tag.
stringBlocks with tag String_tag.
int32Blocks with tag Custom_tag.
int64Blocks with tag Custom_tag.
nativeintBlocks with tag Custom_tag.

3.2 Tuples and records

Tuples are represented by pointers to blocks, with tag ‍0.

Records are also represented by zero-tagged blocks. The ordering of labels in the record type declaration determines the layout of the record fields: the value associated to the label declared first is stored in field ‍0 of the block, the value associated to the second label goes in field ‍1, and so on.

As an optimization, records whose fields all have static type float are represented as arrays of floating-point numbers, with tag Double_array_tag. (See the section below on arrays.)

As another optimization, unboxable record types are represented specially; unboxable record types are the immutable record types that have only one field. An unboxable type will be represented in one of two ways: boxed or unboxed. Boxed record types are represented as described above (by a block with tag 0 or Double_array_tag). An unboxed record type is represented directly by the value of its field (i.e. there is no block to represent the record itself).

The representation is chosen according to the following, in decreasing order of priority:

3.3 Arrays

Arrays of integers and pointers are represented like tuples and records, that is, as pointers to blocks tagged ‍0. They are accessed with the Field macro for reading and the caml_modify function for writing.

Values of type floatarray (as manipulated by the Float.Array module), as well as records whose declaration contains only float fields, use an efficient unboxed representation: blocks with tag Double_array_tag whose content consist of raw double values, which are not themselves valid OCaml values. They should be accessed using the Double_flat_field and Store_double_flat_field macros.

Finally, arrays of type float array may use either the boxed or the unboxed representation depending on the how the compiler is configured. They currently use the unboxed representation by default, but can be made to use the boxed representation by passing the --disable-flat-float-array flag to the ‘configure‘ script. They should be accessed using the Double_array_field and Store_double_array_field macros, which will work correctly under both modes.

3.4 Concrete data types

Constructed terms are represented either by unboxed integers (for constant constructors) or by blocks whose tag encode the constructor (for non-constant constructors). The constant constructors and the non-constant constructors for a given concrete type are numbered separately, starting from 0, in the order in which they appear in the concrete type declaration. A constant constructor is represented by the unboxed integer equal to its constructor number. A non-constant constructor declared with n arguments is represented by a block of size n, tagged with the constructor number; the n fields contain its arguments. Example:

Constructed termRepresentation
h::tBlock with size = 2 and tag = 0; first field contains h, second field t.

As a convenience, caml/mlvalues.h defines the macros Val_unit, Val_false, Val_true and Val_emptylist to refer to (), false, true and [].

The following example illustrates the assignment of integers and block tags to constructors:

type t =
  | A             (* First constant constructor -> integer "Val_int(0)" *)
  | B of string   (* First non-constant constructor -> block with tag 0 *)
  | C             (* Second constant constructor -> integer "Val_int(1)" *)
  | D of bool     (* Second non-constant constructor -> block with tag 1 *)
  | E of t * t    (* Third non-constant constructor -> block with tag 2 *)

As an optimization, unboxable concrete data types are represented specially; a concrete data type is unboxable if it has exactly one constructor and this constructor has exactly one argument. Unboxable concrete data types are represented in the same ways as unboxable record types: see the description in section ‍22.3.2.

3.5 Objects

Objects are represented as blocks with tag Object_tag. The first field of the block refers to the object’s class and associated method suite, in a format that cannot easily be exploited from C. The second field contains a unique object ID, used for comparisons. The remaining fields of the object contain the values of the instance variables of the object. It is unsafe to access directly instance variables, as the type system provides no guarantee about the instance variables contained by an object.

One may extract a public method from an object using the C function caml_get_public_method (declared in <caml/mlvalues.h>.) Since public method tags are hashed in the same way as variant tags, and methods are functions taking self as first argument, if you want to do the method call foo#bar from the C side, you should call:

  callback(caml_get_public_method(foo, hash_variant("bar")), foo);

3.6 Polymorphic variants

Like constructed terms, polymorphic variant values are represented either as integers (for polymorphic variants without argument), or as blocks (for polymorphic variants with an argument). Unlike constructed terms, variant constructors are not numbered starting from 0, but identified by a hash value (an OCaml integer), as computed by the C function hash_variant (declared in <caml/mlvalues.h>): the hash value for a variant constructor named, say, VConstr is hash_variant("VConstr").

The variant value `VConstr is represented by hash_variant("VConstr"). The variant value `VConstr(v) is represented by a block of size 2 and tag 0, with field number 0 containing hash_variant("VConstr") and field number 1 containing v.

Unlike constructed values, polymorphic variant values taking several arguments are not flattened. That is, `VConstr(v, w) is represented by a block of size 2, whose field number 1 contains the representation of the pair (v, w), rather than a block of size 3 containing v and w in fields 1 and 2.

4 Operations on values

4.1 Kind tests

4.2 Operations on integers

4.3 Accessing blocks

The expressions Field(v, n), Byte(v, n) and Byte_u(v, n) are valid l-values. Hence, they can be assigned to, resulting in an in-place modification of value v. Assigning directly to Field(v, n) must be done with care to avoid confusing the garbage collector (see below).

4.4 Allocating blocks

Simple interface

Low-level interface

The following functions are slightly more efficient than caml_alloc, but also much more difficult to use.

From the standpoint of the allocation functions, blocks are divided according to their size as zero-sized blocks, small blocks (with size less than or equal to Max_young_wosize), and large blocks (with size greater than Max_young_wosize). The constant Max_young_wosize is declared in the include file mlvalues.h. It is guaranteed to be at least 64 (words), so that any block with constant size less than or equal to 64 can be assumed to be small. For blocks whose size is computed at run-time, the size must be compared against Max_young_wosize to determine the correct allocation procedure.

4.5 Raising exceptions

Two functions are provided to raise two standard exceptions:

Raising arbitrary exceptions from C is more delicate: the exception identifier is dynamically allocated by the OCaml program, and therefore must be communicated to the C function using the registration facility described below in section ‍22.7.3. Once the exception identifier is recovered in C, the following functions actually raise the exception:

5 Living in harmony with the garbage collector

Unused blocks in the heap are automatically reclaimed by the garbage collector. This requires some cooperation from C code that manipulates heap-allocated blocks.

5.1 Simple interface

All the macros described in this section are declared in the memory.h header file.

Rule ‍1 A function that has parameters or local variables of type value must begin with a call to one of the CAMLparam macros and return with CAMLreturn, CAMLreturn0, or CAMLreturnT.

There are six CAMLparam macros: CAMLparam0 to CAMLparam5, which take zero to five arguments respectively. If your function has no more than 5 parameters of type value, use the corresponding macros with these parameters as arguments. If your function has more than 5 parameters of type value, use CAMLparam5 with five of these parameters, and use one or more calls to the CAMLxparam macros for the remaining parameters (CAMLxparam1 to CAMLxparam5).

The macros CAMLreturn, CAMLreturn0, and CAMLreturnT are used to replace the C keyword return; any function using a CAMLparam macro on entry should use CAMLreturn on all exit points. C functions exported as OCaml externals must return a value, and they should use CAMLreturn (x) instead of return x. Some helper functions may manipulate OCaml values yet return void or another datatype. void-returning procedures should explicitly use CAMLreturn0, and not have any implicit return. Helper functions returning C data of some type t should use CAMLreturnT (t, x) instead of return x.


Some C compilers give bogus warnings about unused variables caml__dummy_xxx at each use of CAMLparam and CAMLlocal. You should ignore them.


CAMLprim value my_external (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);
  CAMLreturn (Val_unit);

static void helper_procedure (value v1, value v2)
  CAMLparam2 (v1, v2);

static int helper_function (value v1, value v2)
  CAMLparam2 (v1, v2);
  CAMLreturnT (int, 0);

If your function is a primitive with more than 5 arguments for use with the byte-code runtime, its arguments are not values and must not be declared (they have types value * and int).


CAMLreturn0 should only be used for internal procedures that return void. CAMLreturn(Val_unit) should be used for functions that return an OCaml unit value. Primitives (C functions that can be called from OCaml) should never return void.

Rule ‍2 Local variables of type value must be declared with one of the CAMLlocal macros. Arrays of values are declared with CAMLlocalN. These macros must be used at the beginning of the function, not in a nested block.

The macros CAMLlocal1 to CAMLlocal5 declare and initialize one to five local variables of type value. The variable names are given as arguments to the macros. CAMLlocalN(x, n) declares and initializes a local variable of type value [n]. You can use several calls to these macros if you have more than 5 local variables.


CAMLprim value bar (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = caml_alloc (3, 0);
  CAMLreturn (result);

CAMLlocal (and CAMLxparam) can only be called after CAMLparam. If a function declares local values but takes no value argument, it should start with CAMLparam0 ().

static value foo (int n)
  CAMLparam0 ();;
  CAMLlocal (result);
  CAMLreturn (result);
Rule ‍3 Assignments to the fields of structured blocks must be done with the Store_field macro (for normal blocks), Store_double_array_field macro (for float array values) or Store_double_flat_field (for floatarray values and records of floating-point numbers). Other assignments must not use Store_field, Store_double_array_field nor Store_double_flat_field.

Store_field (b, n, v) stores the value v in the field number n of value b, which must be a block (i.e. Is_block(b) must be true).


CAMLprim value bar (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = caml_alloc (3, 0);
  Store_field (result, 0, v1);
  Store_field (result, 1, v2);
  Store_field (result, 2, v3);
  CAMLreturn (result);

The first argument of Store_field and Store_double_field must be a variable declared by CAMLparam* or a parameter declared by CAMLlocal* to ensure that a garbage collection triggered by the evaluation of the other arguments will not invalidate the first argument after it is computed.

Use with CAMLlocalN:

Arrays of values declared using CAMLlocalN must not be written to using Store_field. Use the normal C array syntax instead.

Rule ‍4 Global variables containing values must be registered with the garbage collector using the caml_register_global_root function, save that global variables and locations that will only ever contain OCaml integers (and never pointers) do not have to be registered.

The same is true for any memory location outside the OCaml heap that contains a value and is not guaranteed to be reachable—for as long as it contains such value—from either another registered global variable or location, local variable declared with CAMLlocal or function parameter declared with CAMLparam.

Registration of a global variable v is achieved by calling caml_register_global_root(&v) just before or just after a valid value is stored in v for the first time; likewise, registration of an arbitrary location p is achieved by calling caml_register_global_root(p).

You must not call any of the OCaml runtime functions or macros between registering and storing the value. Neither must you store anything in the variable v (likewise, the location p) that is not a valid value.

The registration causes the contents of the variable or memory location to be updated by the garbage collector whenever the value in such variable or location is moved within the OCaml heap. In the presence of threads care must be taken to ensure appropriate synchronisation with the OCaml runtime to avoid a race condition against the garbage collector when reading or writing the value. (See section 22.12.2.)

A registered global variable v can be un-registered by calling caml_remove_global_root(&v).

If the contents of the global variable v are seldom modified after registration, better performance can be achieved by calling caml_register_generational_global_root(&v) to register v (after its initialization with a valid value, but before any allocation or call to the GC functions), and caml_remove_generational_global_root(&v) to un-register it. In this case, you must not modify the value of v directly, but you must use caml_modify_generational_global_root(&v,x) to set it to x. The garbage collector takes advantage of the guarantee that v is not modified between calls to caml_modify_generational_global_root to scan it less often. This improves performance if the modifications of v happen less often than minor collections.


The CAML macros use identifiers (local variables, type identifiers, structure tags) that start with caml__. Do not use any identifier starting with caml__ in your programs.

5.2 Low-level interface

We now give the GC rules corresponding to the low-level allocation functions caml_alloc_small and caml_alloc_shr. You can ignore those rules if you stick to the simplified allocation function caml_alloc.

Rule ‍5 After a structured block (a block with tag less than No_scan_tag) is allocated with the low-level functions, all fields of this block must be filled with well-formed values before the next allocation operation. If the block has been allocated with caml_alloc_small, filling is performed by direct assignment to the fields of the block:

        Field(v, n) = vn;
If the block has been allocated with caml_alloc_shr, filling is performed through the caml_initialize function:

        caml_initialize(&Field(v, n), vn);

The next allocation can trigger a garbage collection. The garbage collector assumes that all structured blocks contain well-formed values. Newly created blocks contain random data, which generally do not represent well-formed values.

If you really need to allocate before the fields can receive their final value, first initialize with a constant value (e.g. Val_unit), then allocate, then modify the fields with the correct value (see rule ‍6).

Rule ‍6 Direct assignment to a field of a block, as in

        Field(v, n) = w;
is safe only if v is a block newly allocated by caml_alloc_small; that is, if no allocation took place between the allocation of v and the assignment to the field. In all other cases, never assign directly. If the block has just been allocated by caml_alloc_shr, use caml_initialize to assign a value to a field for the first time:

        caml_initialize(&Field(v, n), w);
Otherwise, you are updating a field that previously contained a well-formed value; then, call the caml_modify function:

        caml_modify(&Field(v, n), w);

To illustrate the rules above, here is a C function that builds and returns a list containing the two integers given as parameters. First, we write it using the simplified allocation functions:

value alloc_list_int(int i1, int i2)
  CAMLparam0 ();
  CAMLlocal2 (result, r);

  r = caml_alloc(2, 0);                   /* Allocate a cons cell */
  Store_field(r, 0, Val_int(i2));         /* car = the integer i2 */
  Store_field(r, 1, Val_emptylist);       /* cdr = the empty list [] */
  result = caml_alloc(2, 0);              /* Allocate the other cons cell */
  Store_field(result, 0, Val_int(i1));    /* car = the integer i1 */
  Store_field(result, 1, r);              /* cdr = the first cons cell */
  CAMLreturn (result);

Here, the registering of result is not strictly needed, because no allocation takes place after it gets its value, but it’s easier and safer to simply register all the local variables that have type value.

Here is the same function written using the low-level allocation functions. We notice that the cons cells are small blocks and can be allocated with caml_alloc_small, and filled by direct assignments on their fields.

value alloc_list_int(int i1, int i2)
  CAMLparam0 ();
  CAMLlocal2 (result, r);

  r = caml_alloc_small(2, 0);             /* Allocate a cons cell */
  Field(r, 0) = Val_int(i2);              /* car = the integer i2 */
  Field(r, 1) = Val_emptylist;            /* cdr = the empty list [] */
  result = caml_alloc_small(2, 0);        /* Allocate the other cons cell */
  Field(result, 0) = Val_int(i1);         /* car = the integer i1 */
  Field(result, 1) = r;                   /* cdr = the first cons cell */
  CAMLreturn (result);

In the two examples above, the list is built bottom-up. Here is an alternate way, that proceeds top-down. It is less efficient, but illustrates the use of caml_modify.

value alloc_list_int(int i1, int i2)
  CAMLparam0 ();
  CAMLlocal2 (tail, r);

  r = caml_alloc_small(2, 0);             /* Allocate a cons cell */
  Field(r, 0) = Val_int(i1);              /* car = the integer i1 */
  Field(r, 1) = Val_int(0);               /* A dummy value
  tail = caml_alloc_small(2, 0);          /* Allocate the other cons cell */
  Field(tail, 0) = Val_int(i2);           /* car = the integer i2 */
  Field(tail, 1) = Val_emptylist;         /* cdr = the empty list [] */
  caml_modify(&Field(r, 1), tail);        /* cdr of the result = tail */
  CAMLreturn (r);

It would be incorrect to perform Field(r, 1) = tail directly, because the allocation of tail has taken place since r was allocated.

5.3 Pending actions and asynchronous exceptions

Since 4.10, allocation functions are guaranteed not to run any OCaml code, including finalisers, signal handlers and other threads running on the same domain. Instead, their execution is delayed to a later safe point.

The function caml_process_pending_actions from <caml/signals.h> executes any pending signal handlers and finalisers, Memprof callbacks, preemptive systhread switching, and requested minor and major garbage collections. In particular, it can raise asynchronous exceptions and cause mutations on the OCaml heap from the same domain. It is recommended to call it regularly at safe points inside long-running non-blocking C code.

The variant caml_process_pending_actions_exn is provided, that returns the exception instead of raising it directly into OCaml code. Its result must be tested using Is_exception_result, and followed by Extract_exception if appropriate. It is typically used for clean up before re-raising:

    exn = caml_process_pending_actions_exn();
    if(Is_exception_result(exn)) {
      exn = Extract_exception(exn);

Correct use of exceptional return, in particular in the presence of garbage collection, is further detailed in Section ‍22.7.1.

6 A complete example

This section outlines how the functions from the Unix curses library can be made available to OCaml programs. First of all, here is the interface that declares the curses primitives and data types:

(* File -- declaration of primitives and data types *)
type window                   (* The type "window" remains abstract *)
external initscr: unit -> window = "caml_curses_initscr"
external endwin: unit -> unit = "caml_curses_endwin"
external refresh: unit -> unit = "caml_curses_refresh"
external wrefresh : window -> unit = "caml_curses_wrefresh"
external newwin: int -> int -> int -> int -> window = "caml_curses_newwin"
external addch: char -> unit = "caml_curses_addch"
external mvwaddch: window -> int -> int -> char -> unit = "caml_curses_mvwaddch"
external addstr: string -> unit = "caml_curses_addstr"
external mvwaddstr: window -> int -> int -> string -> unit
         = "caml_curses_mvwaddstr"
(* lots more omitted *)

To compile this interface:

        ocamlc -c

To implement these functions, we just have to provide the stub code; the core functions are already implemented in the curses library. The stub code file, curses_stubs.c, looks like this:

/* File curses_stubs.c -- stub code for curses */
#include <curses.h>
#include <caml/mlvalues.h>
#include <caml/memory.h>
#include <caml/alloc.h>
#include <caml/custom.h>

/* Encapsulation of opaque window handles (of type WINDOW *)
   as OCaml custom blocks. */

static struct custom_operations curses_window_ops = {

/* Accessing the WINDOW * part of an OCaml custom block */
#define Window_val(v) (*((WINDOW **) Data_custom_val(v)))

/* Allocating an OCaml custom block to hold the given WINDOW * */
static value alloc_window(WINDOW * w)
  value v = caml_alloc_custom(&curses_window_ops, sizeof(WINDOW *), 0, 1);
  Window_val(v) = w;
  return v;

CAMLprim value caml_curses_initscr(value unit)
  CAMLparam1 (unit);
  CAMLreturn (alloc_window(initscr()));

CAMLprim value caml_curses_endwin(value unit)
  CAMLparam1 (unit);
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_refresh(value unit)
  CAMLparam1 (unit);
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_wrefresh(value win)
  CAMLparam1 (win);
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_newwin(value nlines, value ncols, value x0, value y0)
  CAMLparam4 (nlines, ncols, x0, y0);
  CAMLreturn (alloc_window(newwin(Int_val(nlines), Int_val(ncols),
                                  Int_val(x0), Int_val(y0))));

CAMLprim value caml_curses_addch(value c)
  CAMLparam1 (c);
  addch(Int_val(c));            /* Characters are encoded like integers */
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_mvwaddch(value win, value x, value y, value c)
  CAMLparam4 (win, x, y, c);
  mvwaddch(Window_val(win), Int_val(x), Int_val(y), Int_val(c));
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_addstr(value s)
  CAMLparam1 (s);
  CAMLreturn (Val_unit);

CAMLprim value caml_curses_mvwaddstr(value win, value x, value y, value s)
  CAMLparam4 (win, x, y, s);
  mvwaddstr(Window_val(win), Int_val(x), Int_val(y), String_val(s));
  CAMLreturn (Val_unit);

/* This goes on for pages. */

The file curses_stubs.c can be compiled with:

        cc -c -I`ocamlc -where` curses_stubs.c

or, even simpler,

        ocamlc -c curses_stubs.c

(When passed a .c file, the ocamlc command simply calls the C compiler on that file, with the right -I option.)

Now, here is a sample OCaml program that uses the curses module:

(* File -- main program using curses *)
open Curses;;
let main_window = initscr () in
let small_window = newwin 10 5 20 10 in
  mvwaddstr main_window 10 2 "Hello";
  mvwaddstr small_window 4 3 "world";
  Unix.sleep 5;

To compile and link this program, run:

       ocamlc -custom -o prog unix.cma curses.cmo curses_stubs.o -cclib -lcurses

(On some machines, you may need to put -cclib -lcurses -cclib -ltermcap or -cclib -ltermcap instead of -cclib -lcurses.)

7 Advanced topic: callbacks from C to OCaml

So far, we have described how to call C functions from OCaml. In this section, we show how C functions can call OCaml functions, either as callbacks (OCaml calls C which calls OCaml), or with the main program written in C.

7.1 Applying OCaml closures from C

C functions can apply OCaml function values (closures) to OCaml values. The following functions are provided to perform the applications:

If the function f does not return, but raises an exception that escapes the scope of the application, then this exception is propagated to the next enclosing OCaml code, skipping over the C code. That is, if an OCaml function f calls a C function g that calls back an OCaml function h that raises a stray exception, then the execution of g is interrupted and the exception is propagated back into f.

If the C code wishes to catch exceptions escaping the OCaml function, it can use the functions caml_callback_exn, caml_callback2_exn, caml_callback3_exn, caml_callbackN_exn. These functions take the same arguments as their non-_exn counterparts, but catch escaping exceptions and return them to the C code. The return value v of the caml_callback*_exn functions must be tested with the macro Is_exception_result(v). If the macro returns “false”, no exception occurred, and v is the value returned by the OCaml function. If Is_exception_result(v) returns “true”, an exception escaped, and its value (the exception descriptor) can be recovered using Extract_exception(v).


If the OCaml function returned with an exception, Extract_exception should be applied to the exception result prior to calling a function that may trigger garbage collection. Otherwise, if v is reachable during garbage collection, the runtime can crash since v does not contain a valid value.


    CAMLprim value call_caml_f_ex(value closure, value arg)
      CAMLparam2(closure, arg);
      CAMLlocal2(res, tmp);
      res = caml_callback_exn(closure, arg);
      if(Is_exception_result(res)) {
        res = Extract_exception(res);
        tmp = caml_alloc(3, 0); /* Safe to allocate: res contains valid value. */
      CAMLreturn (res);

7.2 Obtaining or registering OCaml closures for use in C functions

There are two ways to obtain OCaml function values (closures) to be passed to the callback functions described above. One way is to pass the OCaml function as an argument to a primitive function. For example, if the OCaml code contains the declaration

    external apply : ('a -> 'b) -> 'a -> 'b = "caml_apply"

the corresponding C stub can be written as follows:

    CAMLprim value caml_apply(value vf, value vx)
      CAMLparam2(vf, vx);
      vy = caml_callback(vf, vx);

Another possibility is to use the registration mechanism provided by OCaml. This registration mechanism enables OCaml code to register OCaml functions under some global name, and C code to retrieve the corresponding closure by this global name.

On the OCaml side, registration is performed by evaluating Callback.register n v. Here, n is the global name (an arbitrary string) and v the OCaml value. For instance:

    let f x = print_string "f is applied to "; print_int x; print_newline()
    let _ = Callback.register "test function" f

On the C side, a pointer to the value registered under name n is obtained by calling caml_named_value(n). The returned pointer must then be dereferenced to recover the actual OCaml value. If no value is registered under the name n, the null pointer is returned. For example, here is a C wrapper that calls the OCaml function f above:

    void call_caml_f(int arg)
        caml_callback(*caml_named_value("test function"), Val_int(arg));

The pointer returned by caml_named_value is constant and can safely be cached in a C variable to avoid repeated name lookups. The value pointed to cannot be changed from C. However, it might change during garbage collection, so must always be recomputed at the point of use. Here is a more efficient variant of call_caml_f above that calls caml_named_value only once:

    void call_caml_f(int arg)
        static const value * closure_f = NULL;
        if (closure_f == NULL) {
            /* First time around, look up by name */
            closure_f = caml_named_value("test function");
        caml_callback(*closure_f, Val_int(arg));

7.3 Registering OCaml exceptions for use in C functions

The registration mechanism described above can also be used to communicate exception identifiers from OCaml to C. The OCaml code registers the exception by evaluating Callback.register_exception n exn, where n is an arbitrary name and exn is an exception value of the exception to register. For example:

    exception Error of string
    let _ = Callback.register_exception "test exception" (Error "any string")

The C code can then recover the exception identifier using caml_named_value and pass it as first argument to the functions raise_constant, raise_with_arg, and raise_with_string (described in section ‍22.4.5) to actually raise the exception. For example, here is a C function that raises the Error exception with the given argument:

    void raise_error(char * msg)
        caml_raise_with_string(*caml_named_value("test exception"), msg);

7.4 Main program in C

In normal operation, a mixed OCaml/C program starts by executing the OCaml initialization code, which then may proceed to call C functions. We say that the main program is the OCaml code. In some applications, it is desirable that the C code plays the role of the main program, calling OCaml functions when needed. This can be achieved as follows:

7.5 Embedding the OCaml code in the C code

The bytecode compiler in custom runtime mode (ocamlc -custom) normally appends the bytecode to the executable file containing the custom runtime. This has two consequences. First, the final linking step must be performed by ocamlc. Second, the OCaml runtime library must be able to find the name of the executable file from the command-line arguments. When using caml_main(argv) as in section ‍22.7.4, this means that argv[0] or argv[1] must contain the executable file name.

An alternative is to embed the bytecode in the C code. The -output-obj and -output-complete-obj options to ocamlc are provided for this purpose. They cause the ocamlc compiler to output a C object file (.o file, .obj under Windows) containing the bytecode for the OCaml part of the program, as well as a caml_startup function. The C object file produced by ocamlc -output-complete-obj also contains the runtime and autolink libraries. The C object file produced by ocamlc -output-obj or ocamlc -output-complete-obj can then be linked with C code using the standard C compiler, or stored in a C library.

The caml_startup function must be called from the main C program in order to initialize the OCaml runtime and execute the OCaml initialization code. Just like caml_main, it takes one argv parameter containing the command-line parameters. Unlike caml_main, this argv parameter is used only to initialize Sys.argv, but not for finding the name of the executable file.

The caml_startup function calls the uncaught exception handler (or enters the debugger, if running under ocamldebug) if an exception escapes from a top-level module initialiser. Such exceptions may be caught in the C code by instead using the caml_startup_exn function and testing the result using Is_exception_result (followed by Extract_exception if appropriate).

The -output-obj and -output-complete-obj options can also be used to obtain the C source file. More interestingly, these options can also produce directly a shared library (.so file, .dll under Windows) that contains the OCaml code, the OCaml runtime system and any other static C code given to ocamlc (.o, .a, respectively, .obj, .lib). This use of -output-obj and -output-complete-obj is very similar to a normal linking step, but instead of producing a main program that automatically runs the OCaml code, it produces a shared library that can run the OCaml code on demand. The three possible behaviors of -output-obj and -output-complete-obj (to produce a C source code .c, a C object file .o, a shared library .so), are selected according to the extension of the resulting file (given with -o).

The native-code compiler ocamlopt also supports the -output-obj and -output-complete-obj options, causing it to output a C object file or a shared library containing the native code for all OCaml modules on the command-line, as well as the OCaml startup code. Initialization is performed by calling caml_startup (or caml_startup_exn) as in the case of the bytecode compiler. The file produced by ocamlopt -output-complete-obj also contains the runtime and autolink libraries.

For the final linking phase, in addition to the object file produced by -output-obj, you will have to provide the OCaml runtime library (libcamlrun.a for bytecode, libasmrun.a for native-code), as well as all C libraries that are required by the OCaml libraries used. For instance, assume the OCaml part of your program uses the Unix library. With ocamlc, you should do:

        ocamlc -output-obj -o camlcode.o unix.cma other .cmo and .cma files
        cc -o myprog C objects and libraries \
           camlcode.o -L‘ocamlc -where‘ -lunix -lcamlrun

With ocamlopt, you should do:

        ocamlopt -output-obj -o camlcode.o unix.cmxa other .cmx and .cmxa files
        cc -o myprog C objects and libraries \
           camlcode.o -L‘ocamlc -where‘ -lunix -lasmrun

For the final linking phase, in addition to the object file produced by -output-complete-obj, you will have only to provide the C libraries required by the OCaml runtime.

For instance, assume the OCaml part of your program uses the Unix library. With ocamlc, you should do:

        ocamlc -output-complete-obj -o camlcode.o unix.cma other .cmo and .cma files
        cc -o myprog C objects and libraries \
           camlcode.o C libraries required by the runtime, eg -lm  -ldl -lcurses -lpthread

With ocamlopt, you should do:

        ocamlopt -output-complete-obj -o camlcode.o unix.cmxa other .cmx and .cmxa files
        cc -o myprog C objects and libraries \
           camlcode.o C libraries required by the runtime, eg -lm -ldl

On some ports, special options are required on the final linking phase that links together the object file produced by the -output-obj and -output-complete-obj options and the remainder of the program. Those options are shown in the configuration file Makefile.config generated during compilation of OCaml, as the variable OC_LDFLAGS.

Stack backtraces.

When OCaml bytecode produced by ocamlc -g is embedded in a C program, no debugging information is included, and therefore it is impossible to print stack backtraces on uncaught exceptions. This is not the case when native code produced by ocamlopt -g is embedded in a C program: stack backtrace information is available, but the backtrace mechanism needs to be turned on programmatically. This can be achieved from the OCaml side by calling Printexc.record_backtrace true in the initialization of one of the OCaml modules. This can also be achieved from the C side by calling caml_record_backtraces(1); in the OCaml-C glue code. (caml_record_backtraces is declared in backtrace.h)

Unloading the runtime.

In case the shared library produced with -output-obj is to be loaded and unloaded repeatedly by a single process, care must be taken to unload the OCaml runtime explicitly, in order to avoid various system resource leaks.

Since 4.05, caml_shutdown function can be used to shut the runtime down gracefully, which equals the following:

As a shared library may have several clients simultaneously, it is made for convenience that caml_startup (and caml_startup_pooled) may be called multiple times, given that each such call is paired with a corresponding call to caml_shutdown (in a nested fashion). The runtime will be unloaded once there are no outstanding calls to caml_startup.

Once a runtime is unloaded, it cannot be started up again without reloading the shared library and reinitializing its static data. Therefore, at the moment, the facility is only useful for building reloadable shared libraries.

Unix signal handling.

Depending on the target platform and operating system, the native-code runtime system may install signal handlers for one or several of the SIGSEGV, SIGTRAP and SIGFPE signals when caml_startup is called, and reset these signals to their default behaviors when caml_shutdown is called. The main program written in ‍C should not try to handle these signals itself.

8 Advanced example with callbacks

This section illustrates the callback facilities described in section ‍22.7. We are going to package some OCaml functions in such a way that they can be linked with C code and called from C just like any C functions. The OCaml functions are defined in the following OCaml source:

(* File -- some "useful" OCaml functions *)

let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2)

let format_result n = Printf.sprintf "Result is: %d\n" n

(* Export those two functions to C *)

let _ = Callback.register "fib" fib
let _ = Callback.register "format_result" format_result

Here is the C stub code for calling these functions from C:

/* File modwrap.c -- wrappers around the OCaml functions */

#include <stdio.h>
#include <string.h>
#include <caml/mlvalues.h>
#include <caml/callback.h>

int fib(int n)
  static const value * fib_closure = NULL;
  if (fib_closure == NULL) fib_closure = caml_named_value("fib");
  return Int_val(caml_callback(*fib_closure, Val_int(n)));

char * format_result(int n)
  static const value * format_result_closure = NULL;
  if (format_result_closure == NULL)
    format_result_closure = caml_named_value("format_result");
  return strdup(String_val(caml_callback(*format_result_closure, Val_int(n))));
  /* We copy the C string returned by String_val to the C heap
     so that it remains valid after garbage collection. */

We now compile the OCaml code to a C object file and put it in a C library along with the stub code in modwrap.c and the OCaml runtime system:

        ocamlc -custom -output-obj -o modcaml.o
        ocamlc -c modwrap.c
        cp `ocamlc -where`/libcamlrun.a mod.a && chmod +w mod.a
        ar r mod.a modcaml.o modwrap.o

(One can also use ocamlopt -output-obj instead of ocamlc -custom -output-obj. In this case, replace libcamlrun.a (the bytecode runtime library) by libasmrun.a (the native-code runtime library).)

Now, we can use the two functions fib and format_result in any C program, just like regular C functions. Just remember to call caml_startup (or caml_startup_exn) once before.

/* File main.c -- a sample client for the OCaml functions */

#include <stdio.h>
#include <caml/callback.h>

extern int fib(int n);
extern char * format_result(int n);

int main(int argc, char ** argv)
  int result;

  /* Initialize OCaml code */
  /* Do some computation */
  result = fib(10);
  printf("fib(10) = %s\n", format_result(result));
  return 0;

To build the whole program, just invoke the C compiler as follows:

        cc -o prog -I `ocamlc -where` main.c mod.a -lcurses

(On some machines, you may need to put -ltermcap or -lcurses -ltermcap instead of -lcurses.)

9 Advanced topic: custom blocks

Blocks with tag Custom_tag contain both arbitrary user data and a pointer to a C struct, with type struct custom_operations, that associates user-provided finalization, comparison, hashing, serialization and deserialization functions to this block.

9.1 The struct custom_operations

The struct custom_operations is defined in <caml/custom.h> and contains the following fields:

Note: the finalize, compare, hash, serialize, and deserialize functions attached to custom blocks descriptors are only allowed limited interactions with the OCaml runtime. Within these functions, do not call any of the OCaml allocation functions, and do not perform any callback into OCaml code. Do not use CAMLparam to register the parameters to these functions, and do not use CAMLreturn to return the result. Do not raise exceptions (to signal an error during deserialization, use caml_deserialize_error). Do not remove global roots. When in doubt, err on the side of caution. Within serialize and deserialize functions, use of the corresponding functions from section ‍22.9.4 is allowed (and even recommended).

9.2 Allocating custom blocks

Custom blocks must be allocated via caml_alloc_custom or caml_alloc_custom_mem:

caml_alloc_custom(ops, size, used, max)

returns a fresh custom block, with room for size bytes of user data, and whose associated operations are given by ops (a pointer to a struct custom_operations, usually statically allocated as a C global variable).

The two parameters used and max are used to control the speed of garbage collection when the finalized object contains pointers to out-of-heap resources. Generally speaking, the OCaml incremental major collector adjusts its speed relative to the allocation rate of the program. The faster the program allocates, the harder the GC works in order to reclaim quickly unreachable blocks and avoid having large amount of “floating garbage” (unreferenced objects that the GC has not yet collected).

Normally, the allocation rate is measured by counting the in-heap size of allocated blocks. However, it often happens that finalized objects contain pointers to out-of-heap memory blocks and other resources (such as file descriptors, X Windows bitmaps, etc.). For those blocks, the in-heap size of blocks is not a good measure of the quantity of resources allocated by the program.

The two arguments used and max give the GC an idea of how much out-of-heap resources are consumed by the finalized block being allocated: you give the amount of resources allocated to this object as parameter used, and the maximum amount that you want to see in floating garbage as parameter max. The units are arbitrary: the GC cares only about the ratio used / max.

For instance, if you are allocating a finalized block holding an X Windows bitmap of w by h pixels, and you’d rather not have more than 1 mega-pixels of unreclaimed bitmaps, specify used = w * h and max = 1000000.

Another way to describe the effect of the used and max parameters is in terms of full GC cycles. If you allocate many custom blocks with used / max = 1 / N, the GC will then do one full cycle (examining every object in the heap and calling finalization functions on those that are unreachable) every N allocations. For instance, if used = 1 and max = 1000, the GC will do one full cycle at least every 1000 allocations of custom blocks.

If your finalized blocks contain no pointers to out-of-heap resources, or if the previous discussion made little sense to you, just take used = 0 and max = 1. But if you later find that the finalization functions are not called “often enough”, consider increasing the used / max ratio.

caml_alloc_custom_mem(ops, size, used)

Use this function when your custom block holds only out-of-heap memory (memory allocated with malloc or caml_stat_alloc) and no other resources. used should be the number of bytes of out-of-heap memory that are held by your custom block. This function works like caml_alloc_custom except that the max parameter is under the control of the user (via the custom_major_ratio, custom_minor_ratio, and custom_minor_max_size parameters) and proportional to the heap sizes. It has been available since OCaml 4.08.0.

9.3 Accessing custom blocks

The data part of a custom block v can be accessed via the pointer Data_custom_val(v). This pointer has type void * and should be cast to the actual type of the data stored in the custom block.

The contents of custom blocks are not scanned by the garbage collector, and must therefore not contain any pointer inside the OCaml heap. In other terms, never store an OCaml value in a custom block, and do not use Field, Store_field nor caml_modify to access the data part of a custom block. Conversely, any C data structure (not containing heap pointers) can be stored in a custom block.

9.4 Writing custom serialization and deserialization functions

The following functions, defined in <caml/intext.h>, are provided to write and read back the contents of custom blocks in a portable way. Those functions handle endianness conversions when e.g. data is written on a little-endian machine and read back on a big-endian machine.

caml_serialize_int_1Write a 1-byte integer
caml_serialize_int_2Write a 2-byte integer
caml_serialize_int_4Write a 4-byte integer
caml_serialize_int_8Write a 8-byte integer
caml_serialize_float_4Write a 4-byte float
caml_serialize_float_8Write a 8-byte float
caml_serialize_block_1Write an array of 1-byte quantities
caml_serialize_block_2Write an array of 2-byte quantities
caml_serialize_block_4Write an array of 4-byte quantities
caml_serialize_block_8Write an array of 8-byte quantities
caml_deserialize_uint_1Read an unsigned 1-byte integer
caml_deserialize_sint_1Read a signed 1-byte integer
caml_deserialize_uint_2Read an unsigned 2-byte integer
caml_deserialize_sint_2Read a signed 2-byte integer
caml_deserialize_uint_4Read an unsigned 4-byte integer
caml_deserialize_sint_4Read a signed 4-byte integer
caml_deserialize_uint_8Read an unsigned 8-byte integer
caml_deserialize_sint_8Read a signed 8-byte integer
caml_deserialize_float_4Read a 4-byte float
caml_deserialize_float_8Read an 8-byte float
caml_deserialize_block_1Read an array of 1-byte quantities
caml_deserialize_block_2Read an array of 2-byte quantities
caml_deserialize_block_4Read an array of 4-byte quantities
caml_deserialize_block_8Read an array of 8-byte quantities
caml_deserialize_errorSignal an error during deserialization; input_value or Marshal.from_... raise a Failure exception after cleaning up their internal data structures

Serialization functions are attached to the custom blocks to which they apply. Obviously, deserialization functions cannot be attached this way, since the custom block does not exist yet when deserialization begins! Thus, the struct custom_operations that contain deserialization functions must be registered with the deserializer in advance, using the register_custom_operations function declared in <caml/custom.h>. Deserialization proceeds by reading the identifier off the input stream, allocating a custom block of the size specified in the input stream, searching the registered struct custom_operation blocks for one with the same identifier, and calling its deserialize function to fill the data part of the custom block.

9.5 Choosing identifiers

Identifiers in struct custom_operations must be chosen carefully, since they must identify uniquely the data structure for serialization and deserialization operations. In particular, consider including a version number in the identifier; this way, the format of the data can be changed later, yet backward-compatible deserialisation functions can be provided.

Identifiers starting with _ (an underscore character) are reserved for the OCaml runtime system; do not use them for your custom data. We recommend to use a URL ( or a Java-style package name (com.mydomain.mymachine.mylibrary.version-number) as identifiers, to minimize the risk of identifier collision.

9.6 Finalized blocks

Custom blocks generalize the finalized blocks that were present in OCaml prior to version 3.00. For backwards compatibility, the format of custom blocks is compatible with that of finalized blocks, and the caml_alloc_final function is still available to allocate a custom block with a given finalization function, but default comparison, hashing and serialization functions. (In particular, the finalization function must not access the OCaml runtime.)

caml_alloc_final(n, f, used, max) returns a fresh custom block of size n+1 words, with finalization function f. The first word is reserved for storing the custom operations; the other n words are available for your data. The two parameters used and max are used to control the speed of garbage collection, as described for caml_alloc_custom.

10 Advanced topic: Bigarrays and the OCaml-C interface

This section explains how C stub code that interfaces C or Fortran code with OCaml code can use Bigarrays.

10.1 Include file

The include file <caml/bigarray.h> must be included in the C stub file. It declares the functions, constants and macros discussed below.

10.2 Accessing an OCaml bigarray from C or Fortran

If v is a OCaml value representing a Bigarray, the expression Caml_ba_data_val(v) returns a pointer to the data part of the array. This pointer is of type void * and can be cast to the appropriate C type for the array (e.g. double [], char [][10], etc).

Various characteristics of the OCaml Bigarray can be consulted from C as follows:

C expressionReturns
Caml_ba_array_val(v)->num_dimsnumber of dimensions
Caml_ba_array_val(v)->dim[i]i-th dimension
Caml_ba_array_val(v)->flags & CAML_BA_KIND_MASKkind of array elements

The kind of array elements is one of the following constants:

ConstantElement kind
CAML_BA_FLOAT1616-bit half-precision floats
CAML_BA_FLOAT3232-bit single-precision floats
CAML_BA_FLOAT6464-bit double-precision floats
CAML_BA_SINT88-bit signed integers
CAML_BA_UINT88-bit unsigned integers
CAML_BA_SINT1616-bit signed integers
CAML_BA_UINT1616-bit unsigned integers
CAML_BA_INT3232-bit signed integers
CAML_BA_INT6464-bit signed integers
CAML_BA_CAML_INT31- or 63-bit signed integers
CAML_BA_NATIVE_INT32- or 64-bit (platform-native) integers
CAML_BA_COMPLEX3232-bit single-precision complex numbers
CAML_BA_COMPLEX6464-bit double-precision complex numbers
CAML_BA_CHAR8-bit characters

Caml_ba_array_val(v) must always be dereferenced immediately and not stored anywhere, including local variables. It resolves to a derived pointer: it is not a valid OCaml value but points to a memory region managed by the GC. For this reason this value must not be stored in any memory location that could be live cross a GC.

The following example shows the passing of a two-dimensional Bigarray to a C function and a Fortran function.

    extern void my_c_function(double * data, int dimx, int dimy);
    extern void my_fortran_function_(double * data, int * dimx, int * dimy);

    CAMLprim value caml_stub(value bigarray)
      int dimx = Caml_ba_array_val(bigarray)->dim[0];
      int dimy = Caml_ba_array_val(bigarray)->dim[1];
      /* C passes scalar parameters by value */
      my_c_function(Caml_ba_data_val(bigarray), dimx, dimy);
      /* Fortran passes all parameters by reference */
      my_fortran_function_(Caml_ba_data_val(bigarray), &dimx, &dimy);
      return Val_unit;

10.3 Wrapping a C or Fortran array as an OCaml Bigarray

A pointer p to an already-allocated C or Fortran array can be wrapped and returned to OCaml as a Bigarray using the caml_ba_alloc or caml_ba_alloc_dims functions.

The following example illustrates how statically-allocated C and Fortran arrays can be made available to OCaml.

    extern long my_c_array[100][200];
    extern float my_fortran_array_[300][400];

    CAMLprim value caml_get_c_array(value unit)
      long dims[2];
      dims[0] = 100; dims[1] = 200;
      return caml_ba_alloc(CAML_BA_NATIVE_INT | CAML_BA_C_LAYOUT,
                           2, my_c_array, dims);

    CAMLprim value caml_get_fortran_array(value unit)
      return caml_ba_alloc_dims(CAML_BA_FLOAT32 | CAML_BA_FORTRAN_LAYOUT,
                                2, my_fortran_array_, 300L, 400L);

11 Advanced topic: cheaper C call

This section describe how to make calling C functions cheaper.

Note: This only applies to the native compiler. So whenever you use any of these methods, you have to provide an alternative byte-code stub that ignores all the special annotations.

11.1 Passing unboxed values

We said earlier that all OCaml objects are represented by the C type value, and one has to use macros such as Int_val to decode data from the value type. It is however possible to tell the OCaml native-code compiler to do this for us and pass arguments unboxed to the C function. Similarly it is possible to tell OCaml to expect the result unboxed and box it for us.

The motivation is that, by letting ‘ocamlopt‘ deal with boxing, it can often decide to suppress it entirely.

For instance let’s consider this example:

external foo : float -> float -> float = "foo"

let f a b =
  let len = Array.length a in
  assert (Array.length b = len);
  let res = Array.make len 0. in
  for i = 0 to len - 1 do
    res.(i) <- foo a.(i) b.(i)

Float arrays are unboxed in OCaml, however the C function foo expect its arguments as boxed floats and returns a boxed float. Hence the OCaml compiler has no choice but to box a.(i) and b.(i) and unbox the result of foo. This results in the allocation of 3 * len temporary float values.

Now if we annotate the arguments and result with [@unboxed], the native-code compiler will be able to avoid all these allocations:

external foo
  :  (float [@unboxed])
  -> (float [@unboxed])
  -> (float [@unboxed])
  = "foo_byte" "foo"

In this case the C functions must look like:

CAMLprim double foo(double a, double b)

CAMLprim value foo_byte(value a, value b)
  return caml_copy_double(foo(Double_val(a), Double_val(b)))

For convenience, when all arguments and the result are annotated with [@unboxed], it is possible to put the attribute only once on the declaration itself. So we can also write instead:

external foo : float -> float -> float = "foo_byte" "foo" [@@unboxed]

The following table summarize what OCaml types can be unboxed, and what C types should be used in correspondence:

OCaml typeC type

Similarly, it is possible to pass untagged OCaml integers between OCaml and C. This is done by annotating the arguments and/or result with [@untagged]:

external f : string -> (int [@untagged]) = "f_byte" "f"

The corresponding C type must be intnat.

Note: Do not use the C int type in correspondence with (int [@untagged]). This is because they often differ in size.

It is possible to annotate with [@untagged] any immediate type, i.e. types that are represented like int. This includes bool, char, any variant type with only constant constructors. Note: this does not include Unix.file_descr which is not represented as an interger on all platforms.

11.2 Direct C call

In order to be able to run the garbage collector in the middle of a C function, the OCaml native-code compiler generates some bookkeeping code around C calls. Technically it wraps every C call with the C function caml_c_call which is part of the OCaml runtime.

For small functions that are called repeatedly, this indirection can have a big impact on performances. However this is not needed if we know that the C function doesn’t allocate, doesn’t raise exceptions, and doesn’t release the domain lock (see section ‍22.12.2). We can instruct the OCaml native-code compiler of this fact by annotating the external declaration with the attribute [@@noalloc]:

external bar : int -> int -> int = "foo" [@@noalloc]

In this case calling bar from OCaml is as cheap as calling any other OCaml function, except for the fact that the OCaml compiler can’t inline C functions...

11.3 Example: calling C library functions without indirection

Using these attributes, it is possible to call C library functions with no indirection. For instance many math functions are defined this way in the OCaml standard library:

external sqrt : float -> float = "caml_sqrt_float" "sqrt"
  [@@unboxed] [@@noalloc]
(** Square root. *)

external exp : float -> float = "caml_exp_float" "exp" [@@unboxed] [@@noalloc]
(** Exponential. *)

external log : float -> float = "caml_log_float" "log" [@@unboxed] [@@noalloc]
(** Natural logarithm. *)

12 Advanced topic: multithreading

Using multiple threads (shared-memory concurrency) in a mixed OCaml/C application requires special precautions, which are described in this section.

12.1 Registering threads created from C

Callbacks from C to OCaml are possible only if the calling thread is known to the OCaml run-time system. Threads created from OCaml (through the Thread.create function of the system threads library) are automatically known to the run-time system. If the application creates additional threads from C and wishes to callback into OCaml code from these threads, it must first register them with the run-time system. The following functions are declared in the include file <caml/threads.h>.

12.2 Parallel execution of long-running C code with systhreads

Domains are the unit of parallelism for OCaml programs. When using the systhreads library, multiple threads might be attached to the same domain. However, at any time, at most one of those thread can be executing OCaml code or C code that uses the OCaml run-time system by domain. Technically, this is enforced by a “domain lock” that any thread must hold while executing such code within a domain.

When OCaml calls the C code implementing a primitive, the domain lock is held, therefore the C code has full access to the facilities of the run-time system. However, no other thread in the same domain can execute OCaml code concurrently with the C code of the primitive. See also chapter ‍9.6 for the behaviour with multiple domains.

If a C primitive runs for a long time or performs potentially blocking input-output operations, it can explicitly release the domain lock, enabling other OCaml threads in the same domain to run concurrently with its operations. The C code must re-acquire the domain lock before returning to OCaml. This is achieved with the following functions, declared in the include file <caml/threads.h>.

These functions poll for pending signals by calling asynchronous callbacks (section ‍22.5.3) before releasing and after acquiring the lock. They can therefore execute arbitrary OCaml code including raising an asynchronous exception.

After caml_release_runtime_system() was called and until caml_acquire_runtime_system() is called, the C code must not access any OCaml data, nor call any function of the run-time system, nor call back into OCaml code. Consequently, arguments provided by OCaml to the C primitive must be copied into C data structures before calling caml_release_runtime_system(), and results to be returned to OCaml must be encoded as OCaml values after caml_acquire_runtime_system() returns.

Example: the following C primitive invokes gethostbyname to find the IP address of a host name. The gethostbyname function can block for a long time, so we choose to release the OCaml run-time system while it is running.

CAMLprim stub_gethostbyname(value vname)
  CAMLparam1 (vname);
  CAMLlocal1 (vres);
  struct hostent * h;
  char * name;

  /* Copy the string argument to a C string, allocated outside the
     OCaml heap. */
  name = caml_stat_strdup(String_val(vname));
  /* Release the OCaml run-time system */
  /* Resolve the name */
  h = gethostbyname(name);
  /* Free the copy of the string, which we might as well do before
     acquiring the runtime system to benefit from parallelism. */
  /* Re-acquire the OCaml run-time system */
  /* Encode the relevant fields of h as the OCaml value vres */
  ... /* Omitted */
  /* Return to OCaml */
  CAMLreturn (vres);

The macro Caml_state evaluates to the domain state variable, and checks in debug mode that the domain lock is held. Such a check is also placed in normal mode at key entry points of the C API; this is why calling some of the runtime functions and macros without correctly owning the domain lock can result in a fatal error: no domain lock held. The variant Caml_state_opt does not perform any check but evaluates to NULL when the domain lock is not held. This lets you determine whether a thread belonging to a domain currently holds its domain lock, for various purposes.

Callbacks from C to OCaml must be performed while holding the domain lock to the OCaml run-time system. This is naturally the case if the callback is performed by a C primitive that did not release the run-time system. If the C primitive released the run-time system previously, or the callback is performed from other C code that was not invoked from OCaml (e.g. an event loop in a GUI application), the run-time system must be acquired before the callback and released after:

  /* Resolve OCaml function vfun to be invoked */
  /* Build OCaml argument varg to the callback */
  vres = callback(vfun, varg);
  /* Copy relevant parts of result vres to C data structures */

Note: the acquire and release functions described above were introduced in OCaml 3.12. Older code uses the following historical names, declared in <caml/signals.h>:

Intuition: a “blocking section” is a piece of C code that does not use the OCaml run-time system, typically a blocking input/output operation.

13 Advanced topic: interfacing with Windows Unicode APIs

This section contains some general guidelines for writing C stubs that use Windows Unicode APIs.

The OCaml system under Windows can be configured at build time in one of two modes:

In what follows, we say that a string has the OCaml encoding if it is encoded in UTF-8 when in Unicode mode, in the current code page in legacy mode, or is an arbitrary string under Unix. A string has the platform encoding if it is encoded in UTF-16 under Windows or is an arbitrary string under Unix.

From the point of view of the writer of C stubs, the challenges of interacting with Windows Unicode APIs are twofold:

The native C character type under Windows is WCHAR, two bytes wide, while under Unix it is char, one byte wide. A type char_os is defined in <caml/misc.h> that stands for the concrete C character type of each platform. Strings in the platform encoding are of type char_os *.

The following functions are exposed to help write compatible C stubs. To use them, you need to include both <caml/misc.h> and <caml/osdeps.h>.

Note: The strings returned by caml_stat_strdup_to_os and caml_stat_strdup_of_os are allocated using caml_stat_alloc, so they need to be deallocated using caml_stat_free when they are no longer needed.


We want to bind the function getenv in a way that works both under Unix and Windows. Under Unix this function has the prototype:

    char *getenv(const char *);

While the Unicode version under Windows has the prototype:

    WCHAR *_wgetenv(const WCHAR *);

In terms of char_os, both functions take an argument of type char_os * and return a result of the same type. We begin by choosing the right implementation of the function to bind:

#ifdef _WIN32
#define getenv_os _wgetenv
#define getenv_os getenv

The rest of the binding is the same for both platforms:

#include <caml/mlvalues.h>
#include <caml/misc.h>
#include <caml/alloc.h>
#include <caml/fail.h>
#include <caml/osdeps.h>
#include <stdlib.h>

CAMLprim value stub_getenv(value var_name)
  char_os *var_name_os, *var_value_os;

  var_name_os = caml_stat_strdup_to_os(String_val(var_name));
  var_value_os = getenv_os(var_name_os);

  if (var_value_os == NULL)

  var_value = caml_copy_string_of_os(var_value_os);


14 Building mixed C/OCaml libraries: ocamlmklib

The ocamlmklib command facilitates the construction of libraries containing both OCaml code and C code, and usable both in static linking and dynamic linking modes. This command is available under Windows since Objective Caml 3.11 and under other operating systems since Objective Caml 3.03.

The ocamlmklib command takes three kinds of arguments:

It generates the following outputs:

In addition, the following options are recognized:

-cclib, -ccopt, -I, -linkall
These options are passed as is to ocamlc or ocamlopt. See the documentation of these commands.
-rpath, -R, -Wl,-rpath, -Wl,-R
These options are passed as is to the C compiler. Refer to the documentation of the C compiler.
Force the construction of a statically linked library only, even if dynamic linking is supported.
Fall back to building a statically linked library if a problem occurs while building the shared library (e.g. some of the support libraries are not available as shared libraries).
Add dir to the search path for support libraries (-llib).
-ocamlc cmd
Use cmd instead of ocamlc to call the bytecode compiler.
-ocamlopt cmd
Use cmd instead of ocamlopt to call the native-code compiler.
-o output
Set the name of the generated OCaml library. ocamlmklib will generate output.cma and/or output.cmxa. If not specified, defaults to a.
-oc outputc
Set the name of the generated C library. ocamlmklib will generate (if shared libraries are supported) and liboutputc.a. If not specified, defaults to the output name given with -o.

Consider an OCaml interface to the standard libz C library for reading and writing compressed files. Assume this library resides in /usr/local/zlib. This interface is composed of an OCaml part zip.cmo/zip.cmx and a C part zipstubs.o containing the stub code around the libz entry points. The following command builds the OCaml libraries zip.cma and zip.cmxa, as well as the companion C libraries and libzip.a:

ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib

If shared libraries are supported, this performs the following commands:

ocamlc -a -o zip.cma zip.cmo -dllib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
gcc -shared -o zipstubs.o -lz -L/usr/local/zlib
ar rc libzip.a zipstubs.o

Note: This example is on a Unix system. The exact command lines may be different on other systems.

If shared libraries are not supported, the following commands are performed instead:

ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ar rc libzip.a zipstubs.o

Instead of building simultaneously the bytecode library, the native-code library and the C libraries, ocamlmklib can be called three times to build each separately. Thus,

ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib

builds the bytecode library zip.cma, and

ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib

builds the native-code library zip.cmxa, and

ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib

builds the C libraries and libzip.a. Notice that the support libraries (-lz) and the corresponding options (-L/usr/local/zlib) must be given on all three invocations of ocamlmklib, because they are needed at different times depending on whether shared libraries are supported.

15 Cautionary words: the internal runtime API

Not all header available in the caml/ directory were described in previous sections. All those unmentioned headers are part of the internal runtime API, for which there is no stability guarantee. If you really need access to this internal runtime API, this section provides some guidelines that may help you to write code that might not break on every new version of OCaml.


Programmers which come to rely on the internal API for a use-case which they find realistic and useful are encouraged to open a request for improvement on the bug tracker.

15.1 Internal variables and CAML_INTERNALS

Since OCaml 4.04, it is possible to get access to every part of the internal runtime API by defining the CAML_INTERNALS macro before loading caml header files. If this macro is not defined, parts of the internal runtime API are hidden.

If you are using internal C variables, do not redefine them by hand. You should import those variables by including the corresponding header files. The representation of those variables has already changed once in OCaml 4.10, and is still under evolution. If your code relies on such internal and brittle properties, it will be broken at some point in time.

For instance, rather than redefining caml_young_limit:

extern int caml_young_limit;

which breaks in OCaml ≥ 4.10, you should include the minor_gc header:

#include <caml/minor_gc.h>

15.2 OCaml version macros

Finally, if including the right headers is not enough, or if you need to support version older than OCaml 4.04, the header file caml/version.h should help you to define your own compatibility layer. This file provides few macros defining the current OCaml version. In particular, the OCAML_VERSION macro describes the current version, its format is MmmPP. For example, if you need some specific handling for versions older than 4.10.0, you could write

#include <caml/version.h>
#if OCAML_VERSION >= 41000