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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_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 memory_address : memory -> nativeint
Returns the start address of the buffer

Allocation and memory-mapping


val getpagesize : unit -> int
Returns the size of a page.

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 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.


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 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 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_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.

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.
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.

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.

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,free) = 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_block_size : memory_pool -> int
Returns the size of the memory blocks in bytes
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 pool_report : memory_pool -> string
Returns a report describing the memory allocation in the pool
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