# Module Netsys_mem

    module Netsys_mem: sig .. end

Bigarrays as memory buffers

    type memory = Netsys_types.memory

We consider 1-dimensional bigarrays of chars as memory buffers. They have the useful property that the garbage collector cannot relocate them, i.e. the address is fixed. Also, one can mmap a file, and connect the bigarray with shared memory.

## General

    val blit_memory_to_string : memory -> int -> string -> int -> int -> unit

`blit_memory_to_string src srcoff dst dstoff len` copies `len` characters from buffer `src`, starting at character number `srcoff`, to string `dst`, starting at character number `dstoff`

Raise `Invalid_argument` if `srcoff` and `len` do not designate a valid subbuffer of `src`, or if `dstoff` and `len` do not designate a valid substring of `dst`.

    val blit_memory_to_string_unsafe : memory -> int -> string -> int -> int -> unit

Unsafe version

    val blit_string_to_memory : string -> int -> memory -> int -> int -> unit

`blit_string_to_memory src srcoff dst dstoff len` copies `len` characters from string `src`, starting at character number `srcoff`, to buffer `dst`, starting at character number `dstoff`

Raise `Invalid_argument` if `srcoff` and `len` do not designate a valid substring of `src`, or if `dstoff` and `len` do not designate a valid subbuffer of `dst`.

    val blit_string_to_memory_unsafe : string -> int -> memory -> int -> int -> unit

Unsafe version

    val memory_address : memory -> nativeint

Returns the start address of the buffer

    val memory_of_bigarray : ('a, 'b, 'c) Bigarray.Genarray.t -> memory

    val memory_of_bigarray_1 : ('a, 'b, 'c) Bigarray.Array1.t -> memory

    val memory_of_bigarray_2 : ('a, 'b, 'c) Bigarray.Array2.t -> memory

    val memory_of_bigarray_3 : ('a, 'b, 'c) Bigarray.Array3.t -> memory

These functions return an arbitrary bigarray as `memory`.

Due to a bug in the OCaml runtime, this is for now limited to arrays of up to 2G size (in bytes). (The bug exists at least up to OCaml-3.12.1.)

## Allocation and memory-mapping

    val getpagesize : unit -> int

Returns the size of a page as reported by `sysconf`.

On many systems, a page has 4096 bytes, but this cannot be relied upon.

This function is only available if the system has `sysconf`.

    val pagesize : int

The best guess at the page size

    val alloc_memory_pages : ?addr:nativeint -> int -> memory

Allocates memory in units of pages. The memory buffer will start on a page boundary.

The passed int is the requested number of bytes. The size of the buffer is rounded up so a whole number of pages is allocated.

Optionally, one can request a certain address `addr` (which must be a multiple of the page size). There is, however, no guarantee that this wish can be fulfilled. In any way, one should check with `memory_address` what the start address really is.

This function is only available if the system has `sysconf`, `mmap`, and allows to allocate anonymous memory with `mmap` (outside POSIX but common).

    val alloc_aligned_memory : int -> int -> memory

`alloc_aligned_memory alignment size`: Allocates a buffer of `size` whose start address is a multiple of `alignment`. The `alignment` must be a power of two, and at least `Sys.word_size/8`.

Aligned memory can be useful for ensuring that the whole memory block is in the same cache line. A cache line typically has 64 or 128 bytes - but this is very platform-specific. (Linux: look at `/proc/cpuinfo`.)

This function is only available if the system has `posix_memalign`.

    val memory_map_file : Unix.file_descr ->
           ?pos:int64 -> ?addr:nativeint -> bool -> int -> memory

`memory_map_file fd shared size`: Maps `size` bytes of the file `fd` into memory, and returns the memory buffer like `Bigarray.Array1.map_file`. `pos` and `shared` have the same meaning as there. In `addr` one can suggest a start address. There is, however, no guarantee that this wish can be fulfilled.

    val memory_unmap_file : memory -> unit

Unmaps the file. The memory block must have been allocated with `memory_map_file` or with `Bigarray.Array1.map_file`.

Note that the data pointer of the bigarray is set to NULL, and that any further access of the array will trigger a segmentation violation! The intention of this function is to control when the file mapping is removed. Normally, this is done first when the GC finalizer is run.

It is required that there are no subarrays at the time of calling this function. (If so, the function does nothing.)

    val zero_pages : memory -> int -> int -> unit

`zero_pages m pos len`: If possible, the memory pages in the range `pos` to `pos+len-1` of `m` are allocated again, so that they replace the previous pages.

It is required that the start address of the range is a multiple of the page size, and the `len` is a multiple of the page size. Fails with `Invalid_argument` if the requirements are not met, or the function is otherwise unavailable.

Calling `zero_pages` is sometimes an optimization when old memory pages can be dropped, and when the alternative of overwriting these pages would imply a copy-on-write operation.

    val grab : nativeint -> int -> memory

`grab addr len`: Interprets the address range from `addr` to `addr+len-1` as `memory` bigarray.

This function does not allocate! It assumes that the given address range points to valid memory.

## Interpreting memory as values

    val as_value : memory -> int -> 'a

`as_value mem offset`: Returns a pointer to `mem+offset`. There must be a valid boxed value at this address (i.e. at the word preceding `mem+offset` there must be a valid block header, followed by a valid value of the right type). However, this is not checked:

This is an unsafe function that may crash the program if used in the wrong way!

It is possible that the memory block is deallocated while the returned value still exists. Any attempt to access the value will result into undefined behavior (anything from funny results to crashes may happen).

Some Ocaml primitives might not work on the returned values (polymorphic equality, marshalling, hashing) unless `Netsys_mem.value_area` is called for the memory block.

    val as_obj : memory -> int -> Obj.t

Same as `as_value` but returns the value as `Obj.t`

    val value_area : memory -> unit

Marks the memory block as value area. This enables that the value primitives (polymorphic equality, marshalling, hashing) return meaningful results. The memory area is automatically unmarked when the finaliser for the memory block is run.

Be careful when marking sub arrays.

This function is first available since O'Caml 3.11.

    val obj_address : Obj.t -> nativeint

    val hdr_address : Obj.t -> nativeint

These two functions return the address of the `Obj.t` and the address of the header of the `Obj.t`, respectively.

Note that this can only be relied upon if the input object cannot be moved around by the garbage collector!

    val cmp_string : string -> string -> int

Compares two strings like `String.compare`. This also works when the strings reside outside the O'Caml heap, e.g. in a `memory` block.

    exception Out_of_space

    val init_header : memory -> int -> int -> int -> unit

`init_header mem offset tag size`: Initializes the word at `mem+offset` as an Ocaml value header with the given `tag` and the given `size` (in words). The GC color is always set to "white".

    val init_string : memory -> int -> int -> int * int

`let voffset, bytelen = init_string mem offset len`: Initializes the memory at `offset` and following bytes as Ocaml string with length `len`. Returns in `voffset` the offset where the value starts (i.e. `offset` plus one word), and in `bytelen` the number of bytes used in `mem`.

`offset` must be a multiple of the word size in bytes.

The string can be accessed with

     let s = (as_value mem voffset : string)

The function is useful for initializing shared memory as string so that several processes can directly access the string.

The string has the GC color `White`.

Raises `Out_of_space` if the memory block is too small.

    val init_string_bytelen : int -> int

Returns `bytelen` if `init_string` was called with the passed `len`.

    val init_array : memory -> int -> int -> int * int

`let voffset, bytelen = init_array mem offset size`: Initializes the memory at `offset` and following bytes as Ocaml array with `size` elements. Returns in `voffset` the offset where the value starts (i.e. `offset` plus one word), and in `bytelen` the number of bytes used in `mem`.

The array cannot be used as float array.

`offset` must be a multiple of the word size in bytes.

The array can be accessed with

     let a = (as_value mem voffset : _ array)

The elements of the array have a value but it might not be valid for the element type of the array. Because of this, it is unwise to access the elements before setting them for the first time.

The array has the GC color `White`.

Raises `Out_of_space` if the memory block is too small.

    val init_float_array : memory -> int -> int -> int * int

Same for arrays of floats

    val init_array_bytelen : int -> int

Returns `bytelen` if `init_array` was called with the passed `size`.

    val init_float_array_bytelen : int -> int

Same for arrays of floats

    type custom_ops = nativeint

`type ``init_value_flag =`

`|` | `Copy_bigarray`
`|` | `Copy_custom_int`
`|` | `Copy_atom`
`|` | `Copy_simulate`

    val init_value : ?targetaddr:nativeint ->
           ?target_custom_ops:(string * custom_ops) list ->
           memory ->
           int -> 'a -> init_value_flag list -> int * int

`let voffset, bytelen = init_value mem offset v flags`: Initializes the memory at `offset` and following bytes as copy of the boxed value `v`. Returns in `voffset` the offset where the value starts (i.e. `offset` plus one word), and in `bytelen` the number of bytes used in `mem`.

The copied value can then be accessed with

     let v' = (as_value mem voffset : 'a)

`offset` must be a multiple of the word size in bytes.

The input value `v` must be heap-allocated. Also, a number of restrictions and caveats apply:

- Objects, closures, and lazy values are not supported
- Bigarrays are only supported if the `Copy_bigarray` flag is given. In this case, a copy of the bigarray is also made and appended to the value copy (i.e. it is also placed into the buffer `mem`).
- Abstract and custom values need to be enabled. For `int32`, `int64`, and `nativeint` the flag `Copy_custom_int` enables copying, and for bigarrays the flag `Copy_bigarray`. Generally, there is a function pointer in such data blocks which might be invalid when the memory buffer is loaded into a different executable. This specific problem can be fixed by passing `target_custom_ops` with the right pointers.
- Atoms (i.e. zero-sized blocks such as empty arrays) are only supported if the `Copy_atom` flag is present. It is, however, illegal to copy atoms because they lose then their atomic property. This breaks comparisons, e.g. `if array=[| |] then...`. Unfortunately there is nothing we can do about it.
- The input value may reside outside the Ocaml heap. This may break badly written C wrappers that do not use abstract or custom tags to mark foreign data.

The function raises `Out_of_space` if the memory block is too small. Cyclic input values are supported.

If the `Copy_simulate` flag is given, `mem` is not modified. In simulation mode, it is pretended that `mem` is as large as necessary to hold the value, no matter how large `mem` really is. The returned values `voffset` and `bytelen` reflect how much of `mem` would have been used.

If the `targetaddr` argument is passed, it is assumed that the memory block is mapped at this address and not at the address it is really mapped. This is useful for preparing memory that is going to be mapped at a different address than it is right now.

The new value has the GC color `White`.

    val get_custom_ops : 'a -> string * custom_ops

Returns custom ops for a sample value (or `Invalid_argument`)

    val copy_value : init_value_flag list -> 'a -> 'a

`copy_value flags v`: Creates a deep copy of `v` and returns it. The copy is allocated in the normal Ocaml heap.

Restrictions:

- Objects, closures, and lazy values are not supported (FIXME)
- Bigarrays are only supported if the `Copy_bigarray` flag is given. In this case, a copy of bigarrays are also made, and placed into additional buffers obtained via `stat_alloc`.
- Abstract and custom values need to be enabled. For `int32`, `int64`, and `nativeint` the flag `Copy_custom_int` enables copying, and for bigarrays the flag `Copy_bigarray`.
- Atoms are automatically fixed. `Copy_atoms` is ignored.

Cyclic input values are supported. `Copy_simulate` is ignored.
`type ``color =`

`|` | `White`
`|` | `Gray`
`|` | `Blue`
`|` | `Black` | `(*` | GC colors | `*)`

    val color : Obj.t -> color

Return the GC color

    val set_color : Obj.t -> color -> unit

Set the GC color

## I/O using `memory` as buffers

    val mem_read : Unix.file_descr -> memory -> int -> int -> int

A version of `Unix.read` that uses a `memory` buffer. Some OS allow faster I/O when `memory` is page-aligned (see `alloc_memory_pages`). Also, a copy in the stub function can be avoided. Both effects can result in a considerable speedup.

    val mem_write : Unix.file_descr -> memory -> int -> int -> int

A version of `Unix.single_write` that uses a `memory` buffer.

    val mem_recv : Unix.file_descr ->
           memory -> int -> int -> Unix.msg_flag list -> int

    val mem_send : Unix.file_descr ->
           memory -> int -> int -> Unix.msg_flag list -> int

Versions of `Unix.recv`, and `Unix.send` using `memory` buffers.

## Buffer pools

    type memory_pool

A pool of `memory` blocks that are all the same size and page-aligned (if the OS supports this). The pool tries to bundle memory allocations so that not for every block a system call is required. This reduces the number of system calls, and the number of entries in the process page table. Also, unused blocks are automatically returned to the pool.

    val create_pool : int -> memory_pool

Create a new pool. The argument is the size of the memory blocks (must be a multiple of the page size)

    val pool_alloc_memory : memory_pool -> memory

`let m = pool_alloc_memory p`: Gets a memory block `m` from the pool `p`. If required, new blocks are automatically allocated and added to the pool. This function is thread-safe.

The memory block is automatically garbage-collected.

    val pool_alloc_memory2 : memory_pool -> memory * (unit -> unit)

`let m, free = pool_alloc_memory2 p`: Gets a memory block `m` from the pool `p` like `pool_alloc_memory`. This function also returns the function `free` marking the block as free again. The block can then be immediately recycled for another use.

If `free` is not called, the block `m` is first recycled when it is not referenced any more (like in `pool_alloc_memory`).

    val pool_reclaim : memory_pool -> unit

Reclaim as much memory as possible

    val pool_block_size : memory_pool -> int

Returns the size of the memory blocks in bytes

    val default_block_size : int

The default block size, normally 64 K (or better, 16 times the page size)

    val default_pool : memory_pool

The default pool with the default block size. This pool is used by Ocamlnet itself as much as possible

    val small_block_size : int

The block size of `small_pool`, normally 4K (or better, the page size)

    val small_pool : memory_pool

Another standard pool where the blocks are smaller than in `default_pool`.

    val pool_report : memory_pool -> string

Returns a report describing the memory allocation in the pool