Erlang MMAP `emmap`

This Erlang library provides a wrapper that allows you to memory map files into the Erlang memory space.

Authors

Basic Usage

The basic usage is

{ok, Mem} = emmap:open("filename", [read, shared, direct]),
{ok, Binary} = file:pread(Mem, 100, 40),
...
ok = file:close(Mem).

The open options is a list containing zero or more options.

From this point, Mem can be used with the file operations:

{ok, Binary} = file:pread(Mem, Position, Length) read Length bytes at Position in the file.
ok = file:pwrite(Mem, Position, Binary) writes to the given position.
{ok, Binary} = file:read(Mem, Length) read 1..Length bytes from current position, or return eof if pointer is at end of file.
{ok, Pos} = file:position(Mem, Where) see file:position/2 documentation.
ok = file:close(Mem)

Additionally, the read/write operations can be performed by the corresponding functions in the emmap module.

Atomic operations on the memory mapped file

The emmap application offers a way to do atomic ADD, SUB, XCHG as well as bitwise AND, OR, XOR operations using emmap:patomic/4 function.

Effectively this directly changes the content of the underlying memory and is thread-safe.

{ok, OldValue} = emmap:patomic(Mem, Position, add, 1).

This approach allows to implement persistent atomic counters that survive node restarts.

Atomic persistent counters

The emmap application allows a user to maintain atomic persistent counters. This could be useful for continuous numbering of some events in the system which could be efficiently shared among processes in a thread-safe way and at the same time being persistent. This is a very light-weight approach compared to using mnesia or other form of persistent storage.

Here is an example:

F  = emmap:open_counters("/tmp/mem.bin", 2),
N1 = emmap:inc_counter(F, 0),
N2 = emmap:inc_counter(F, 0, 5),
N3 = emmap:inc_counter(F, 0),
N4 = emmap:inc_counter(F, 0, 2),
N5 = emmap:set_counter(F, 0, 15),
N6 = emmap:read_counter(F, 0),
emmap:close_counters(F),
io_format("N1=~w, N2=~w, N3=~w, N4=~w, N5=~w, N6=~w\n",
          [N1, N2, N3, N4, N5, N6]).  % Prints: N1=0, N2=1, N3=6, N4=7, N5=9, N6=15

Shared memory and using mutable binaries

While Erlang goes at length to achieve immutability, sometimes applications might need to have access to mutable memory. This can be accomplished by using the direct shared access to the memory mapped file.

Example:

shell1> {ok, MM} = emmap:open("/tmp/mem.data", 0, 8, [create, direct, read, write, shared, nolock]).
shell2> {ok, MM} = emmap:open("/tmp/mem.data", 0, 8, [create, direct, read, write, shared, nolock]).

shell1> emmap:pwrite(MM, 0, <<"test1">>).
shell2> {ok, Bin} = emmap:pread(MM, 0, 5).
{ok, <<"test1">>}

shell1> emmap:pwrite(MM, 0, <<"test2">>).
shell2> Bin.
<<"test2">>

shell1> emmap:pwrite(MM, 0, <<"test3">>).
shell2> Bin.
<<"test3">>

Notes

Using the option direct has the effect that the mmap file is not closed until all references to binaries coming out of read/pread have been garbage collected. This is a consequence of that such binaries are referring directly to the mmap'ed memory.