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Chapter 18 Interfacing C with Objective Caml

This chapter describes how user-defined primitives, written in C, can be linked with Caml code and called from Caml functions.

18.1 Overview and compilation information

18.1.1 Declaring primitives

User primitives are declared in an implementation file or struct...end 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 input primitive is declared in the standard library module Pervasives:
        external input : in_channel -> string -> int -> int -> int
                       = "input"
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 sig...end signatures either as regular values
        val name : type
thus hiding their implementation as a C function, 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 Caml function.

The arity (number of arguments) of a primitive is automatically determined from its Caml 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 * string * int * int -> int = "input2"
has arity 1, and the input2 C function receives one argument (which is a quadruple of Caml 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.

18.1.2 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 Caml values. It encodes objects of several base types (integers, floating-point numbers, strings, ...), as well as Caml data structures. The type value and the associated conversion functions and macros are described in details 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 a Caml 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 Caml 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 Caml 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 Caml values, and encoding the return value as a Caml 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 Caml values to C values, call the first function, and convert the returned C value to Caml 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 following it is exported and accessible from Caml.) 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 Objective Caml 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 Caml 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 18.4.5)
caml/callback.h callback from C to Caml (see section 18.7).
caml/custom.h operations on custom blocks (see section 18.9).
caml/intext.h operations for writing user-defined serialization and deserialization functions for custom blocks (see section 18.9).

These files reside in the caml/ subdirectory of the Objective Caml standard library directory (usually /usr/local/lib/ocaml).

18.1.3 Statically linking C code with Caml code

The Objective Caml 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 Caml 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 18.1.4 below.)

In the ``custom runtime'' mode, the Caml linker scans the object files and determines the set of required primitives. Then, it builds a suitable runtime system, by calling the native code linker with: This builds a runtime system with the required primitives. The Caml linker generates bytecode for this custom runtime system. The bytecode is appended to the end of the custom runtime system, so that it will be automatically executed when the output file (custom runtime + bytecode) is launched.

To link in ``custom runtime'' mode, execute the ocamlc command with: If you are using the native-code compiler ocamlopt, the -custom flag is not needed, as the final linking phase of ocamlopt always builds a standalone executable. To build a mixed Caml/C executable, execute the ocamlopt command with: Starting with OCaml 3.00, it is possible to record the -custom option as well as the names of C libraries in a Caml library file .cma or .cmxa. For instance, consider a Caml library mylib.cma, built from the Caml 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, of course, is 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.

18.1.4 Dynamically linking C code with Caml code

Starting with OCaml 3.03, an alternative to static linking of C code using the -custom code is provided. In this mode, the Caml 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 Caml runtime system ocamlrun then loads dynamically these libraries, and resolves references to the required primitives, before executing the bytecode.

This facility is currently supported and known to work well under Linux and Windows (the native Windows port). It is supported, but not fully tested yet, under FreeBSD, Tru64, Solaris and Irix. It is not supported yet under other Unixes, Cygwin for Windows, and MacOS.

To dynamically link C code with Caml 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, 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 10.3). Finally (step 3), execute the ocamlc command with Do not set the -custom flag, otherwise you're back to static linking as described in section 18.1.3. Under Unix, the ocamlmklib tool (see section 18.10) 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 a Caml .cmo library archive. Consider again a Caml library mylib.cma, built from the Caml object files a.cmo and b.cmo, which reference C code in If the library is built as follows:
        ocamlc -a -o mylib.cma a.cmo b.cmo -dllib -lmylib
users of the library can simply link with mylib.cma:
        ocamlc -o myprog mylib.cma ...
and the system will automatically add the -dllib -lmylib option, achieving the same effect as
        ocamlc -o myprog a.cmo b.cmo ... -dllib -lmylib
Using this mechanism, users of the library mylib.cma do not need to known that it references C code, nor whether this C code must be statically linked (using -custom) or dynamically linked.

18.1.5 Choosing between static linking and dynamic linking

After having described two different ways of linking C code with Caml code, we now review the pros and cons of each, to help developers of mixed Caml/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 18.10) 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.

18.1.6 Building standalone custom runtime systems

It is sometimes inconvenient to build a custom runtime system each time Caml 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).

18.2 The value type

All Caml 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:
18.2.1 Integer values

Integer values encode 31-bit signed integers (63-bit on 64-bit architectures). They are unboxed (unallocated).

18.2.2 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 benefits 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 Caml 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.
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 atttached.

18.2.3 Pointers outside the heap

Any word-aligned pointer to an address outside the heap can be safely cast to and from the type value. This includes pointers returned by malloc, and pointers to C variables (of size at least one word) obtained with the & operator.

Caution: if a pointer returned by malloc is cast to the type value and returned to Caml, explicit deallocation of the pointer using free is potentially dangerous, because the pointer may still be accessible from the Caml world. Worse, the memory space deallocated by free can later be reallocated as part of the Caml heap; the pointer, formerly pointing outside the Caml heap, now points inside the Caml heap, and this can confuse the garbage collector. To avoid these problems, it is preferable to wrap the pointer in a Caml block with tag Abstract_tag or Custom_tag.

18.3 Representation of Caml data types

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

18.3.1 Atomic types

Caml type Encoding
int Unboxed integer values.
char Unboxed integer values (ASCII code).
float Blocks with tag Double_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.

18.3.2 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 label declared next 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.)

18.3.3 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 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.

18.3.4 Concrete 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. Constant constructors are represented by unboxed integers equal to the constructor number. Non-constant constructors declared with a n-tuple as argument are represented by a block of size n, tagged with the constructor number; the n fields contain the components of its tuple argument. Other non-constant constructors are represented by a block of size 1, tagged with the constructor number; the field 0 contains the value of the constructor argument. 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.

18.3.5 Objects

Objects are represented as zero-tagged blocks. The first field of the block refers to the object class and associated method suite, in a format that cannot easily be exploited from C. The remaining fields of the object contain the values of the instance variables of the object. Instance variables are stored in the order in which they appear in the class definition (taking inherited classes into account).

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);
18.3.6 Variants

Like constructed terms, values of variant types are represented either as integers (for variants without arguments), or as blocks (for variants with an argument). Unlike constructed terms, variant constructors are not numbered starting from 0, but identified by a hash value (a Caml 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, variant values taking several arguments are not flattened. That is, `VConstr(v, v') is represented by a block of size 2, whose field number 1 contains the representation of the pair (v, v'), but not as a block of size 3 containing v and v' in fields 1 and 2.

18.4 Operations on values

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

18.4.4 Allocating blocks

Simple interface
Low-level interface

The following functions are slightly more efficient than 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.
18.4.5 Raising exceptions

Two functions are provided to raise two standard exceptions: Raising arbitrary exceptions from C is more delicate: the exception identifier is dynamically allocated by the Caml program, and therefore must be communicated to the C function using the registration facility described below in section 18.7.3. Once the exception identifier is recovered in C, the following functions actually raise the exception:
18.5 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.

18.5.1 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 or CAMLreturn0.

There are six CAMLparam macros: CAMLparam0 to CAMLparam5, which take zero to five arguments respectively. If your function has fewer 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 and CAMLreturn0 are used to replace the C keyword return. Every occurence of return x must be replaced by CAMLreturn (x), every occurence 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.

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. You can also use them in nested C blocks within the function.

value bar (value v1, value v2, value v3)
  CAMLparam3 (v1, v2, v3);
  CAMLlocal1 (result);
  result = 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 = 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.
Rule 4   Global variables containing values must be registered with the garbage collector using the register_global_root function.

Registration of a global variable v is achieved by calling register_global_root(&v) just before a valid value is stored in v for the first time.

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

Note: 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.

18.5.2 Low-level interface

We now give the GC rules corresponding to the low-level allocation functions alloc_small and alloc_shr. You can ignore those rules if you stick to the simplified allocation function 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 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 alloc_shr, filling is performed through the initialize function:

        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 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 alloc_shr, use initialize to assign a value to a field for the first time:

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

        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 = 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 = 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 alloc_small, and filled by direct assignments on their fields.
value alloc_list_int(int i1, int i2)
  CAMLparam0 ();
  CAMLlocal2 (result, r);

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

  r = 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 = 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 [] */
  modify(&Field(r, 1), tail);             /* cdr of the result = tail */
  return r;
It would be incorrect to perform Field(r, 1) = tail directly, because the allocation of tail has taken place since r was allocated. tail is not registered as a root because there is no allocation between the assignment where it takes its value and the modify statement that uses the value.

18.6 A complete example

This section outlines how the functions from the Unix curses library can be made available to Objective Caml programs. First of all, here is the interface curses.mli that declares the curses primitives and data types:
type window                   (* The type "window" remains abstract *)
external initscr: unit -> window = "curses_initscr"
external endwin: unit -> unit = "curses_endwin"
external refresh: unit -> unit = "curses_refresh"
external wrefresh : window -> unit = "curses_wrefresh"
external newwin: int -> int -> int -> int -> window = "curses_newwin"
external mvwin: window -> int -> int -> unit = "curses_mvwin"
external addch: char -> unit = "curses_addch"
external mvwaddch: window -> int -> int -> char -> unit = "curses_mvwaddch"
external addstr: string -> unit = "curses_addstr"
external mvwaddstr: window -> int -> int -> string -> unit = "curses_mvwaddstr"
(* lots more omitted *)
To compile this interface:
        ocamlc -c curses.mli
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.o, looks like:
#include <curses.h>
#include <mlvalues.h>

value curses_initscr(value unit)
  CAMLparam1 (unit);
  CAMLreturn ((value) initscr());  /* OK to coerce directly from WINDOW * to
                              value since that's a block created by malloc() */

value curses_wrefresh(value win)
  CAMLparam1 (win);
  wrefresh((WINDOW *) win);
  CAMLreturn (Val_unit);

value curses_newwin(value nlines, value ncols, value x0, value y0)
  CAMLparam4 (nlines, ncols, x0, y0);
  CAMLreturn ((value) newwin(Int_val(nlines), Int_val(ncols),
                             Int_val(x0), Int_val(y0)));

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

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

/* This goes on for pages. */
The file curses.c can be compiled with:
        cc -c -I/usr/local/lib/ocaml curses.c
or, even simpler,
        ocamlc -c curses.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 Caml program that uses the curses module:
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";
  for i = 1 to 100000 do () done;
To compile this program, run:
        ocamlc -c
Finally, to link everything together:
        ocamlc -custom -o test test.cmo curses.o -cclib -lcurses
(On some machines, you may need to put -cclib -ltermcap or -cclib -lcurses -cclib -ltermcap instead of -cclib -lcurses.)

18.7 Advanced topic: callbacks from C to Caml

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

18.7.1 Applying Caml closures from C

C functions can apply Caml functional values (closures) to Caml values. The following functions are provided to perform the applications: If the function f does not return, but raises an exception that escapes the scope of the application, then this exception is propagated to the next enclosing Caml code, skipping over the C code. That is, if a Caml function f calls a C function g that calls back a Caml 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 Caml function, it can use the functions callback_exn, callback2_exn, callback3_exn, 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 callback*_exn functions must be tested with the macro Is_exception_result(v). If the macro returns ``false'', no exception occured, and v is the value returned by the Caml function. If Is_exception_result(v) returns ``true'', an exception escaped, and its value (the exception descriptor) can be recovered using Extract_exception(v).

18.7.2 Registering Caml closures for use in C functions

The main difficulty with the callback functions described above is obtaining a closure to the Caml function to be called. For this purpose, Objective Caml provides a simple registration mechanism, by which Caml code can register Caml functions under some global name, and then C code can retrieve the corresponding closure by this global name.

On the Caml side, registration is performed by evaluating Callback.register n v. Here, n is the global name (an arbitrary string) and v the Caml value. For instance:
    let f x = print_string "f is applied to "; print_int n; 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 Caml 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 Caml function f above:
    void call_caml_f(int arg)
        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. On the other hand, the value pointed to can change during garbage collection and 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 value * closure_f = NULL;
        if (closure_f == NULL) {
            /* First time around, look up by name */
            closure_f = caml_named_value("test function");
        callback(*closure_f, Val_int(arg));
18.7.3 Registering Caml exceptions for use in C functions

The registration mechanism described above can also be used to communicate exception identifiers from Caml to C. The Caml 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 18.4.5) 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)
        raise_with_string(*caml_named_value("test exception"), msg);
18.7.4 Main program in C

In normal operation, a mixed Caml/C program starts by executing the Caml initialization code, which then may proceed to call C functions. We say that the main program is the Caml code. In some applications, it is desirable that the C code plays the role of the main program, calling Caml functions when needed. This can be achieved as follows:
18.7.5 Embedding the Caml 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 Caml 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 18.7.4, 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) containing the bytecode for the Caml 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 Caml runtime and execute the Caml 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 native-code compiler ocamlopt also supports the -output-obj option, causing it to output a C object file containing the native code for all Caml modules on the command-line, as well as the Caml startup code. Initialization is performed by calling caml_startup 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 Objective Caml runtime library (libcamlrun.a for bytecode, libasmrun.a for native-code), as well as all C libraries that are required by the Caml libraries used. For instance, assume the Caml 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/usr/local/lib/ocaml -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/usr/local/lib/ocaml -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 config/Makefile generated during compilation of Objective Caml, as the variables BYTECCLINKOPTS (for object files produced by ocamlc -output-obj) and NATIVECCLINKOPTS (for object files produced by ocamlopt -output-obj). Currently, the only ports that require special attention are:
18.8 Advanced example with callbacks

This section illustrates the callback facilities described in section 18.7. We are going to package some Caml functions in such a way that they can be linked with C code and called from C just like any C functions. The Caml functions are defined in the following Caml source:
(* File -- some ``useful'' Caml 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 Caml functions */

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

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

char * format_result(int n)
  static value * format_result_closure = NULL;
  if (format_result_closure == NULL)
    format_result_closure = caml_named_value("format_result");
  return strdup(String_val(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 Caml code to a C object file and put it in a C library along with the stub code in modwrap.c and the Caml runtime system:
        ocamlc -custom -output-obj -o modcaml.o
        ocamlc -c modwrap.c
        cp /usr/local/lib/ocaml/libcamlrun.a 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 fonctions fib and format_result in any C program, just like regular C functions. Just remember to call caml_startup once before.
/* File main.c -- a sample client for the Caml functions */

#include <stdio.h>

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

  /* Initialize Caml 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 main.c mod.a -lcurses
(On some machines, you may need to put -ltermcap or -lcurses -ltermcap instead of -lcurses.)

18.9 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.

18.9.1 The struct custom_operations

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

Custom blocks must be allocated via the alloc_custom function. 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 Caml 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.

18.9.3 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 Caml heap. In other terms, never store a Caml value in a custom block, and do not use Field, Store_field nor 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.

18.9.4 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
serialize_int_1 Write a 1-byte integer
serialize_int_2 Write a 2-byte integer
serialize_int_4 Write a 4-byte integer
serialize_int_8 Write a 8-byte integer
serialize_float_4 Write a 4-byte float
serialize_float_8 Write a 8-byte float
serialize_block_1 Write an array of 1-byte quantities
serialize_block_2 Write an array of 2-byte quantities
serialize_block_4 Write an array of 4-byte quantities
serialize_block_8 Write an array of 8-byte quantities
deserialize_uint_1 Read an unsigned 1-byte integer
deserialize_sint_1 Read a signed 1-byte integer
deserialize_uint_2 Read an unsigned 2-byte integer
deserialize_sint_2 Read a signed 2-byte integer
deserialize_uint_4 Read an unsigned 4-byte integer
deserialize_sint_4 Read a signed 4-byte integer
deserialize_uint_8 Read an unsigned 8-byte integer
deserialize_sint_8 Read a signed 8-byte integer
deserialize_float_4 Read a 4-byte float
deserialize_float_8 Read an 8-byte float
deserialize_block_1 Read an array of 1-byte quantities
deserialize_block_2 Read an array of 2-byte quantities
deserialize_block_4 Read an array of 4-byte quantities
deserialize_block_8 Read an array of 8-byte quantities
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.

18.9.5 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 Objective Caml runtime system; do not use them for your custom data. We recommend to use a URL ( or a Java-style package name (com.mydomain.mymachine.mylibrary.version-number) as identifiers, to minimize the risk of identifier collision.

18.9.6 Finalized blocks

Custom blocks generalize the finalized blocks that were present in Objective Caml 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. alloc_final(n, f, used, max) returns a fresh custom block of size n words, with finalization function f. The first word is reserved for storing the custom operations; the other n-1 words are available for your data. The two parameters used and max are used to control the speed of garbage collection, as described for alloc_custom.

18.10 Building mixed C/Caml libraries: ocamlmklib

The ocamlmklib command facilitates the construction of libraries containing both Caml code and C code, and usable both in static linking and dynamic linking modes.

This command is available only under Cygwin, but not for the native Win32 port.
The ocamlmklib command takes three kinds of arguments: It generates the following outputs: In addition, the following options are recognized:
-cclib, -ccopt, -I, -linkall
These options are passed as is to ocamlc or ocamlopt. See the documentation of these commands.
-pthread, -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 Caml library. ocamlmklib will generate output.cma and/or output.cmxa. If not specified, defaults to a.
-oc outputc
Set the name of the generated C library. ocamlmklib will generate (if shared libraries are supported) and liboutputc.a. If not specified, defaults to the output name given with -o.
Consider a Caml 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 a Caml 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 Caml libraries zip.cma and zip.cmxa, as well as the companion C libraries and libzip.a:
ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib
If shared libraries are supported, this performs the following commands:
ocamlc -a -o zip.cma zip.cmo -dllib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \
        -cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
gcc -shared -o zipstubs.o -lz -L/usr/local/zlib
ar rc libzip.a zipstubs.o
If shared libraries are not supported, the following commands are performed instead:
ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -lzip \
        -cclib -lz -ccopt -L/usr/local/zlib
ar rc libzip.a zipstubs.o
Instead of building simultaneously the bytecode library, the native-code library and the C libraries, ocamlmklib can be called three times to build each separately. Thus,
ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib
builds the bytecode library zip.cma, and
ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib
builds the native-code library zip.cmxa, and
ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib
builds the C libraries and libzip.a. Notice that the support libraries (-lz) and the corresponding options (-L/usr/local/zlib) must be given on all three invocations of ocamlmklib, because they are needed at different times depending on whether shared libraries are supported.

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