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.

blit_memory_to_bytes 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.

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blit_bytes_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.

blit_string_to_memory src srcoff dst dstoff len: A version for immutable strings

Return a new bigarray as a copy of the string

Return a new bigarray as a copy of the string

Return a new string as a copy of the bigarray

Return a new string as a copy of the bigarray

Returns the start address of the buffer

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.)

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.

The best guess at the page size

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.

If exec, the memory region is marked as executable.

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

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.

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.

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.)

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.

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.

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.

Same as as_value but returns the value as Obj.t

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!

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

A version for immutable strings

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".

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.

Returns bytelen if init_string was called with the passed len.

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.

Same for arrays of floats

Returns bytelen if init_array was called with the passed size.

Same for arrays of floats

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.

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

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

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.

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

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.

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).

Reclaim as much memory as possible

Returns the size of the memory blocks in bytes

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

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

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

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

Returns a report describing the memory allocation in the pool