The bigarray library implements large, multi-dimensional, numerical
arrays. These arrays are called ``big arrays'' to distinguish them
from the standard Caml arrays described in
Module Array.
The main differences between ``big arrays'' and standard Caml arrays
are as follows:
-
Big arrays are not limited in size, unlike Caml arrays
(float array are limited to 2097151 elements on a 32-bit platform,
other array types to 4194303 elements).
- Big arrays are multi-dimensional. Any number of dimensions
between 1 and 16 is supported. In contrast, Caml arrays are
mono-dimensional and require encoding multi-dimensional arrays as
arrays of arrays.
- Big arrays can only contain integers and floating-point
numbers, while Caml arrays can contain arbitrary Caml data types.
However, big arrays provide more space-efficient storage of integer
and floating-point elements, in particular because they support
``small'' types such as single-precision floats and 8 and 16-bit
integers, in addition to the standard Caml types of double-precision
floats and 32 and 64-bit integers.
- The memory layout of big arrays is entirely compatible with that
of arrays in C and Fortran, allowing large arrays to be passed back
and forth between Caml code and C / Fortran code with no data copying
at all.
- Big arrays support interesting high-level operations that normal
arrays do not provide efficiently, such as extracting sub-arrays and
``slicing'' a multi-dimensional array along certain dimensions, all
without any copying.
Programs that use the bigarray library must be linked as follows:
ocamlc other options bigarray.cma other files
ocamlopt other options bigarray.cmxa other files
For interactive use of the bigarray library, do:
ocamlmktop -o mytop bigarray.cma
./mytop
or (if dynamic linking of C libraries is supported on your platform),
start ocaml and type #load "bigarray.cma";;.
| 29.1 |
Module Bigarray: large, multi-dimensional, numerical arrays |
|
Module Bigarray
| 29.2 |
Big arrays in the Caml-C interface |
|
C stub code that interface C or Fortran code with Caml code, as
described in chapter 18, can exploit big arrays as
follows.
The include file <caml/bigarray.h> must be included in the C stub
file. It declares the functions, constants and macros discussed
below.
| 29.2.2 |
Accessing a Caml bigarray from C or Fortran |
|
If v is a Caml value representing a big array, the expression
Data_bigarray_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 Caml big array can be consulted from C
as follows:
| C expression |
Returns |
|
Bigarray_val(v)->num_dims |
number of dimensions |
| Bigarray_val(v)->dim[i] |
i-th dimension |
| Bigarray_val(v)->flags & BIGARRAY_KIND_MASK |
kind of array elements |
The kind of array elements is one of the following constants:
| Constant |
Element kind |
|
BIGARRAY_FLOAT32 |
32-bit single-precision floats |
| BIGARRAY_FLOAT64 |
64-bit double-precision floats |
| BIGARRAY_SINT8 |
8-bit signed integers |
| BIGARRAY_UINT8 |
8-bit unsigned integers |
| BIGARRAY_SINT16 |
16-bit signed integers |
| BIGARRAY_UINT16 |
16-bit unsigned integers |
| BIGARRAY_INT32 |
32-bit signed integers |
| BIGARRAY_INT64 |
64-bit signed integers |
| BIGARRAY_CAML_INT |
31- or 63-bit signed integers |
| BIGARRAY_NATIVE_INT |
32- or 64-bit (platform-native) integers |
The following example shows the passing of a two-dimensional big array
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 = Bigarray_val(bigarray)->dim[0];
int dimy = Bigarray_val(bigarray)->dim[1];
/* C passes scalar parameters by value */
my_c_function(Data_bigarray_val(bigarray), dimx, dimy);
/* Fortran passes all parameters by reference */
my_fortran_function_(Data_bigarray_val(bigarray), &dimx, &dimy);
return Val_unit;
}
| 29.2.3 |
Wrapping a C or Fortran array as a Caml big array |
|
A pointer p to an already-allocated C or Fortran array can be
wrapped and returned to Caml as a big array using the alloc_bigarray
or alloc_bigarray_dims functions.
-
alloc_bigarray(kind | layout, numdims, p, dims)
Return a Caml big array wrapping the data pointed to by p.
kind is the kind of array elements (one of the BIGARRAY_
kind constants above). layout is BIGARRAY_C_LAYOUT for an
array with C layout and BIGARRAY_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.
- alloc_bigarray_dims(kind | layout, numdims,
p, (long) dim1, (long) dim2, ..., (long) dimnumdims)
Same as alloc_bigarray, 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 Caml.
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 alloc_bigarray(BIGARRAY_NATIVEINT | BIGARRAY_C_LAYOUT,
2, my_c_array, dims);
}
value caml_get_fortran_array(value unit)
{
return alloc_bigarray_dims(BIGARRAY_FLOAT32 | BIGARRAY_FORTRAN_LAYOUT,
2, my_fortran_array_, 300L, 400L);
}