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Chapter 20 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.

20.1 Overview and compilation information

20.1.1 Declaring primitives

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 int_of_string primitive is declared in the standard library module Stdlib:

        external int_of_string : string -> int = "caml_int_of_string"

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, input above has arity 4, and the input C function is called with four arguments. Similarly,

    external input2 : in_channel * bytes * int * int -> int = "input2"

has arity 1, and the input2 C function receives one argument (which is a quadruple 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.

20.1.2 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 input primitive:

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, call the first function, and convert the returned C value to OCaml value. For instance, here is the stub code for the input primitive:

CAMLprim value input(value channel, value buffer, value offset, value length)
  return Val_long(getblock((struct channel *) channel,
                           &Byte(buffer, Long_val(offset)),

(Here, Val_long, Long_val and so on are conversion 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 getblock, which is declared as:

long getblock(struct channel * channel, char * p, long n)

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

Include file Provides
caml/mlvalues.h definition of the value type, and conversion macros
caml/alloc.h allocation functions (to create structured OCaml objects)
caml/memory.h miscellaneous memory-related functions and macros (for GC interface, in-place modification of structures, etc).
caml/fail.h functions for raising exceptions (see section 20.4.5)
caml/callback.h callback from C to OCaml (see section 20.7).
caml/custom.h operations on custom blocks (see section 20.9).
caml/intext.h operations for writing user-defined serialization and deserialization functions for custom blocks (see section 20.9).
caml/threads.h operations for interfacing in the presence of multiple threads (see section 20.12).

Before including any of these files, you should define the OCAML_NAME_SPACE macro. For instance,

#include "caml/mlvalues.h"
#include "caml/fail.h"

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).

Note: Including the header files without first defining CAML_NAME_SPACE introduces in scope short names for most functions. Those short names are deprecated, and may be removed in the future because they usually produce clashes with names defined by other C libraries.

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 20.1.4 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:

  • the table of the required primitives;
  • a library that provides the bytecode interpreter, the memory manager, and the standard primitives;
  • libraries and object code files (.o files) mentioned on the command line for the OCaml linker, that provide implementations for the user’s primitives.

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:

  • the -custom option;
  • the names of the desired OCaml object files (.cmo and .cma files) ;
  • the names of the C object files and libraries (.o and .a files) that implement the required primitives. Under Unix and Windows, a library named libname.a (respectively, .lib) residing in one of the standard library directories can also be specified as -cclib -lname.

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:

  • the names of the desired OCaml native object files (.cmx and .cmxa files);
  • the names of the C object files and libraries (.o, .a, .so or .dll files) that implement the required primitives.

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 11.3). Finally (step 3), execute the ocamlc command with

  • the names of the desired OCaml object files (.cmo and .cma files) ;
  • the names of the C shared libraries (.so or .dll files) that implement the required primitives. Under Unix and Windows, a library named dllname.so (respectively, .dll) residing in one of the standard library directories can also be specified as -dllib -lname.

Do not set the -custom flag, otherwise you’re back to static linking as described in section 20.1.3. The ocamlmklib tool (see section 20.14) 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 dllmylib.so. 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 known that it references C code, nor whether this C code must be statically linked (using -custom) or dynamically linked.

20.1.5 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 20.14) 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.

20.1.6 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).

20.2 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:

  • an unboxed integer;
  • or a pointer to a block inside the heap, allocated through one of the caml_alloc_* functions described in section 20.4.4.

20.2.1 Integer values

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

20.2.2 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:

Tag Contents of the block
0 to No_scan_tag−1 A structured block (an array of OCaml objects). Each field is a value.
Closure_tag A 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_tag A character string or a byte sequence.
Double_tag A double-precision floating-point number.
Double_array_tag An array or record of double-precision floating-point numbers.
Abstract_tag A block representing an abstract datatype.
Custom_tag A block representing an abstract datatype with user-defined finalization, comparison, hashing, serialization and deserialization functions attached.

20.2.3 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. Starting with OCaml 4.11, this usage is deprecated and will stop being supported in OCaml 5.00.

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 20.6 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);

20.3 Representation of OCaml data types

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

20.3.1 Atomic types

OCaml type Encoding
int Unboxed integer values.
char Unboxed integer values (ASCII code).
float Blocks with tag Double_tag.
bytes Blocks with tag String_tag.
string Blocks with tag String_tag.
int32 Blocks with tag Custom_tag.
int64 Blocks with tag Custom_tag.
nativeint Blocks with tag Custom_tag.

20.3.2 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:

  • An attribute ([@@boxed] or [@@unboxed]) on the type declaration.
  • A compiler option (-unboxed-types or -no-unboxed-types).
  • The default representation. In the present version of OCaml, the default is the boxed representation.

20.3.3 Arrays

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

Arrays of floating-point numbers (type float array) have a special, unboxed, more efficient representation. These arrays are represented by pointers to blocks with tag Double_array_tag. They should be accessed with the Double_field and Store_double_field macros.

20.3.4 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 term Representation
() Val_int(0)
false Val_int(0)
true Val_int(1)
[] Val_int(0)
h::t Block 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 and Val_true to refer to (), false and true.

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 20.3.2.

20.3.5 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);

20.3.6 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.

20.4 Operations on values

20.4.1 Kind tests

  • Is_long(v) is true if value v is an immediate integer, false otherwise
  • Is_block(v) is true if value v is a pointer to a block, and false if it is an immediate integer.

20.4.2 Operations on integers

  • Val_long(l) returns the value encoding the long int l.
  • Long_val(v) returns the long int encoded in value v.
  • Val_int(i) returns the value encoding the int i.
  • Int_val(v) returns the int encoded in value v.
  • Val_bool(x) returns the OCaml boolean representing the truth value of the C integer x.
  • Bool_val(v) returns 0 if v is the OCaml boolean false, 1 if v is true.
  • Val_true, Val_false represent the OCaml booleans true and false.

20.4.3 Accessing blocks

  • Wosize_val(v) returns the size of the block v, in words, excluding the header.
  • Tag_val(v) returns the tag of the block v.
  • Field(v, n) returns the value contained in the nth field of the structured block v. Fields are numbered from 0 to Wosize_val(v)−1.
  • Store_field(b, n, v) stores the value v in the field number n of value b, which must be a structured block.
  • Code_val(v) returns the code part of the closure v.
  • caml_string_length(v) returns the length (number of bytes) of the string or byte sequence v.
  • Byte(v, n) returns the nth byte of the string or byte sequence v, with type char. Bytes are numbered from 0 to string_length(v)−1.
  • Byte_u(v, n) returns the nth byte of the string or byte sequence v, with type unsigned char. Bytes are numbered from 0 to string_length(v)−1.
  • String_val(v) returns a pointer to the first byte of the string v, with type char * or, when OCaml is configured with -force-safe-string, with type const char *. This pointer is a valid C string: there is a null byte after the last byte in the string. However, OCaml strings can contain embedded null bytes, which will confuse the usual C functions over strings.
  • Bytes_val(v) returns a pointer to the first byte of the byte sequence v, with type unsigned char *.
  • Double_val(v) returns the floating-point number contained in value v, with type double.
  • Double_field(v, n) returns the nth element of the array of floating-point numbers v (a block tagged Double_array_tag).
  • Store_double_field(v, n, d) stores the double precision floating-point number d in the nth element of the array of floating-point numbers v.
  • Data_custom_val(v) returns a pointer to the data part of the custom block v. This pointer has type void * and must be cast to the type of the data contained in the custom block.
  • Int32_val(v) returns the 32-bit integer contained in the int32 v.
  • Int64_val(v) returns the 64-bit integer contained in the int64 v.
  • Nativeint_val(v) returns the long integer contained in the nativeint v.
  • caml_field_unboxed(v) returns the value of the field of a value v of any unboxed type (record or concrete data type).
  • caml_field_boxed(v) returns the value of the field of a value v of any boxed type (record or concrete data type).
  • caml_field_unboxable(v) calls either caml_field_unboxed or caml_field_boxed according to the default representation of unboxable types in the current version of OCaml.

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).

20.4.4 Allocating blocks

Simple interface

  • Atom(t) returns an “atom” (zero-sized block) with tag t. Zero-sized blocks are preallocated outside of the heap. It is incorrect to try and allocate a zero-sized block using the functions below. For instance, Atom(0) represents the empty array.
  • caml_alloc(n, t) returns a fresh block of size n with tag t. If t is less than No_scan_tag, then the fields of the block are initialized with a valid value in order to satisfy the GC constraints.
  • caml_alloc_tuple(n) returns a fresh block of size n words, with tag 0.
  • caml_alloc_string(n) returns a byte sequence (or string) value of length n bytes. The sequence initially contains uninitialized bytes.
  • caml_alloc_initialized_string(n, p) returns a byte sequence (or string) value of length n bytes. The value is initialized from the n bytes starting at address p.
  • caml_copy_string(s) returns a string or byte sequence value containing a copy of the null-terminated C string s (a char *).
  • caml_copy_double(d) returns a floating-point value initialized with the double d.
  • caml_copy_int32(i), caml_copy_int64(i) and caml_copy_nativeint(i) return a value of OCaml type int32, int64 and nativeint, respectively, initialized with the integer i.
  • caml_alloc_array(f, a) allocates an array of values, calling function f over each element of the input array a to transform it into a value. The array a is an array of pointers terminated by the null pointer. The function f receives each pointer as argument, and returns a value. The zero-tagged block returned by alloc_array(f, a) is filled with the values returned by the successive calls to f. (This function must not be used to build an array of floating-point numbers.)
  • caml_copy_string_array(p) allocates an array of strings or byte sequences, copied from the pointer to a string array p (a char **). p must be NULL-terminated.
  • caml_alloc_float_array(n) allocates an array of floating point numbers of size n. The array initially contains uninitialized values.
  • caml_alloc_unboxed(v) returns the value (of any unboxed type) whose field is the value v.
  • caml_alloc_boxed(v) allocates and returns a value (of any boxed type) whose field is the value v.
  • caml_alloc_unboxable(v) calls either caml_alloc_unboxed or caml_alloc_boxed according to the default representation of unboxable types in the current version of OCaml.

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.

  • caml_alloc_small(n, t) returns a fresh small block of size nMax_young_wosize words, with tag t. If this block is a structured block (i.e. if t < No_scan_tag), then the fields of the block (initially containing garbage) must be initialized with legal values (using direct assignment to the fields of the block) before the next allocation.
  • caml_alloc_shr(n, t) returns a fresh block of size n, with tag t. The size of the block can be greater than Max_young_wosize. (It can also be smaller, but in this case it is more efficient to call caml_alloc_small instead of caml_alloc_shr.) If this block is a structured block (i.e. if t < No_scan_tag), then the fields of the block (initially containing garbage) must be initialized with legal values (using the caml_initialize function described below) before the next allocation.

20.4.5 Raising exceptions

Two functions are provided to raise two standard exceptions:

  • caml_failwith(s), where s is a null-terminated C string (with type char *), raises exception Failure with argument s.
  • caml_invalid_argument(s), where s is a null-terminated C string (with type char *), raises exception Invalid_argument with argument s.

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 20.7.3. Once the exception identifier is recovered in C, the following functions actually raise the exception:

  • caml_raise_constant(id) raises the exception id with no argument;
  • caml_raise_with_arg(id, v) raises the exception id with the OCaml value v as argument;
  • caml_raise_with_args(id, n, v) raises the exception id with the OCaml values v[0], …, v[n-1] as arguments;
  • caml_raise_with_string(id, s), where s is a null-terminated C string, raises the exception id with a copy of the C string s as argument.

20.5 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.

20.5.1 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. In particular, CAMLlocal and CAMLxparam can only be called after CAMLparam.

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. Every occurrence of return x must be replaced by CAMLreturn (x) if x has type value, or CAMLreturnT (t, x) (where t is the type of x); every occurrence of return without argument must be replaced by CAMLreturn0. If your C function is a procedure (i.e. if it returns void), you must insert CAMLreturn0 at the end (to replace C’s implicit return).


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


void foo (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);

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).

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.


value bar (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = caml_alloc (3, 0);
  CAMLreturn (result);
Rule 3 Assignments to the fields of structured blocks must be done with the Store_field macro (for normal blocks) or Store_double_field macro (for arrays and records of floating-point numbers). Other assignments must not use Store_field nor Store_double_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).


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 20.12.2.)

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.

20.5.2 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_int(0));          /* 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_int(0);               /* 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_int(0);            /* 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.

20.5.3 Pending actions and asynchronous exceptions

Since 4.10, allocation functions are guaranteed not to call any OCaml callbacks from C, including finalisers and signal handlers, and delay their execution instead.

The function caml_process_pending_actions from <caml/signals.h> executes any pending signal handlers and finalisers, Memprof callbacks, and requested minor and major garbage collections. In particular, it can raise asynchronous exceptions. 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 20.7.1.

20.6 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 curses.ml that declares the curses primitives and data types:

(* File curses.ml -- 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 curses.ml

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;

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

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

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

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

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))));

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

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);

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

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 prog.ml that uses the curses module:

(* File prog.ml -- 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 prog.ml curses_stubs.o -cclib -lcurses

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

20.7 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.

20.7.1 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:

  • caml_callback(f, a) applies the functional value f to the value a and returns the value returned by f.
  • caml_callback2(f, a, b) applies the functional value f (which is assumed to be a curried OCaml function with two arguments) to a and b.
  • caml_callback3(f, a, b, c) applies the functional value f (a curried OCaml function with three arguments) to a, b and c.
  • caml_callbackN(f, n, args) applies the functional value f to the n arguments contained in the array of values args.

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.


    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);

20.7.2 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));

20.7.3 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 20.4.5) 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);

20.7.4 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:

  • The C part of the program must provide a main function, which will override the default main function provided by the OCaml runtime system. Execution will start in the user-defined main function just like for a regular C program.
  • At some point, the C code must call caml_main(argv) to initialize the OCaml code. The argv argument is a C array of strings (type char **), terminated with a NULL pointer, which represents the command-line arguments, as passed as second argument to main. The OCaml array Sys.argv will be initialized from this parameter. For the bytecode compiler, argv[0] and argv[1] are also consulted to find the file containing the bytecode.
  • The call to caml_main initializes the OCaml runtime system, loads the bytecode (in the case of the bytecode compiler), and executes the initialization code of the OCaml program. Typically, this initialization code registers callback functions using Callback.register. Once the OCaml initialization code is complete, control returns to the C code that called caml_main.
  • The C code can then invoke OCaml functions using the callback mechanism (see section 20.7.1).

20.7.5 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 20.7.4, 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 option to ocamlc is provided for this purpose. It causes 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-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 option can also be used to obtain the C source file. More interestingly, the same option 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 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 are selected according to the extension of the resulting file (given with -o).

The native-code compiler ocamlopt also supports the -output-obj option, 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.

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

On some ports, special options are required on the final linking phase that links together the object file produced by the -output-obj option 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.

  • Windows with the MSVC compiler: the object file produced by OCaml have been compiled with the /MD flag, and therefore all other object files linked with it should also be compiled with /MD.
  • other systems: you may have to add one or more of -lcurses, -lm, -ldl, depending on your OS and C compiler.
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_backtrace(Val_int(1)); in the OCaml-C glue code.

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:

  • Running the functions that were registered with Stdlib.at_exit.
  • Triggering finalization of allocated custom blocks (see section 20.9). For example, Stdlib.in_channel and Stdlib.out_channel are represented by custom blocks that enclose file descriptors, which are to be released.
  • Unloading the dependent shared libraries that were loaded by the runtime, including dynlink plugins.
  • Freeing the memory blocks that were allocated by the runtime with malloc. Inside C primitives, it is advised to use caml_stat_* functions from memory.h for managing static (that is, non-moving) blocks of heap memory, as all the blocks allocated with these functions are automatically freed by caml_shutdown. For ensuring compatibility with legacy C stubs that have used caml_stat_* incorrectly, this behaviour is only enabled if the runtime is started with a specialized caml_startup_pooled function.

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.

20.8 Advanced example with callbacks

This section illustrates the callback facilities described in section 20.7. 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 mod.ml OCaml source:

(* File mod.ml -- 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 mod.ml
        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.)

20.9 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.

20.9.1 The struct custom_operations

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

  • char *identifier
    A zero-terminated character string serving as an identifier for serialization and deserialization operations.
  • void (*finalize)(value v)
    The finalize field contains a pointer to a C function that is called when the block becomes unreachable and is about to be reclaimed. The block is passed as first argument to the function. The finalize field can also be custom_finalize_default to indicate that no finalization function is associated with the block.
  • int (*compare)(value v1, value v2)
    The compare field contains a pointer to a C function that is called whenever two custom blocks are compared using OCaml’s generic comparison operators (=, <>, <=, >=, <, > and compare). The C function should return 0 if the data contained in the two blocks are structurally equal, a negative integer if the data from the first block is less than the data from the second block, and a positive integer if the data from the first block is greater than the data from the second block.

    The compare field can be set to custom_compare_default; this default comparison function simply raises Failure.

  • int (*compare_ext)(value v1, value v2)
    (Since 3.12.1) The compare_ext field contains a pointer to a C function that is called whenever one custom block and one unboxed integer are compared using OCaml’s generic comparison operators (=, <>, <=, >=, <, > and compare). As in the case of the compare field, the C function should return 0 if the two arguments are structurally equal, a negative integer if the first argument compares less than the second argument, and a positive integer if the first argument compares greater than the second argument.

    The compare_ext field can be set to custom_compare_ext_default; this default comparison function simply raises Failure.

  • intnat (*hash)(value v)
    The hash field contains a pointer to a C function that is called whenever OCaml’s generic hash operator (see module Hashtbl) is applied to a custom block. The C function can return an arbitrary integer representing the hash value of the data contained in the given custom block. The hash value must be compatible with the compare function, in the sense that two structurally equal data (that is, two custom blocks for which compare returns 0) must have the same hash value.

    The hash field can be set to custom_hash_default, in which case the custom block is ignored during hash computation.

  • void (*serialize)(value v, uintnat * bsize_32, uintnat * bsize_64)
    The serialize field contains a pointer to a C function that is called whenever the custom block needs to be serialized (marshaled) using the OCaml functions output_value or Marshal.to_.... For a custom block, those functions first write the identifier of the block (as given by the identifier field) to the output stream, then call the user-provided serialize function. That function is responsible for writing the data contained in the custom block, using the serialize_... functions defined in <caml/intext.h> and listed below. The user-provided serialize function must then store in its bsize_32 and bsize_64 parameters the sizes in bytes of the data part of the custom block on a 32-bit architecture and on a 64-bit architecture, respectively.

    The serialize field can be set to custom_serialize_default, in which case the Failure exception is raised when attempting to serialize the custom block.

  • uintnat (*deserialize)(void * dst)
    The deserialize field contains a pointer to a C function that is called whenever a custom block with identifier identifier needs to be deserialized (un-marshaled) using the OCaml functions input_value or Marshal.from_.... This user-provided function is responsible for reading back the data written by the serialize operation, using the deserialize_... functions defined in <caml/intext.h> and listed below. It must then rebuild the data part of the custom block and store it at the pointer given as the dst argument. Finally, it returns the size in bytes of the data part of the custom block. This size must be identical to the wsize_32 result of the serialize operation if the architecture is 32 bits, or wsize_64 if the architecture is 64 bits.

    The deserialize field can be set to custom_deserialize_default to indicate that deserialization is not supported. In this case, do not register the struct custom_operations with the deserializer using register_custom_operations (see below).

  • const struct custom_fixed_length* fixed_length
    (Since 4.08.0) Normally, space in the serialized output is reserved to write the bsize_32 and bsize_64 fields returned by serialize. However, for very short custom blocks, this space can be larger than the data itself! As a space optimisation, if serialize always returns the same values for bsize_32 and bsize_64, then these values may be specified in the fixed_length structure, and do not consume space in the serialized output.

Note: the finalize, compare, hash, serialize and deserialize functions attached to custom block descriptors must never trigger a garbage collection. Within these functions, do not call any of the OCaml allocation functions, and do not perform a callback into OCaml code. Do not use CAMLparam to register the parameters to these functions, and do not use CAMLreturn to return the result.

20.9.2 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.

20.9.3 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.

20.9.4 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.

Function Action
caml_serialize_int_1 Write a 1-byte integer
caml_serialize_int_2 Write a 2-byte integer
caml_serialize_int_4 Write a 4-byte integer
caml_serialize_int_8 Write a 8-byte integer
caml_serialize_float_4 Write a 4-byte float
caml_serialize_float_8 Write a 8-byte float
caml_serialize_block_1 Write an array of 1-byte quantities
caml_serialize_block_2 Write an array of 2-byte quantities
caml_serialize_block_4 Write an array of 4-byte quantities
caml_serialize_block_8 Write an array of 8-byte quantities
caml_deserialize_uint_1 Read an unsigned 1-byte integer
caml_deserialize_sint_1 Read a signed 1-byte integer
caml_deserialize_uint_2 Read an unsigned 2-byte integer
caml_deserialize_sint_2 Read a signed 2-byte integer
caml_deserialize_uint_4 Read an unsigned 4-byte integer
caml_deserialize_sint_4 Read a signed 4-byte integer
caml_deserialize_uint_8 Read an unsigned 8-byte integer
caml_deserialize_sint_8 Read a signed 8-byte integer
caml_deserialize_float_4 Read a 4-byte float
caml_deserialize_float_8 Read an 8-byte float
caml_deserialize_block_1 Read an array of 1-byte quantities
caml_deserialize_block_2 Read an array of 2-byte quantities
caml_deserialize_block_4 Read an array of 4-byte quantities
caml_deserialize_block_8 Read an array of 8-byte quantities
caml_deserialize_error Signal 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.

20.9.5 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 (http://mymachine.mydomain.com/mylibrary/version-number) or a Java-style package name (com.mydomain.mymachine.mylibrary.version-number) as identifiers, to minimize the risk of identifier collision.

20.9.6 Finalized blocks

Custom blocks generalize the finalized blocks that were present in OCaml prior to version 3.00. For backward compatibility, the format of custom blocks is compatible with that of finalized blocks, and the alloc_final function is still available to allocate a custom block with a given finalization function, but default comparison, hashing and serialization functions. 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.

20.10 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.

20.10.1 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.

20.10.2 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 expression Returns
Caml_ba_array_val(v)->num_dims number of dimensions
Caml_ba_array_val(v)->dim[i] i-th dimension
Caml_ba_array_val(v)->flags & BIGARRAY_KIND_MASK kind of array elements

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

Constant Element kind
CAML_BA_FLOAT32 32-bit single-precision floats
CAML_BA_FLOAT64 64-bit double-precision floats
CAML_BA_SINT8 8-bit signed integers
CAML_BA_UINT8 8-bit unsigned integers
CAML_BA_SINT16 16-bit signed integers
CAML_BA_UINT16 16-bit unsigned integers
CAML_BA_INT32 32-bit signed integers
CAML_BA_INT64 64-bit signed integers
CAML_BA_CAML_INT 31- or 63-bit signed integers
CAML_BA_NATIVE_INT 32- or 64-bit (platform-native) integers

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);

    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;

20.10.3 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.

  • caml_ba_alloc(kind | layout, numdims, p, dims)

    Return an OCaml Bigarray wrapping the data pointed to by p. kind is the kind of array elements (one of the CAML_BA_ kind constants above). layout is CAML_BA_C_LAYOUT for an array with C layout and CAML_BA_FORTRAN_LAYOUT for an array with Fortran layout. numdims is the number of dimensions in the array. dims is an array of numdims long integers, giving the sizes of the array in each dimension.

  • caml_ba_alloc_dims(kind | layout, numdims, p, (long) dim1, (long) dim2, …, (long) dimnumdims)

    Same as caml_ba_alloc, but the sizes of the array in each dimension are listed as extra arguments in the function call, rather than being passed as an array.

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];

    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);

    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);

20.11 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.

20.11.1 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 convenicence, 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 type C type
float double
int32 int32_t
int64 int64_t
nativeint intnat

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.

20.11.2 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 master lock (see section 20.12.2). 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...

20.11.3 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. *)

20.12 Advanced topic: multithreading

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

20.12.1 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>.

  • caml_c_thread_register() registers the calling thread with the OCaml run-time system. Returns 1 on success, 0 on error. Registering an already-registered thread does nothing and returns 0.
  • caml_c_thread_unregister() must be called before the thread terminates, to unregister it from the OCaml run-time system. Returns 1 on success, 0 on error. If the calling thread was not previously registered, does nothing and returns 0.

20.12.2 Parallel execution of long-running C code

The OCaml run-time system is not reentrant: at any time, at most one thread can be executing OCaml code or C code that uses the OCaml run-time system. Technically, this is enforced by a “master lock” that any thread must hold while executing such code.

When OCaml calls the C code implementing a primitive, the master lock is held, therefore the C code has full access to the facilities of the run-time system. However, no other thread can execute OCaml code concurrently with the C code of the primitive.

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

  • caml_release_runtime_system() The calling thread releases the master lock and other OCaml resources, enabling other threads to run OCaml code in parallel with the execution of the calling thread.
  • caml_acquire_runtime_system() The calling thread re-acquires the master lock and other OCaml resources. It may block until no other thread uses the OCaml run-time system.

These functions poll for pending signals by calling asynchronous callbacks (section 20.5.3) 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);

Callbacks from C to OCaml must be performed while holding the master 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>:

  • caml_enter_blocking_section as an alias for caml_release_runtime_system
  • caml_leave_blocking_section as an alias for caml_acquire_runtime_system

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.

20.13 Advanced topic: interfacing with Windows Unicode APIs

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

Note: This is an experimental feature of OCaml: the set of APIs below, as well as their exact semantics are not final and subject to change in future releases.

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

  • legacy mode: All path names, environment variables, command line arguments, etc. on the OCaml side are assumed to be encoded using the current 8-bit code page of the system.
  • Unicode mode: All path names, environment variables, command line arguments, etc. on the OCaml side are assumed to be encoded using UTF-8.

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 Windows API uses the UTF-16 encoding to support Unicode. The runtime system performs the necessary conversions so that the OCaml programmer only needs to deal with the OCaml encoding. C stubs that call Windows Unicode APIs need to use specific runtime functions to perform the necessary conversions in a compatible way.
  • When writing stubs that need to be compiled under both Windows and Unix, the stubs need to be written in a way that allow the necessary conversions under Windows but that also work under Unix, where typically nothing particular needs to be done to support Unicode.

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>.

  • char_os* caml_stat_strdup_to_os(const char *) copies the argument while translating from OCaml encoding to the platform encoding. This function is typically used to convert the char * underlying an OCaml string before passing it to an operating system API that takes a Unicode argument. Under Unix, it is equivalent to caml_stat_strdup.

    Note: For maximum backwards compatibility in Unicode mode, if the argument is not a valid UTF-8 string, this function will fall back to assuming that it is encoded in the current code page.

  • char* caml_stat_strdup_of_os(const char_os *) copies the argument while translating from the platform encoding to the OCaml encoding. It is the inverse of caml_stat_strdup_to_os. This function is typically used to convert a string obtained from the operating system before passing it on to OCaml code. Under Unix, it is equivalent to caml_stat_strdup.
  • value caml_copy_string_of_os(char_os *) allocates an OCaml string with contents equal to the argument string converted to the OCaml encoding. This function is essentially equivalent to caml_stat_strdup_of_os followed by caml_copy_string, except that it avoids the allocation of the intermediate string returned by caml_stat_strdup_of_os. Under Unix, it is equivalent to caml_copy_string.

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:

/* The following define is necessary because the API is experimental */

#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);


20.14 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:

  • OCaml source files and object files (.cmo, .cmx, .ml) comprising the OCaml part of the library;
  • C object files (.o, .a, respectively, .obj, .lib) comprising the C part of the library;
  • Support libraries for the C part (-llib).

It generates the following outputs:

  • An OCaml bytecode library .cma incorporating the .cmo and .ml OCaml files given as arguments, and automatically referencing the C library generated with the C object files.
  • An OCaml native-code library .cmxa incorporating the .cmx and .ml OCaml files given as arguments, and automatically referencing the C library generated with the C object files.
  • If dynamic linking is supported on the target platform, a .so (respectively, .dll) shared library built from the C object files given as arguments, and automatically referencing the support libraries.
  • A C static library .a(respectively, .lib) built from the C object files.

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 liboutputc.so (if shared libraries are supported) and liboutputc.a. If not specified, defaults to the output name given with -o.

On native Windows, the following environment variable is also consulted:

Alternative executable to use instead of the configured value. Primarily used for bootstrapping.

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 dllzip.so 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 dllzip.so 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 dllzip.so 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.

20.15 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.

20.15.1 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>

20.15.2 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

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