Element kinds

Bigarrays can contain elements of the following kinds:

• IEEE half precision (16 bits) floating-point numbers (Bigarray.float16_elt),
• IEEE single precision (32 bits) floating-point numbers (Bigarray.float32_elt),
• IEEE double precision (64 bits) floating-point numbers (Bigarray.float64_elt),
• IEEE single precision (2 * 32 bits) floating-point complex numbers (Bigarray.complex32_elt),
• IEEE double precision (2 * 64 bits) floating-point complex numbers (Bigarray.complex64_elt),
• 8-bit integers (signed or unsigned) (Bigarray.int8_signed_elt or Bigarray.int8_unsigned_elt),
• 16-bit integers (signed or unsigned) (Bigarray.int16_signed_elt or Bigarray.int16_unsigned_elt),
• OCaml integers (signed, 31 bits on 32-bit architectures, 63 bits on 64-bit architectures) (Bigarray.int_elt),
• 32-bit signed integers (Bigarray.int32_elt),
• 64-bit signed integers (Bigarray.int64_elt),
• platform-native signed integers (32 bits on 32-bit architectures, 64 bits on 64-bit architectures) (Bigarray.nativeint_elt).

Each element kind is represented at the type level by one of the *_elt types defined below (defined with a single constructor instead of abstract types for technical injectivity reasons).

• since 4.07 Moved from otherlibs to stdlib.

• since 5.2 Added float16_elt element kind.

Array layouts

Supported layouts

The GADT type 'a layout represents one of the two supported memory layouts: C-style or Fortran-style. Its constructors are re-exported as values below for backward-compatibility reasons.

Generic arrays (of arbitrarily many dimensions)

Zero-dimensional arrays

One-dimensional arrays

Two-dimensional arrays

Three-dimensional arrays

Coercions between generic Bigarrays and fixed-dimension Bigarrays

Re-shaping Bigarrays

Bigarrays and concurrency safety

Care must be taken when concurrently accessing bigarrays from multiple domains: accessing a bigarray will never crash a program, but unsynchronized accesses might yield surprising (non-sequentially-consistent) results.

Atomicity

Every bigarray operation that accesses more than one array element is not atomic. This includes slicing, bliting, and filling bigarrays.

For example, consider the following program:

open Bigarray
let size = 100_000_000
let a = Array1.init Int C_layout size (fun _ -> 1)
let update f a () =
  for i = 0 to size - 1 do a.{i} <- f a.{i} done
let d1 = Domain.spawn (update (fun x -> x + 1) a)
let d2 = Domain.spawn (update (fun x -> 2 * x + 1) a)
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the bigarray a is either 2, 3, 4 or 5. If atomicity is required, then the user must implement their own synchronization (for example, using Mutex.t).

Data races

If two domains only access disjoint parts of the bigarray, then the observed behaviour is the equivalent to some sequential interleaving of the operations from the two domains.

A data race is said to occur when two domains access the same bigarray element without synchronization and at least one of the accesses is a write. In the absence of data races, the observed behaviour is equivalent to some sequential interleaving of the operations from different domains.

Whenever possible, data races should be avoided by using synchronization to mediate the accesses to the bigarray elements.

Indeed, in the presence of data races, programs will not crash but the observed behaviour may not be equivalent to any sequential interleaving of operations from different domains.

Tearing

Bigarrays have a distinct caveat in the presence of data races: concurrent bigarray operations might produce surprising values due to tearing. More precisely, the interleaving of partial writes and reads might create values that would not exist with a sequential execution. For instance, at the end of

let res = Array1.init Complex64 c_layout size (fun _ -> Complex.zero)
let d1 = Domain.spawn (fun () -> Array1.fill res Complex.one)
let d2 = Domain.spawn (fun () -> Array1.fill res Complex.i)
let () = Domain.join d1; Domain.join d2

the res bigarray might contain values that are neither Complex.i nor Complex.one (for instance 1 + i).